% Copyright (c) 2000 The PARI Group
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% This file is part of the PARI/\kbd{gp} documentation
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\def\TITLE{A Tutorial for Pari/GP}
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\centerline{\mine A Tutorial}
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\centerline{\mine for}
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\centerline{\mine PARI / GP}
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\centerline{\sectiontitlebf (version \vers)}
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\noindent This booklet is a guided tour and a tutorial to the \kbd{gp}
calculator. Many examples will be given, but each time a new function is
used, the reader should look at the appropriate section in the \emph{User's
Manual to PARI/GP} for detailed explanations. This chapter can be read
independently, for example to get acquainted with the possibilities of
\kbd{gp} without having to read the whole manual. At this point.
\section{Greetings!}
So you are sitting in front of your workstation (or terminal, or PC\dots),
and you type \kbd{gp} to get the program started (or click on the relevant
icon, or select some menu item). It says hello in its particular manner, and
then waits for you after its \kbd{prompt}, initially \kbd{?} (or something
like {\bf gp}~\kbd{>}). Type
\bprog
2 + 2
@eprog\noindent What happens? Maybe not what you expect. First of all, of
course, you should tell \kbd{gp} that your input is finished, and this is
done by hitting the \kbd{Return} (or \kbd{Newline}, or \kbd{Enter}) key. If
you do exactly this, you will get the expected answer. However some of you
may be used to other systems like Gap, Macsyma, Magma or Maple. In this case,
you will have subconsciously ended the line with a semicolon ``\kbd{;}''
before hitting \kbd{Return}, since this is how it is done on those systems.
In that case, you will simply see \kbd{gp} answering you with a smug
expression, i.e.~a new prompt and no answer! This is because a semicolon at
the end of a line tells \kbd{gp} not to print the result (it is still stored
in the result history). You will certainly want to use this feature if the
output is several pages long. Try
\bprog
27 * 37
@eprog\noindent Wow! even multiplication works. Actually, maybe those
spaces are not necessary after all. Let's try \kbd{27*37}. Seems to be ok. We
will still insert them in this document since it makes things easier to read,
but as \kbd{gp} does not care about them, you don't have to type them all.
Now this session is getting lengthy, so the second thing one needs to learn
is to quit. Each system has its quit signal. In \kbd{gp}, you can use
\kbd{quit} or \b{q} (backslash q), the \kbd{q} being of course for quit.
Try it.
Now you've done it! You're out of \kbd{gp}, so how do you want to continue
studying this tutorial? Get back in please.
Let's get to more serious stuff. I seem to remember that the decimal
expansion of $1/7$ has some interesting properties. Let's see what \kbd{gp}
has to say about this. Type
\bprog
1 / 7
@eprog\noindent
What? This computer is making fun of me, it just spits back to me my own
input, that's not what I want!
Now stop complaining, and think a little. Mathematically, $1/7$ is an element
of the field $\Q$ of rational numbers, so how else but $1/7$ can the computer
give the answer to you? Well maybe $2/14$ or $7^{-1}$, but why complicate
matters? Seriously, the basic point here is that PARI, hence \kbd{gp}, will
almost always try to give you a result which is as precise as possible (we
will see why ``almost'' later). Hence since here the result can be
represented exactly, that's what it gives you.
But I still want the decimal expansion of $1/7$. No problem. Type one of
the following:
\bprog
1./ 7
1 / 7.
1./ 7.
1 / 7 + 0.
@eprog\noindent
Immediately a number of decimals of this fraction appear, 38 on most systems,
28 on the others, and the repeating pattern is $142857$. The reason is that
you have included in the operations numbers like \kbd{0.}, \kbd{1.} or \kbd{7.}
which are \emph{imprecise} real numbers, hence \kbd{gp} cannot give you an
exact result.
Why 28 / 38 decimals by the way? Well, it is the default initial precision.
This has been chosen so that the computations are very fast, and gives
already 12 decimals more accuracy than conventional double precision floating
point operations. The precise value depends on a technical reason: if your
machine supports 64-bit integers (the standard C library can handle integers
up to $2^{64}$), the default precision is 38 decimals, and 28 otherwise.
For definiteness, we will assume the former henceforth. Of course, you can
extend the precision (almost) as much as you like as we will see in a moment.
I'm getting bored, why don't we get on with some more exciting stuff? Well,
try \kbd{exp(1)}. Presto, comes out the value of $e$ to 38 digits. Try
\kbd{log(exp(1))}. Well, we get a floating point number and not an exact $1$,
but pretty close! That's what you lose by working numerically.
What could we try now? Hum, \kbd{pi}? The answer is not that
enlightening. \kbd{Pi}? Ah. This works better. But let's remember that
\kbd{gp} distinguishes between uppercase and lowercase letters. \kbd{pi} was
as meaningless to it as \kbd{stupid garbage} would have been: in both cases
\kbd{gp} will just create a variable with that funny unknown name you just
used. Try it! Note that it is actually equivalent to type
\kbd{stupidgarbage}: all spaces are suppressed from the input. In the
\kbd{27~*~37} example it was not so conspicuous as we had an operator to
separate the two operands. This has important consequences for the writing of
\kbd{gp} scripts. More about this later.
By the way, you can ask \kbd{gp} about any identifier you think it might know
about: just type it, prepending a question mark ``\kbd{?}''. Try \kbd{?Pi}
and \kbd{?pi} for instance. On most systems, an extended online help should
be available: try doubling the question mark to check whether it's the case
on yours: \kbd{??Pi}. In fact the \kbd{gp} header already gave you that
information if it was the case, just before the copyright message. As well,
if it says something like ``\kbd{readline enabled}'' then you should have a
look at the \kbd{readline} introduction in the User's Manual before you go
on: it will be much easier to type in examples and correct typos after you've
done that.
Now try \kbd{exp(Pi * sqrt(163))}. Hmmm, we suspect that the last digit may
be wrong, can this really be an integer? This is the time to change
precision. Type \kbd{\b{p} 50}, then try \kbd{exp(Pi * sqrt(163))} again. We
were right to suspect that the last decimal was incorrect, since we get quite
a few nines in its place, but it is now convincingly clear that this is not
an integer. Maybe it's a bug in PARI, and the result is really an integer?
Type
\bprog
(log(%) / Pi)^2
@eprog\noindent
immediately after the preceding computation; \kbd{\%} means the result of the
last computed expression. More generally, the results are numbered \kbd{\%1,
\%2, \dots} \emph{including} the results
that you do not want to see printed by putting a semicolon at the end of the
line, and you can evidently use all these quantities in any further
computations. The result seems to be indistinguishable from $163$, hence it
does not seem to be a bug.
In fact, it is known that $\exp(\pi*\sqrt{n})$ not only is not an integer or
a rational number, but is even a transcendental number when $n$ is a nonzero
rational number.
So \kbd{gp} is just a fancy calculator, able to give me more decimals than I
will ever need? Not so, \kbd{gp} is incredibly more powerful than an ordinary
calculator, independently of its arbitrary precision possibilities.
\misctitle{Additional comments} (you are supposed to skip this at first,
and come back later)
1) If you are a PARI old timer, say the last version of PARI you used was
released around 1996, you have certainly noticed already that many many
things changed between the older 1.39.xx versions and this one.
Conspicuously, most function names have been changed. To know how a specific
function was changed, type \kbd{whatnow({\rm function})}.
2) It seems that the text implicitly says that as soon as an imprecise number
is entered, the result will be imprecise. Is this always true? There is a
unique exception: when you multiply an imprecise number by the exact number
0, you will get the exact 0. Compare \kbd{0 * 1.4} and \kbd{0.~*~1.4}.
\smallskip
%
3) Not only can the number of decimal places of real numbers be large, but
the number of digits of integers also. Try \kbd{1000!}. It is never necessary
to tell \kbd{gp} in advance the size of the integers that it will encounter.
The same is true for real numbers, although most computations with floating
point assume a default precision and truncate their results to this accuracy;
initially 38 decimal digits, but we may change that with \b{p} of course.
\smallskip
%
4) Come back to 38 digits of precision (\kbd{\b{p} 38}), and type
\kbd{exp(100)}. As you can see the result is printed in exponential format.
This is because \kbd{gp} never wants you to believe that a result is correct
when it is not. We are working with 38 digits of precision, but the integer
part of $\exp(100)$ has 44 decimal digits. Hence if \kbd{gp} had dutifully
printed out 44 digits, the last few digits would have been wrong. Hence
\kbd{gp} wants to print only 38 significant digits, but to do so it has to
print in exponential format. \smallskip
%
5) There are two ways to avoid this. One is of course to increase the
precision. Let's try it. To give it a wide margin, we set the precision to 50
decimals. Then we recall our last result (\kbd{\%}
or \kbd{\%n} where \kbd{n} is the number of the result). What? We still have
an exponential format! Do you understand why?
Again let's try to see what's happening. The number you recalled had been
computed only to 38 decimals, and even if you set the precision to 1000
decimals, \kbd{gp} knows that your number has only 38 digits of accuracy but
an integral part with 44 digits. So you haven't improved things by increasing
the precision. Or have you? What if we retype \kbd{exp(100)} now that we
have 50 digits? Try it. Now we no longer have an exponential format.
\medskip
%
6) What if I forget what the current precision is and I don't feel like
counting all the decimals? Well, you can type \b{p} by itself. You may also
learn about \kbd{gp} internal variables (and change them!) using
\kbd{default}. Type \kbd{default(realprecision)}, then
\kbd{default(realprecision, 38)}. Huh? In fact this last command is strictly
equivalent to \kbd{\b{p} 38}! (Admittedly more cumbersome to type.) There are
more ``defaults'' than just \kbd{format} and \kbd{realprecision}: type
\kbd{default} by itself now, they are all there. \smallskip
%
7) Note that the \kbd{default} command reacts differently according to the
number of input arguments. This is not an uncommon behavior for \kbd{gp}
functions. You can see this from the online help, or the complete description
in Chapter~3: any argument surrounded by braces \kbd{\obr\cbr} in the
function prototype is optional, which really means that a \emph{default}
argument will be supplied by \kbd{gp}. You can then check out from the text
what effect a given value will have, and in particular the default one.
\smallskip
%
8) Try the following: starting in precision 38, type first
\kbd{default(format, "e0.100")}, then \kbd{exp(1)}. Where are my 100
significant digits? Well, \kbd{default(format,)} only changes the output
format, but \emph{not} the default precision. On the other hand, the \b{p}
command changes both the precision and the output format.
\section{Warming up}
Another thing you better get used to pretty fast is error messages. Try
typing \kbd{1/0}. Could not be clearer. But why has the prompt
become funny, turning from \kbd{?} to \kbd{break>} ? When an error occurs, we
enter a so-called \emph{break loop}, where you get a chance, e.g to inspect
(and save!) values of variables before the prompt returns and all
computations so far are lost. In fact you can run an arbitrary command at
this point, and this mechanism is a tremendous help in debugging. To get out
of the break loop, type \kbd{break}, as instructed in the error message
last line.
\misctitle{Comment} You can enter the break loop at any time using
\kbd{Control-C}: this freezes the current computation and gets you a new
prompt so that you may e.g., increase debugging level, inspect or modify
variables (again, run arbitrary commands), before letting the program go
on.
\medskip
Now, back to our favorite example, in precision 38, type
\bprog
floor(exp(100))
@eprog\noindent
\kbd{floor} is the mathematician's integer part, not to be confused with
\kbd{truncate}, which is the computer scientist's: \kbd{floor(-3.4)} is equal
to $-4$ whereas \kbd{truncate(-3.4)} is equal to $-3$. You get a more
cryptic error message, which you would immediately understand if you had read
the additional comments of the preceding section. Since you were told not to
read them, here's the explanation: \kbd{gp} is unable to compute the
integer part of \kbd{exp(100)} given only 38 decimals of accuracy, since
it has 44 digits.
Some error messages are more cryptic and sometimes not so easy to understand.
For instance, try \kbd{log(x)}. It simply tells you that \kbd{gp} does not
understand what \kbd{log(x)} is, although it does know the \kbd{log}
function, as \kbd{?log} will readily tell us.
Now let's try \kbd{sqrt(-1)} to see what error message we get now. Haha!
\kbd{gp} even knows about complex numbers, so impossible to trick it that
way. Similarly, try typing \kbd{log(-2)}, \kbd{exp(I*Pi)}, \kbd{I\pow
I}\dots\ So we have a lot of real and complex analysis at our disposal.
There always is a specific branch of multivalued complex transcendental
functions which is taken, specified in the manual. Again, beware that
\kbd{I} and \kbd{i} are not the same thing. Compare \kbd{I\pow2} with
\kbd{i\pow2} for instance.
Just for fun, let's try \kbd{6*zeta(2) / Pi\pow2}. Pretty close, no?
\medskip
Now \kbd{gp} didn't seem to know what \kbd{log(x)} was, although it did know
how to compute numerical values of \kbd{log}. This is annoying. Maybe it
knows the exponential function? Let's give it a try. Type \kbd{exp(x)}.
What's this? If you had any experience with other computer algebra systems,
the answer should have simply been \kbd{exp(x)} again. But here the answer is
the Taylor expansion of the function around $\kbd{x}=0$, to 17 terms. Note
the \kbd{O(x\pow17)} which ends the series, and which is trademark of power
series in \kbd{gp}. It is the familiar ``big--oh'' notation of analysis.
Why 17 terms? This is governed by the \kbd{seriesprecision}, which
can be changed by typing \kbd{\b{ps} $n$} or \kbd{default(seriesprecision,
$n$)} where $n$ is the number of terms that you want in your power series
and is $16$ by default. Converting a polynomial or rational function to a
power series will yield 16 significant terms: so $x$ gets converted to $x +
O(x^{17})$; this is completely analogous to \kbd{realprecision} when an eact
integer or rational number is converted to a floating point real number.
Then we take the exponential of this new object and, since it has positive
valuation, we can actually deduce $17$ significant terms from the given $16$.
This is in keeping with PARI's philosophy of always returning a result which
is as precise as possible from a given input.
You thus automatically get the Taylor expansion of any function that can be
expanded around $0$, and incidentally this explains why we weren't able to do
anything with \kbd{log(x)} which is not defined at $0$. (In fact \kbd{gp}
knows about Laurent series, but \kbd{log(x)} is not meromorphic either at
$0$.) If we try \kbd{log(1+x)}, then it works, but this time we \emph{lose}
one significant term: the result is $x - 1/2\*x^2 + \dots + 1/15\*x^{15} +
O(x^{16}))$. (Do you understand why ?)
But what if we wanted the expansion around a point different from 0? Well,
you're able to change $x$ into $x+a$, aren't you? So for instance you can
type \kbd{log(x+2)} to have the expansion of \kbd{log} around $\kbd{x}=2$. As
exercises you can try
\bprog
cos(x)
cos(x)^2 + sin(x)^2
exp(cos(x))
gamma(1 + x)
exp(exp(x) - 1)
1 / tan(x)
@eprog\noindent
for different values of \kbd{serieslength} (change it using \b{ps}
\var{newvalue}).
Let's try something else: type \kbd{(1 + x)\pow 3}. No \kbd{O(x)} here, since
the result is a polynomial. Haha, but I have learnt that if you do not take
exponents which are integers greater or equal to 0, you obtain a power series
with an infinite number of nonzero terms. Let's try. Type
\kbd{(1 + x)\pow (-3)} (the parentheses around \kbd{-3} are not necessary but
make things easier to read). Surprise! Contrary to what we expected, we don't
get a power series but a rational function. Again this is for the same reason
that \kbd{1 / 7} just gave you $1/7$: the result being exact, PARI doesn't see
any reason to make it inexact.
But I still want that power series. To obtain it, you can do as in the $1/7$
example and force a conversion using the $O(x^n)$ notation:
\bprog
(1 + x)^(-3) + O(x^16)
(1 + x)^(-3) * (1 + O(x^16))
(1 + x + O(x^16))^(-3)
@eprog\noindent
(Not on this example, but there is a difference between the first $2$
methods. Do you spot it?) Better yet, use the series constructor which
transforms any object into a power series, using the current
\kbd{seriesprecision}, and simply type
\bprog
Ser( (1 + x)^(-3) )
@eprog
Now try \kbd{(1 + x)\pow (1/2)}: we obtain a power series, since the
result is an object which PARI does not know how to represent exactly. (We
could teach PARI about algebraic functions, but then take \kbd{(1 + x)\pow Pi}
as another example.) This gives us still another solution to our preceding
exercise: we can type \kbd{(1 + x)\pow (-3.)}. Since \kbd{-3.} is not an exact
quantity, PARI has no means to know that we are dealing with a rational
function, and will instead give you the power series, this time with real
instead of integer coefficients.
\smallskip
To summarize, in this section we have seen that in addition to integers, real
numbers and rational numbers, PARI can handle complex numbers, polynomials,
rational functions and power series. A large number of functions exist which
handle these types, but in this tutorial we will only look at a few.
\misctitle{Additional comments} (as before, you are supposed to skip this
at first reading)
1) In almost all cases, there is no loss of information in PARI output: what
you see is all that PARI knows about the object, and you can happily
copy-paste it into another session. There are exceptions, though. Type
\kbd{n = 3 + 0*x}, then \kbd{n} is not the integer 3 but a constant polynomial
equal to $3 x^0$. Check it with \kbd{type(n)}.
However, it \emph{looks} like an integer without being one, and this may
cause some confusion in programs which actually expect integers. Hence if you
try to \kbd{factor(n)}, you obtain an empty factorization ! (Because, once
considered as a polynomial, \kbd{n} is a unit in $\Q[x]$.)
If you try to apply more general arithmetic functions, say the Euler totient
function (known as \kbd{eulerphi} to \kbd{gp}), you get an error message
worrying about integer arguments. You would have guessed yourself, but the
message is difficult to understand since 3 looks like a genuine integer!
Please make sure you understand the above, it is a common source of
incomprehension.
2) If you want the final expression to be in the simplest form possible (for
example before applying an arithmetic function, or simply because things will
go faster afterwards), apply the function \kbd{simplify} to the result.
This is done automatically at the end of a \kbd{gp} command, but
\emph{not} in intermediate expressions. Hence \kbd{n} above is not an
integer, but the final result stored in the output history is! So
if you type \kbd{type(\%)} instead of \kbd{type(n)} the answer is
\typ{INT}, adding to the confusion.
3) As already stated, power series expansions are always implicitly around
$\kbd{x} = 0$. When we wante them around $\kbd{x} = \kbd{a}$, we replace
\kbd{x} by \kbd{z + a} in the function we want to expand. For complicated
functions, it may be simpler to use the substitution function \kbd{subst}.
For example, if \kbd{p~= 1 / (x\pow 4 + 3*x\pow 3 + 5*x\pow 2 - 6*x + 7)},
you may not want to retype this, replacing \kbd{x} by \kbd{z~+ a}, so you can
write \kbd{subst(p, x, z+a)} (look up the exact description of the
\kbd{subst} function).
4) The valuation at $\kbd{x} = 0$ for a power series \kbd{p} is obtained
as \kbd{valuation(p, x)}.
\section{The Remaining PARI Types}
Let's talk some more about the basic PARI types.
Type \kbd{p = x * exp(-x)}. As expected, you get the power series expansion
to 17 terms (if you have not changed the default). Now type
\kbd{pr = serreverse(p)}. You are asking here for the \emph{reversion} of the
power series \kbd{p}, in other words the expansion of the inverse function.
This is possible only for power series whose first nonzero coefficient is
that of $x^1$. To check the correctness of the result, you can type
\kbd{subst(p, x, pr)} or \kbd{ subst(pr, x, p)} and you should get back
\kbd{x + O(x\pow 18)}.
Now the coefficients of \kbd{pr} obey a very simple formula. First, we would
like to multiply the coefficient of \kbd{x\pow n} by \kbd{n!} (in the case of
the exponential function, this would simplify things considerably!). The PARI
function \kbd{serlaplace} does just that. So type \kbd{ps = serlaplace(pr)}.
The coefficients now become integers, which can be immediately recognized by
inspection. The coefficient of $x^n$ is now equal to
$n^{n-1}$. In other words, we have
%
$$\kbd{pr} = \sum_{n\ge1}\dfrac{n^{n-1}}{n!} X^{n}.$$
%
Do you know how to prove this? (The proof is difficult.)
\smallskip
%
Of course PARI knows about vectors (rows and columns are distinguished, even
though mathematically there is no difference) and matrices. Type for example
\kbd{[1,2,3,4]}. This gives the row vector whose coordinates are 1, 2, 3 and
4. If you want a column vector, type \kbd{[1,2,3,4]\til}, the tilde meaning
of course transpose. You don't see much difference in the output, except for
the tilde at the end. However, now type \b{B}: lo and behold, the column
vector appears as a proper vertical thingy now. The \b{B} command is used
mainly for this purpose. The length of a vector is given by, well
\kbd{length} of course. The shorthand ``cardinality'' notation \kbd{\#v} for
\kbd{length(v)} is also available, for instance \kbd{v[\#v]} is the last
element of \kbd{v}.
Type \kbd{m = [a,b,c; d,e,f]}. You have just entered a matrix with 2 rows and
3 columns. Note that the matrix is entered by \emph{rows} and the rows are
separated by semicolons ``\kbd{;}''. The matrix is printed naturally in a
rectangle shape. If you want it printed horizontally just as you typed it,
type \b{a}, or if you want this type of printing to be the permanent default
type \kbd{default(output, 0)}. Type \kbd{default(output, 1)} if you want to
come back to the original output mode.
Now type \kbd{m[1,2]}, \kbd{m[1,]}, \kbd{m[,2]}. Are explanations necessary?
(In an expression such as \kbd{m[j,k]}, the \kbd{j} always refers to the
row number, and the \kbd{k} to the column number, and the first index is
always 1, never 0. This default cannot be changed.)
Even better, type \kbd{m[1,2] = 5; m}. The semicolon also allows us to put
several instructions on the same line; the final result is the output of
the last statement on the line. Now type \kbd{m[1,] = [15,-17,8]}. No
problem. Finally type \kbd{m[,2] = [j,k]}. You have an error message since you
have typed a row vector, while \kbd{m[,2]} is a column vector. If you type
instead \kbd{m[,2] = [j,k]\til} it works. \smallskip
%
\label{se:types}
Type now \kbd{h = mathilbert(20)}. You get the so-called ``Hilbert matrix''
whose coefficient of row $i$ and column $j$ is equal to $(i+j-1)^{-1}$.
Incidentally, the matrix \kbd{h} takes too much room. If you don't want to
see it, simply type a semi-colon ``\kbd{;}'' at the end of the line
(\kbd{h = mathilbert(20);}). This is an example of a ``precomputed'' matrix,
built into PARI. We will see a more general construction later.
What is interesting about Hilbert matrices is that first their inverses and
determinants can be computed explicitly (and the inverse has integer
coefficients), and second they are numerically very unstable, which make them
a severe test for linear algebra packages in numerical analysis. Of course
with PARI, no such problem can occur: since the coefficients are given as
rational numbers, the computation will be done exactly, so there cannot be
any numerical error. Try it. Type \kbd{d~=~matdet(h)}. The result is a
rational number (of course) of numerator equal to 1 and denominator having
226 digits. How do I know, by the way? Well, type \kbd{sizedigit(1/d)}. Or
\kbd{\#Str(1/d)}. (The length of the character string representing the
result.)
Now type \kbd{hr = 1.* h;} (do not forget the semicolon, we don't want to see
the result!), then \kbd{dr = matdet(hr)}. You notice two things. First the
computation, is much faster than in the rational case. (If your computer is
too fast for you to notice, try again with \kbd{h = mathilbert(40)}, or
even some larger value.) The reason for this is that PARI is handling real
numbers with 38 digits of accuracy, while in the rational case it is
handling integers having up to 226 decimal digits.
The second, more important, fact is that the result is terribly wrong. If you
compare with \kbd{1.$*$d} computed earlier, which is the correct answer, you
will see that few decimals agree! (None agree if you replaced 20 by 40 as
suggested above.) This catastrophic instability is as already mentioned one
of the characteristics of Hilbert matrices. In fact, the situation is
worse than that. Type \kbd{norml2(1/h - 1/hr)} (the function \kbd{norml2}
gives the square of the $L^2$ norm, i.e.~the sum of the squares of the
coefficients). The result is larger than $10^{32}$, showing that some
coefficients of \kbd{1/hr} are wrong by as much as $10^{16}$. To obtain the
correct result after rounding for the inverse, we have to use a default
precision of 57 digits (try it).
Although vectors and matrices can be entered manually, by typing explicitly
their elements, very often the elements satisfy a simple law and one uses a
different syntax. For example, assume that you want a vector whose $i$-th
coordinate is equal to $i^2$. No problem, type for example
\kbd{vector(10,i, i\pow 2)} if you want a vector of length 10. Similarly, if
you type
\bprog
matrix(5,5, i,j, 1 / (i+j-1))
@eprog\noindent
you will get the Hilbert matrix of order 5, hence the \kbd{mathilbert}
function is in fact redundant. The \kbd{i} and \kbd{j} represent dummy
variables which are used to number the rows and columns respectively (in
the case of a vector only one is present of course). You must not forget,
in addition to the dimensions of the vector or matrix, to indicate
explicitly the names of these variables. You may omit the variables and
the final expression to get zero entries, as in \kbd{matrix(10,20)}.
\misctitle{Warning} The letter \kbd{I} is reserved for the complex number
equal to the square root of $-1$. Hence it is forbidden to use it as a
variable. Try typing \kbd{vector(10,I, I\pow 2)}, the error message that you
get clearly indicates that \kbd{gp} does not consider \kbd{I} as a variable.
There are other reserved variable names: \kbd{Pi}, \kbd{Euler},
\kbd{Catalan} and \kbd{oo}. All function names are forbidden as well. On the
other hand there is nothing special about \kbd{i}, \kbd{pi}, \kbd{euler} or
\kbd{catalan}.
When creating vectors or matrices, it is often useful to use Boolean
operators and the \kbd{if()} statement. Indeed, an \kbd{if} expression has a
value, which is of course equal to the evaluated part of the \kbd{if}. So for
example you can type
\bprog
matrix(8,8, i,j, if ((i-j)%2, 1, 0))
@eprog\noindent
to get a checkerboard matrix of \kbd{1} and \kbd{0}. Note however
that a vector or matrix must be \emph{created} first before being used. For
example, it is possible to write
\bprog
v = vector(5);
for (i = 1, 5, v[i] = 1/i)
@eprog\noindent
but this would fail if the vector \kbd{v} had not been created beforehand.
Of course, the above example is better written as
\bprog
v = vector(5, i, 1/i);
@eprog
Another useful way to create vectors and matrices is to extract them from
larger ones. For instance, if \kbd{h} is the $20\times 20$ Hilbert matrix as above,
\bprog
h = mathilbert(20);
h[11..20, 11..20]
@eprog\noindent is its lower right quadrant.
\medskip The last PARI types which we have not yet played with are closely
linked to number theory. People not interested in number theory can skip
ahead.
The first is the type ``integer--modulo''. Let us see an example. Type
\bprog
n = 10^15 + 3
@eprog
We want to know whether this number is prime or not. Of course we could make
use of the built-in facilities of PARI, but let us do otherwise. We first
trial divide by the built-in table of primes. We slightly cheat here and use
a variant of the function \kbd{factor} which does exactly this. So type
\kbd{factor(n, 200000)}. The last argument tells \kbd{factor} to trial divide
up to the given bound and stop at this point. Set it to 0 to trial divide by
the full set of built-in primes, which goes up to $500000$ by default.
As for all factoring functions, the result is a 2 column matrix: the first
column gives the primes and the second their exponents. Here we get a single
row, telling us that if primes stopped at $200000$ as we made \kbd{factor}
believe, \kbd{n} would be prime. (Or is that a contradiction?) More
seriously, \kbd{n} is not divisible by any prime up to $200000$.
We could now trial divide further, or cheat and call the PARI function
\kbd{factor} without the optional second argument, but before we do this let
us see how to get an answer ourselves.
By Fermat's little theorem, if $n$ is prime we must have $a^{n-1}\equiv 1
\pmod{n}$ for all $a$ not divisible by $n$. Hence we could try this with $a=2$
for example. But $2^{n-1}$ is a number with approximately $3\cdot10^{14}$
digits, hence impossible to write down, let alone to compute. But instead type
\kbd{a = Mod(2,n)}. This creates the number $2$ considered now as an element
of the ring $R = \Z/\kbd{n}\Z$. The elements of $R$, called intmods, can
always be represented by numbers smaller than \kbd{n}, hence small. Fermat's
theorem can be rewritten
%
$\kbd{a}^{n-1} = \kbd{Mod(1,n)}$
%
in the ring $R$, and this can be computed very efficiently. Elements of $R$
may be lifted back to $\Z$ with either \kbd{lift} or \kbd{centerlift}. Type
\kbd{a\pow (n-1)}. The result is definitely \emph{not} equal to
\kbd{Mod(1,n)}, thus \emph{proving} that \kbd{n} is not a prime. If we had
obtained \kbd{Mod(1,n)} on the other hand, it would have given us a hint that
\kbd{n} is maybe prime, but not a proof.
To find the factors is another story. In this case, the integer $n$ is small
enough to let trial division run to completion. Type \kbd{\#} to turn on the
\kbd{gp} timer, then
\bprog
for (i = 2, ceil(sqrt(n)), if (n%i==0, print(i); break))
@eprog\noindent
This should take less than 5 seconds. In general, one must use less naive
techniques than trial division, or be very patient. Type \kbd{fa = factor(n)}
to let the factoring engine find all prime factors. You may stop the timer by
typing \kbd{\#} again.
Note that, as is the case with most ``prime''-producing functions, the
``prime'' factors given by \kbd{factor} are only strong pseudoprimes, and not
\emph{proven} primes. Use \kbd{isprime( fa[,1] )} to rigorously prove
primality of the factors. The latter command applies \kbd{isprime} to all
entries in the first column of \kbd{fa}, i.e to all pseudoprimes, and returns
the column vector of results: all equal to 1, so our pseudoprimes were
true primes. All arithmetic functions can be applied in this way to the entries
of a vector or matrix. In fact, it has been checked that the strong
pseudoprimes output by \kbd{factor} (Baillie-Pomerance-Selfridge-Wagstaff
pseudoprimes, without small divisors) are true primes at least up to
$2^{64}$, and no explicit counter-example is known.\smallskip
The second specifically number-theoretic type is the $p$-adic numbers. I have
no room for definitions, so please skip ahead if you have no use for such
beasts. A $p$-adic number is entered as a rational or integer valued
expression to which is added \kbd{O(p\pow n)}, or simply \kbd{O(p)} if
$\kbd{n}=1$, where \kbd{p} is the prime and \kbd{n} the $p$-adic precision.
Note that you have to explicitly type in \kbd{3\pow 2} for instance, \kbd{9}
will not do. Unless you want to cheat \kbd{gp} into believing that \kbd{9}
is prime, but you had better know what you are doing in this case: most
computations will yield a wrong result.
Apart from the usual arithmetic operations, you can apply a number of
transcendental functions. For example, type \kbd{n = 569 + O(7\pow 8)}, then
\kbd{s~=~sqrt(n)}, you obtain one of the square roots of \kbd{n}; to check
this, type \kbd{s\pow 2 - n}). Type now \kbd{s = log(n)}, then \kbd{e =
exp(s)}. If you know about $p$-adic logarithms, you will not be surprised
that \kbd{e} is not equal to \kbd{n}. Type \kbd{(n/e)\pow 6}: \kbd{e} is in
fact equal to \kbd{n} times the $(p-1)$-st root of unity \kbd{teichmuller(n)}.
Incidentally, if you want to get back the integer 569 from the $p$-adic
number \kbd{n}, type \kbd{lift(n)} or \kbd{truncate(n)}.
\smallskip
The third number-theoretic type is the type ``quadratic number''. This type
is specially tailored so that we can easily work in a quadratic extension of
a base field, usually $\Q$. It is a generalization of the type
``complex''. To start, we must specify which quadratic field we want to work
in. For this, we use the function \kbd{quadgen} applied to the
\emph{discriminant} \kbd{d} (as opposed to the radicand) of the quadratic
field. This returns a number equal to
$(\kbd{d}+a) / 2$ where $a$ is equal to 0 or 1 according to whether \kbd{d} is
even or odd. The function \kbd{quadgen} takes an extra parameter which is how
the number will be printed. To avoid confusion, this number should be
set to a variable of the same name, i.e. do \kbd{w = quadgen(d, 'w)}.
So type \kbd{w = quadgen(-163,'w)}, then \kbd{charpoly(w)} which asks for the
characteristic polynomial of \kbd{w}. The result shows what \kbd{w} will
represent. You may ask for \kbd{1.*w} to see which root of the quadratic has
been taken, but this is rarely necessary. We can now play in the field
$\Q(\sqrt{-163})$. Type for example \kbd{w\pow 10}, \kbd{norm(3 + 4*w)},
\kbd{1 / (4+w)}. More interesting, type \kbd{a = Mod(1,23) * w} then \kbd{b =
a\pow 264}. This is a generalization of Fermat's theorem to quadratic fields.
If you do not want to see the modulus 23 all the time, type \kbd{lift(b)}.
Another example: type \kbd{p = x\pow 2 + w*x + 5*w + 7}, then \kbd{norm(p)}. We
thus obtain the quartic equation over $\Q$ corresponding to the relative
quadratic extension over $\Q(\kbd{w})$ defined by \kbd{p}.
On the other hand, if you type \kbd{wr = sqrt(w\pow 2)}, do not expect to get
back \kbd{w}. Instead, you get the numerical value, the function \kbd{sqrt}
being considered as a ``transcendental'' function, even though it is
algebraic. Type \kbd{algdep(wr,2)}: this looks for algebraic relations
involving the powers of \kbd{w} up to degree 2. This is one way to get
\kbd{w} back. Similarly, type \kbd{algdep(sqrt(3*w + 5), 4)}. See the user's
manual for the function \kbd{algdep}.\smallskip
The fourth number-theoretic type is the type ``polynomial--modulo'', i.e.
polynomial modulo another polynomial. This type is used to work in general
algebraic extensions, for example elements of number fields (if the base
field is $\Q$), or elements of finite fields (if the base field is
$\Z/p\Z$ for a prime $p$). In a sense it is a generalization of the type
quadratic number. The syntax used is the same as for intmods. For example,
instead of typing \kbd{w = quadgen(-163,'w)}, you can type
\bprog
w = Mod(x, quadpoly(-163))
@eprog\noindent
Then, exactly as in the quadratic case, you can type \kbd{w\pow 10},
\kbd{norm(3 + 4*w)}, \kbd{1 / (4+w)}, \kbd{a = Mod(1,23)*w}, \kbd{b = a\pow
264}, obtaining of course the same results. (Type \kbd{lift(\dots)} if you
don't want to see the polynomial \kbd{x\pow 2 - x + 41} repeated all the
time.) Of course, you can work in any degree, not only quadratic. For the
latter, the corresponding elementary operations will be slower than
with quadratic numbers. Start the timer, then compare
\bprog
w = quadgen(-163,'w); W = Mod(x, quadpoly(-163));
a = 2 + w; A = 2 + W;
b = 3 + w; B = 3 + W;
for (i=1,10^5, a+b)
for (i=1,10^5, A+B)
for (i=1,10^5, a*b)
for (i=1,10^5, A*B)
for (i=1,10^5, a/b)
for (i=1,10^5, A/B)
@eprog\noindent
Don't retype everything, use the arrow keys!
There is however a slight difference in behavior. Keeping our polmod \kbd{w},
type \kbd{1.*w}. As you can see, the result is not the same. Type
\kbd{sqrt(w)}. Here, we obtain a vector with 2 components, the two components
being the principal branch of the square root of all the possible embeddings
of \kbd{w} in $\C$. More generally, if
\kbd{w} was of degree $n$, we would get an $n$-component vector, and similarly
for all transcendental functions.
We have at our disposal the usual arithmetic functions, plus a few others.
Type \kbd{a = Mod(x, x\pow 3 - x - 1)} defining a cubic extension. We can for
example ask for \kbd{b = a\pow 5}. Now assume we want to express \kbd{a}
as a polynomial in \kbd{b}. This is possible since \kbd{b} is also a
generator of the same field. No problem, type \kbd{modreverse(b)}. This gives
a new defining polynomial for the same field, i.e.~the characteristic
polynomial of \kbd{b}, and expresses \kbd{a} in terms of this new polmod,
i.e.~in terms of \kbd{a}. We will see this in more detail in the number
field section.
An important special case of the above construction allows to work in finite
fields, by choosing an irreducible polynomial $T$ of degree $f$ over $\F_p$
and considering $\F_p[t]/(T)$. As in
\bprog
T = ffinit(5, 6, 't); \\ @com degree 6, irreducible over $\F_5$
g = Mod(t, T)
@eprog\noindent Try a few elementary operations involving $g$, such as
$g^{100}$. This special case of \typ{POLMOD}s is in fact so important that we
now introduce a final dedicated number theoretical type \typ{FFELT}, for
``finite field element'', to simplify work with finite fields: \kbd{g =
ffgen(5\pow6, 't)} computes a suitable polynomial $T$ as above and returns
the generator $t \mod T(t)$. This has major advantages over the generic
\typ{POLMOD} solution: elements are printed in a simplified way (in lifted
form), and functions can assume that $T$ is indeed irreducible. A few dedicated
functions \kbd{ffprimroot} (analog of \kbd{znprimroot}), \kbd{fforder}
(analog of \kbd{znorder}), \kbd{fflog} (analog of \kbd{znlog}) are available.
Rational expressions in the variable $t$ can be mapped to such a finite
field by substituting $t$ by $g$, for instance
\bprog
? g = ffgen(5^6, 't);
? g.mod \\ @com irreducible over $\F_5$, defines $\F_{5^6}$
%2 = t^6 + t^5 + t^4 + t^3 + t^2 + t + 1
? Q = x^2 + t*x + 1
? factor(subst(Q,t,g))
%3 =
[ x + (t^5 + 3*t^4 + t^3 + 4*t + 1) 1]
[x + (4*t^5 + 2*t^4 + 4*t^3 + 2*t + 4) 1]
@eprog\noindent factors the polynomial $Q \in \F_{5^6}[x]$, where
$\F_{5^6} = \F_5[t]/(\kbd{g.mod})$.
\section{Elementary Arithmetic Functions}
Since PARI is aimed at number theorists, it is not surprising that there
exists a large number of arithmetic functions; see the list by typing
\kbd{?5}. We have already seen several, such as \kbd{factor}. Note that
\kbd{factor} handles not only integers, but also univariate polynomials.
Type for example \kbd{factor(x\pow 200 - 1)}. You can also ask to factor a
polynomial modulo a finite field or a number field !
Evidently, you have functions for computing GCD's (\kbd{gcd}), extended GCD's
(\kbd{bezout}), solving the Chinese remainder theorem (\kbd{chinese}) and so
on.
In addition to the factoring facilities, you have a few functions related to
primality testing such as \kbd{isprime}, \kbd{ispseudoprime},
\kbd{precprime}, and \kbd{nextprime}. As previously mentioned, only strong
pseudoprimes are produced by the latter two (they pass the
\kbd{ispseudoprime} test); the more sophisticated primality tests in
\kbd{isprime}, being so much slower, are not applied by default.
We also have the usual multiplicative arithmetic functions: the M\"obius $\mu$
function (\kbd{moebius}), the Euler $\phi$ function (\kbd{eulerphi}), the
$\omega$ and $\Omega$ functions (\kbd{omega} and \kbd{bigomega}), the
$\sigma_k$ functions (\kbd{sigma}), which compute sums of $k$-th powers of the
positive divisors of a given integer, etc\dots
You can compute continued fractions. For example, type \kbd{\b{p} 1000}, then
\kbd{contfrac(exp(1))}: you obtain the continued fraction of the base of
natural logarithms, which as you can see obeys a very simple pattern. Can
you prove it?
In many cases, one wants to perform some task only when an arithmetic
condition is satisfied. \kbd{gp} gives you the following functions: \kbd{isprime}
as mentioned above, \kbd{issquare}, \kbd{isfundamental} to test whether an
integer is a fundamental discriminant (i.e.~$1$ or the discriminant of a
quadratic field), and the \kbd{forprime}, \kbd{fordiv} and \kbd{sumdiv}
loops. Assume for example that we want to compute the product of all the
divisors of a positive integer \kbd{n}. The easiest way is to write
\bprog
p = 1; fordiv(n,d, p *= d); p
@eprog\noindent
(There is a simple formula for this product in terms of $n$ and the number of
its divisors: find and prove it!) The notation \kbd{p *= d} is just a
shorthand for \kbd{p = p * d}.
If we want to know the list of primes $p$ less than 1000 such that 2 is a
primitive root modulo $p$, one way would be to write:
\bprog
forprime(p=3,1000, if (znprimroot(p) == 2, print(p)))
@eprog\noindent
%
Note that this assumes that \kbd{znprimroot} returns the smallest primitive
root, and this is indeed the case. Had we not known about this, we could
have written
\bprog
forprime(p=3,1000, if (znorder(Mod(2,p)) == p-1, print(p)))
@eprog\noindent
%
(which is actually faster since we only compute the order of $2$ in $\Z/p\Z$,
instead of looking for a generator by trying successive elements whose orders
have to be computed as well.) Once we know a primitive root $g$, we can write
any nonzero element of $\Z/p\Z$ as $g^x$ for some unique $x$ in $\Z/(p-1)\Z$.
Computing such a discrete logarithm is a hard problem in general, performed
by the function \kbd{znlog}.
Arithmetic functions related to quadratic fields, binary quadratic forms and
general number fields will be seen in the next sections.
\section{Performing Linear Algebra}
The standard linear algebra routines are available: \kbd{matdet},
\kbd{mateigen} (eigenvectors), \kbd{matker}, \kbd{matimage}, \kbd{matrank},
\kbd{matsolve} (to solve a linear system), \kbd{charpoly} (characteristic
polynomial), to name a few. Bilinear algebra over $\R$ is also there:
\kbd{qfgaussred} (Gauss reduction), \kbd{qfsign} (signature). You may also
type \kbd{?7}. Can you guess what each of these do?
Let us see how this works. First, a vector space (or module) is given by a
generating set of vectors (often a basis) which are represented as
\emph{column} vectors. This set of vectors is in turn represented by the
columns of a matrix. Quadratic forms are represented by their Gram matrix.
The base field (or ring) can be any ring type PARI supports. However, certain
operations are specifically written for a real or complex base field, while
others are written for $\Z$ as the base ring.
We had some fun with Hilbert matrices and numerical instability a while back,
but most of the linear algebra routines are generic. If as before \kbd{h =
mathilbert(20)}, we may compute
\bprog
matdet(h * Mod(1,101))
matdet(h * (1 + O(101^100)))
@eprog\noindent
in $\Z/101\Z$ and the $p$-adic ring $\Z_{101}$ (to $100$ words of accuracy)
respectively. Let \kbd{H = 1/h} the inverse of \kbd{h}:
\bprog
H = 1/h; \\ @com integral
L = primes([10^5, 10^5 + 1000]); \\ @com pick a few primes
v = vector(#L, i, matdet(H * Mod(1,L[i])));
centerlift( chinese(v) )
@eprog\noindent
returns the determinant of \kbd{H}. (Assuming it is an integer
less than half the product of elements of \kbd{L} in absolute value, which
it is.)
In fact, we computed an homomorphic image of the determinant in a few small
finite fields, which admits a single integer representative given the size
constraints. We could also have made a single determinant computation modulo
a big prime (or pseudoprime) number, e.g \kbd{nextprime(2 * B)} if we know
that the determinant is less than \kbd{B} in absolute value.
(Why is that $2$ necessary?)
By the way, this is how you insert comments in a script: everything
following a double backslash, up to the first newline character, is ignored.
If you want comments which span many lines, you can brace them between
\kbd{/* ... */} pairs. Everything in between will be ignored as well. For
instance as a header for the script above you could insert the
following:
\bprog
/* Homomorphic imaging scheme to compute the determinant of a classical
* integral matrix.
* TODO: Look up the explicit formula
*/
@eprog\noindent
(I hope you did not waste your time copying this nonsense, did you?)
\medskip
In addition, linear algebra over $\Z$, i.e.~work on lattices, can also be
performed. Let us now consider the lattice $\Lambda$ generated by the columns
of \kbd{H} in $\Z^{20}\subset\R^{20}$. Since the determinant is nonzero, we
have in fact a basis. What is the structure of the finite abelian group
$\Z^{20}/\Lambda$? Type \kbd{matsnf(H)}. Wow, 20 cyclic factors.
There is a triangular basis for $\Lambda$ (triangular when expressed in
the canonical basis), perhaps it looks better than our initial one? Type
\kbd{mathnf(H)}. Hum, what if I also want the unimodular transformation
matrix? Simple : \kbd{z = mathnf(H, 1);} \kbd{z[1]} is the triangular HNF
basis, and \kbd{z[2]} is the base change matrix from the canonical basis to
the new one, with determinant $\pm 1$. Try \kbd{matdet(z[2])},
then \kbd{H * z[2] == z[1]}. Fine, it works. And \kbd{z[1]} indeed looks
better than \kbd{H}.
Can we do better? Perhaps, but then we'd better drop the requirement that
the basis be triangular, since the latter is essentially canonical. Type
\bprog
M = H * qflll(H)
@eprog
Its columns give an LLL-reduced basis for $\Lambda$ (\kbd{qflll(H)} itself
gives the base change matrix). The LLL algorithm outputs a nice basis for a
lattice given by an arbitrary basis, where nice means the basis vectors are
almost orthogonal and short, with precise guarantees on their relations to
the shortest vectors. Not really spectacular on this example, though.
Let us try something else, there should be an integer relation between
$\log 3$, $\log 5$ and $\log 15$. How to detect it?
\bprog
u = [log(15), log(5), log(3)];
m = matid(3); m[3,] = round(u * 10^25);
v = qflll(m)[,1] \\@com first vector of the LLL-reduced basis
u * v
@eprog\noindent
Pretty close. In fact, \kbd{lindep} automates this kind of search for integer
relations; try \kbd{lindep(u)}.
Let us come back to $\Lambda$ above, and our LLL basis in \kbd{M}. Type
\bprog
G = M~*M \\@com Gram matrix
m = qfminim(G, norml2(M[,1]), 100, 2);
@eprog\noindent
This enumerates the vectors in $\Lambda$ which are shorter than the first LLL
basis vector, at most 100 of them. The final argument $2$ instructs the
function to use a safe (slower) algorithm, since the matrix entries are
rather large; trying to remove it should produce an error, in this case.
There are $\kbd{m[1]} = 6$ such vectors, and \kbd{m[3]} gives half of them
(\kbd{-m[3]} would complete the lot): they are the first 3 basis vectors! So
these are optimally short, at least with respect to the Euclidean length. Let
us try
\bprog
m = qfminim(G, norml2(M[,4]), 100, 2);
@eprog\noindent
(The flag $2$ instructs \kbd{qfminim} to use a different enumeration
strategy, which is much faster when we expect more short vectors than we want
to store. Without the flag, this example requires several hours. This is an
exponential time algorithm, after all!) This time, we find a slew of short
vectors; \kbd{matrank(m[3])} says the 100 we have are all included in a
2-dimensional space. Let us try
\bprog
m = qfminim(G, norml2(M[,4]) - 1, 100000, 2);
@eprog\noindent
This time we find 50886 vectors of the requested length, spanning a
$4$-dimensional space, which is actually generated by \kbd{M[,1]},
\kbd{M[,2]} \kbd{M[,3]} and \kbd{M[,5]}.
\section{Using Transcendental Functions}
All the elementary transcendental functions and several higher transcendental
functions are provided: $\Gamma$ function, incomplete $\Gamma$ function, error
function, exponential integral, Bessel functions ($H^1$, $H^2$, $I$, $J$,
$K$, $N$), confluent hypergeometric functions, Riemann $\zeta$ function,
polylogarithms, Weber functions, theta functions. More will be written if the
need arises.
In this type of functions, the default precision plays an essential role.
In almost all cases transcendental functions work in the following way.
If the argument is exact, the result is computed using the current
default precision. If the argument is not exact, the precision of the
argument is used for the computation. A note of warning however: even in this
case the \emph{printed} value is the current real format, usually the
same as the default precision. In the present chapter we assume that your
machine works with 64-bit long integers. If it is not the case, we leave it
to you as a good exercise to make the necessary modifications.
Let's assume that we have 38 decimals of default precision (this is what we
get automatically at the start of a \kbd{gp} session on 64-bit machines). Type
\kbd{e = exp(1)}. We get the number $e=2.718\dots$ to 38 decimals. Let us check
how many correct decimals we really have. Change the precision to a
substantially higher value, for example by typing \kbd{\b{p} 100}. Then type
\kbd{e}, then \kbd{exp(1)} once again. This last value is the correct value
of the mathematical constant $e$ to 100 decimals, while the variable \kbd{e}
shows the value that was computed to 38 decimals. Clearly they coincide to
exactly 38 significant digits.
So 38 digits are printed, but how many significant digits are actually
contained in the variable \kbd{e}? Type \kbd{\#e} which indicates we have
exactly $2$ mantissa words. Since $2\ln(2^{64}) / \ln(10)\approx38.5$ we see
that we have 38 or 39 significant digits (on 64-bit machines).
\smallskip
Come back to 38 decimals (\kbd{\b{p} 38}). If we type \kbd{exp(1.)}
you can check that we also obtain 38 decimals. However, type
\kbd{f = exp(1 + 1E-40)}. Although the default precision is still 38,
you can check using the method above that we have in fact 96 significant
digits! The reason is that \kbd{1 + 1E-40} is computed according to the PARI
philosophy, i.e.~to the best possible precision. Since \kbd{1E-40} has 39
significant digits and 1 has ``infinite'' precision, the number \kbd{1 +
1E-40} will have at least $79=39+40$ significant digits, hence \kbd{f} also.
Now type \kbd{cos(1E-19)}. The result is printed as $1.0000\dots$, but
is of course not exactly equal to 1. Using \kbd{\#\%}, we see that the
result has 4 mantissa words, giving us the possibility of having 77
correct significant digits. PARI gives you as much as it can, and since 3
mantissa words would have given you only 57 digits, it uses 4. But why does
it give so precise a result? Well, it is the same reason as before. When $x$
is close to 1, $\cos(x)$ is close to $1-x^2/2$, hence the precision is going
to be approximately the same as when computing this quantity, here
$1-0.5*10^{-38}$ where $0.5*10^{-38}$ is considered with 38 significant digit
accuracy. Hence the result will have approximately $38+38=76$ significant
digits.
This philosophy cannot go too far. For example, when you type \kbd{cos(0)},
\kbd{gp} should give you exactly 1. Since it is reasonable for a program to
assume that a transcendental function never gives you an exact result,
\kbd{gp} gives you $1.000\dots$ with as many mantissa word as the current
precision.
\medskip
Let's see some more transcendental functions at work. Type
\kbd{gamma(10)}. No problem (type \kbd{9!} to check). Type \kbd{gamma(100)}.
The number is now written in exponential format because the default accuracy
is too small to give the correct result. To get all digits, the most natural
solution is to increase the precision; since \kbd{gamma(100)} has 156 decimal
digits, type \kbd{\b{p} 170} to be on the safe side, then \kbd{gamma(100)}
once again. Another one is to compute \kbd{99!} directly.
Try \kbd{gamma(1/2 + 10*I)}. No problem, we have the complex $\Gamma$ function.
Now type
\bprog
t = 1000;
z = gamma(1 + I*t) * t^(-1/2) * exp(Pi/2*t) / sqrt(2*Pi)
norm(z)
@eprog\noindent The latter is very close to 1, in accordance with the complex
Stirling formula.
\smallskip
Let's play now with the Riemann zeta function. First turn on the timer (type
\kbd{\#}). Type \kbd{zeta(2)}, then \kbd{Pi\pow 2/6}. This seems correct. Type
\kbd{zeta(3)}. All this takes essentially no time at all. However, type
\kbd{zeta(3.1)}. You will notice that the time is substantially larger; if
your machine is too fast to see the difference, increase the precision to
\kbd{\b{p}1000}. This is because PARI uses special formulas to compute
\kbd{zeta(n)} when \kbd{n} is an integer.
Type \kbd{zeta(1 + I)}. This also works. Now for fun, let us compute in a
naive way the first complex zero of \kbd{zeta}. We know that it is
of the form $1/2 + i*t$ with $t$ between 14 and 15. Thus, we can use the
following series of instructions. But instead of typing them directly, write
them into a file, say \kbd{zeta.gp}, then type \kbd{\b{r} zeta.gp} under
\kbd{gp} to read it in:
\bprog
{
t1 = 1/2 + 14*I;
t2 = 1/2 + 15*I; eps = 1E-50;
z1 = zeta(t1);
until (norm(z2) < eps,
z2 = zeta(t2);
if (norm(z2) < norm(z1),
t3 = t1; t1 = t2; t2 = t3; z1 = z2
);
t2 = (t1+t2) / 2.;
print(t1 ": " z1)
)
}
@eprog\noindent
Don't forget the braces: they tell \kbd{gp} that a sequence of instructions
is going to span many lines. We thus obtain the first zero to 25 significant
digits.
By the way, you don't need to type in the suffix~\kbd{.gp} in the \b{r}
command: it is supplied by \kbd{gp} if you forget it. The suffix is not
mandatory either, but it is convenient to have all GP scripts labeled in the
same distinctive way. Also, some text editors, e.g. Emacs or Vim, will
recognize GP scripts as such by their suffix and load special colourful
modes. \medskip
%
As mentioned at the beginning of this tutorial, some transcendental functions
can also be applied to $p$-adic numbers. This is as good a time as any to
familiarize yourself with them. Type
\bprog
a = exp(7 + O(7^10))
log(a)
@eprog\noindent All seems in order.
\bprog
b = log(5 + O(7^10))
exp(b)
@eprog\noindent Is something wrong? We don't recover the number we started
with? This is normal: type
\bprog
exp(b) * teichmuller(5 + O(7^10))
@eprog\noindent
and we indeed recover our initial number. The Teichm\"uller
character \kbd{teichmuller(x)} on $\Z_p^*$ is the unique \hbox{$(p-1)$-st}
root of unity which is congruent to \kbd{x} modulo $p$, assuming that \kbd{x}
is a $p$-adic unit.\smallskip
%
Let us come back to real numbers for the moment. Type \kbd{agm(1,sqrt(2))}.
This gives the arithmetic-geometric mean of 1 and $\sqrt2$, and is the basic
method for computing complete elliptic integrals. In fact, type
\kbd{Pi/2 / intnum(t=0,Pi/2, 1 / sqrt(1 + sin(t)\pow 2))},
\noindent and the result is the same. The elementary transformation
\kbd{x = sin(t)} gives the mathematical equality
$$\int_0^1 \dfrac{dx}{\sqrt{1-x^4}} = \dfrac{\pi}{2\text{AGM}(1,\sqrt2)}
\enspace,$$
which was one of Gauss's remarkable discoveries in his youth.
Now type \kbd{2 * agm(1,I) / (1+I)}. As you see, the complex AGM also works,
although one must be careful with its definition. The result found is
almost identical to the previous one. Do you see why?
Finally, type \kbd{agm(1, 1 + 7 + O(7\pow 10))}. So we also have $p$-adic
AGM. Note however that since the square root of a $p$-adic number is not
in general an element of the same $p$-adic field,
only certain $p$-adic AGMs can be computed. In addition,
when $p=2$, the congruence restriction is that \kbd{agm(a,b)} can be computed
only when \kbd{a/b} is congruent to 1 modulo $16$, and not 8 as could be
expected.\smallskip
%
Now type \kbd{?8}. This gives you the list of all transcendental functions.
Instead of continuing with more examples, we suggest that you experiment
yourself with this list. Try integer, real, complex and $p$-adic arguments.
You will notice that some have not been implemented (or do not have a
reasonable definition).
\section{Using Numerical Tools}
Although not written to be a numerical analysis package, PARI can
nonetheless perform some numerical computations. Since linear algebra and
polynomial computations are treated somewhere else, this section focuses on
solving equations and various methods of summation.
You of course know the formula $\pi = 4(1-\dfrac13+\dfrac15-\dfrac17+\cdots)$
which is deduced from the power series expansion of \kbd{atan(x)}. You also
know that $\pi$ cannot be computed from this formula, since the convergence
is so slow. Right? Wrong! Type
\bprog
\p 100
4 * sumalt(k=0, (-1)^k/(2*k + 1))
@eprog\noindent In a split second, we get $\pi$ to 100 significant digits
(type \kbd{Pi} to check).
Similarly, try
\bprog
sumpos(k=1, k^-2)
@eprog\noindent Although once again the convergence is slow, the summation is
rather fast; compare with the exact result \kbd{Pi\pow 2/6}. This is less
impressive because a bit slower than for alternating sums, but still useful.
Even better, \kbd{sumalt} can be used to sum divergent series! Type
\bprog
zet(s) = sumalt(k=1, (-1)^(k-1) / k^s) / (1 - 2^(1-s))
@eprog\noindent
Then for positive values of \kbd{s} different from 1, \kbd{zet(s)} is equal
to \kbd{zeta(s)} and the series converges, albeit slowly; \kbd{sumalt}
doesn't care however. For negative \kbd{s}, the series diverges, but
\kbd{zet(s)} still gives the correct result! (Namely, the value of a suitable
analytic continuation.) Try \kbd{zet(-1)}, \kbd{zet(-2)}, \kbd{zet(-1.5)},
and compare with the corresponding values of \kbd{zeta}. You should not push
the game too far: \kbd{zet(-100)}, for example, gives a completely wrong
answer.
Try \kbd{zet(I)}, and compare with \kbd{zeta(I)}. Even (some) complex values
work, although the sum is not alternating any more! Similarly, try
\bprog
sumalt(n=1, (-1)^n / (n+I))
@eprog
\medskip
More traditional functions are the numerical integration functions. Try
\kbd{intnum(t=1,2, 1/t)} and presto! you get 100 decimals of $\log(2)$. Look
at Chapter 3 to see the available integration functions.
With PARI, however, you can go further since complex types are allowed.
For example, assume that we want to know the location of the zeros of the
function $h(z)=e^z-z$. We use Cauchy's theorem, which tells us that the
number of zeros in a disk of radius $r$ centered around the origin is
equal to
$$\dfrac{1}{2i\pi}\int_{C_r}\dfrac{h'(z)}{h(z)}\,dz\enspace,$$
where $C_r$ is the circle of radius $r$ centered at the origin.
The function we want to integrate is
\bprog
fun(z) = my(u = exp(z)); (u-1) / (u-z)
@eprog\noindent
(Here \kbd{u} is a local variable to the function \kbd{f}: whenever
a function is called, \kbd{gp} fills its argument list with the actual arguments
given, and initializes the other declared parameters and local variables to
0. It will then restore their former values upon exit. If we had not declared
\kbd{u} in the function prototype, it would be considered as a global
variable, whose value would be permanently changed. It is not mandatory to
declare in this way all parameters, but beware of side effects!)
Type now:
\bprog
zero(r) = r/(2*Pi) * intnum(t=0, 2*Pi, real( fun(r*exp(I*t)) * exp(I*t) ))
@eprog
The function \kbd{zero(r)} will count the number of zeros of \kbd{fun}
whose modulus is less than \kbd{r}: we simply made the change of variable
$z = r*\exp(i*t)$, and took the real part to avoid integrating the
imaginary part. Actually, there is a built-in function \kbd{intcirc}
to integrate over a circle, yielding the much simpler:
\bprog
zero2(r) = intcirc(z=0, r, fun(z))
@eprog
(This is a little faster than the previous implementation, and no less
accurate.)
We may type \kbd{\b{p} 9} since we know that the result is a small integer
(but the computations should be instantaneous even at \b{p} 100 or so),
then \kbd{zero(1)}, \kbd{zero(1.5)}. The result tells us that there are
no zeros inside the unit disk, but that there are two (necessarily
complex conjugate) in the annulus $1<|z|<1.5$. For the sake of
completeness, let us compute them. Let $z = x+iy$ be such a zero, with
$x$ and $y$ real. Then the equation $e^z-z=0$ implies, after elementary
transformations, that $e^{2x}=x^2+y^2$ and that $e^x\cos(y)=x$. Hence
$y=\pm\sqrt{e^{2x}-x^2}$ and hence $e^x\cos(\sqrt{e^{2x}-x^2})=x$.
Therefore, type
\bprog
fun(x) = my(u = exp(x)); u * cos(sqrt(u^2 - x^2)) - x
@eprog\noindent
Then \kbd{fun(0)} is positive while \kbd{fun(1)} is negative. Come
back to precision 38 and type
\bprog
x0 = solve(x=0,1, fun(x))
z = x0 + I*sqrt(exp(2*x0) - x0^2)
@eprog\noindent
which is the required zero. As a check, type \kbd{exp(z) - z}.
Of course you can integrate over contours which are more complicated than
circles, but you must perform yourself the variable changes, as we have done
above to reduce the integral to a number of integrals on line segments.
\smallskip
%
The example above also shows the use of the \kbd{solve} function. To use
\kbd{solve} on functions of a complex variable, it is necessary to reduce the
problem to a real one. For example, to find the first complex zero of the
Riemann zeta function as above, we could try typing
\kbd{solve(t=14,15, real( zeta(1/2 + I*t) ))},
\noindent but this does not work because the real part is positive for
$\kbd{t}=14$ and $15$. As it happens, the imaginary part works. Type
\kbd{solve(t=14,15, imag( zeta(1/2 + I*t) ))},
\noindent and this now works. We could also narrow the search interval and
type for instance
\kbd{solve(t=14,14.2, real( zeta(1/2 + I*t) ))}
\noindent which would also work.
\section{Polynomials}
First a word of warning: it is essential to understand the difference between
exact and inexact objects. Try
\bprog
gcd(x - Pi, x^2 - 6*zeta(2))
@eprog\noindent
We return a trivial GCD because the notion of GCD for inexact polynomials
doesn't make much sense. A better quantitative approach is to use
\bprog
polresultant(x - Pi, x^2 - 6*zeta(2))
@eprog\noindent
A result close to zero shows that the GCD is nontrivial for small
deformations of the inputs. Without telling us what it is, of course. This
being said, we will mostly use polynomials (and power series) with exact
coefficients in our examples.\smallskip
The simplest way to input a polynomial, is to simply write it down,
or use an explicit formula for the coefficients and the function \kbd{sum}:
\bprog
T = 1 + x^2 + 27*x^10;
T = sum(i = 1, 100, (i+1) * x^i);
@eprog\noindent but it is in much more efficient to create a vector of
coefficients then convert it to a polynomial using \kbd{Pol} or \kbd{Polrev}
(\kbd{Pol([1,2])} is $x+2$, \kbd{Polrev([1,2]) is $2x + 1$}) :
\bprog
T = Polrev( vector(100, i, i) );
for (i=1, 10^4, Polrev( vector(100, i, i) ) ) \\@com time: 60ms
for (i=1, 10^4, sum(i = 1, 100, (i+1) * x^i) ) \\@com time: 1,74ms
@eprog\noindent The reason for the discrepancy is that the explicit summation
(of densely encoded polynomials) is quadratic in the degree, whereas creating
a vector of coefficients then converting it to a polynomial type is linear.
We also have a few built-in classical polynomial families. Consider the
$15$-th cyclotomic polynomial,
\bprog
pol = polcyclo(15)
@eprog\noindent
which is of degree $\varphi(15)=8$. Now, type
\bprog
r = polroots(pol)
@eprog\noindent We obtain the 8 complex roots of \kbd{pol}, given to 38
significant digits. To see them better, type \b{B}: they are given as pairs
of complex conjugate roots, in some order. In fact, the
function \kbd{polroots} returns the real roots first, in increasing order,
then the other roots by increasing absolute value of their imaginary parts
(so that pairs of complex conjugate roots appear together).
The roots of \kbd{pol} are by definition the primitive $15$-th roots of unity.
To check this, simply type \kbd{rc = r\pow 15}. Why, we get an error message!
Fair enough, vectors cannot be multiplied, even less raised to a power.
However, type
\bprog
rc = r^15.0
@eprog\noindent without forgetting the `\kbd{.}' at the end. Now it works,
because powering to a nonintegral exponent is a transcendental function and
hence is applied termwise. Note that the fact that $15.0$ is a real number
which is representable exactly as an integer has nothing to do with the
problem.
We see that the components of the result are very close to 1. It is however
tedious to look at all these real and imaginary parts. It would be impossible
if we had many more. Let's do it automatically. Type
\bprog
rr = round(rc)
exponent(rc - rr)
@eprog\noindent We see that \kbd{rr} is indeed all 1's, and that the
sup-norm of \kbd{rc - rr} is around $2^{-125}$, reasonable enough when we
work with 128 bits of accuracy! In fact, \kbd{round} itself already provides
a built-in rough approximation of the error:
\bprog
rr = round(rc, &e)
@eprog\noindent Again, \kbd{e} contains the number of error bits when rounding
\kbd{rc} to \kbd{rr}; in other words the sup norm of $\kbd{rc} - \kbd{rr}$
is bounded by $2^{-\kbd{e}}$.
%
\smallskip
Now type
\bprog
pol = x^5 + x^4 + 2*x^3 - 2*x^2 - 4*x - 3
factor(pol)
factor( poldisc(pol) )
fun(p) = factor(pol, O(p^10));
@eprog\noindent The polynomial \kbd{pol} factors over $\Q$ (or $\Z$) as a
product of two factors, and the primes dividing its discriminant are
$11$, $23$ and $37$. We also created a function \kbd{fun(p)} which factors
\kbd{pol} over $\Q_p$ to $p$-adic precision 10. Type
\bprog
fun(5)
fun(11)
fun(23)
fun(37)
@eprog\noindent to see different splittings. You can use \kbd{lift} to
convert $p$-adic numbers to neighbouring rational numbers.
Similarly, type
\bprog
lf(p) = lift( factor(pol, p) );
lf(2)
lf(11)
lf(23)
lf(37)
@eprog\noindent which show the different factorizations, this time over
$\F_p$. In fact, even better: type successively
\bprog
T = ffgen(3^3, 't) \\@com we want \kbd{t} to be a free variable
factor(pol, t)
@eprog\noindent
The element $T$ is printed as \kbd{t} but is actually defined modulo the
irreducible polynomial \kbd{t\pow 3 + 2t + 2} over $\F_{3}$ (see
\kbd{t.mod}): it defines the finite field $\F_{27}$. Note that we introduced
a new variable $t$ to express elements in this nonprime field.
More generally, the generic \kbd{factor} function allows a second argument
which describes the domain over which we want to factor, here $\F_{27}$
(and, above, $\Q_p$ to $10$ digits of accuracy and $\F_p$).
Typing \kbd{factor(pol)} directly would factor it over $\Q$, not what we
wanted.
Similarly, type
\bprog
pol2 = x^4 - 4*x^2 + 16
fn = lift( factor(pol2, t^2 + 1) )
@eprog\noindent and we get the factorization of the polynomial \kbd{pol2}
over the number field $\Q[t]/(t^2+1)$, i.e.~over $\Q(i)$. Without the
\kbd{lift}, the result would involve number field elements as \typ{POLMOD}s
of the form \kbd{Mod(1+t, t\pow2+1)}, which are more explicit but less
readable.
\smallskip
To summarize, in addition to being able to factor integers, you can factor
polynomials over $\C$ using \kbd{polroots}, over $\R$ using
\kbd{factor(,1.0)}, over finite fields using
\kbd{factor(, p)}, over $\Q_p$ using \kbd{factor(, O(p\pow $n$))},
over $\Q$ using \kbd{factor}, and over the number field $\Q[t]/(T)$ using
\kbd{factor(, T)}.
Note however that \kbd{factor} itself will guess intelligently over
which ring you want to factor: try
\bprog
pol = x^2 + 1;
factor(pol)
factor(pol *1.)
factor(pol * (1 + 0*I))
factor(pol * (1 + 0.*I))
factor(pol * Mod(1,2))
factor(pol * Mod(1, Mod(1,3)*(t^2+1)))
pol2 = x^2 + y^2;
factor(pol2)
factor(pol2 * Mod(1,5))
@eprog\noindent
In the present version \vers{}, it is not possible to factor over other
base rings than the ones mentioned above, but multivariate polynomials over
those rings are allowed as shown in the last examples. Other functions
related to factoring are \kbd{padicappr}, \kbd{polrootsmod},
\kbd{polrootspadic}, \kbd{polsturm}. Play with them a little.
Finally, type
\bprog
polsym(pol2, 20)
@eprog\noindent where \kbd{pol2} was defined above. This gives the sum of the
$k$-th powers of the roots of \kbd{pol2} up to $k=20$, of course computed
using Newton's formula and not using \kbd{polroots}. You notice that every
odd sum is zero (expected, since the polynomial is even), but also that
the signs follow a regular pattern and that the (nonzero) absolute values
are powers of 2. This is true: prove it, and more precisely find an explicit
formula for the $k$-th symmetric power not involving (nonrational) algebraic
numbers.
\section{Power Series}
Now let's play with power series as we have done at the beginning. Type
\bprog
N = 39;
8*x + prod(n=1,N, if(n%4, 1 - x^n, 1), 1 + O(x^(N+1)))^8
@eprog\noindent
Apparently, only even powers of \kbd{x} appear. This is surprising, but can
be proved using the theory of modular forms. Note that we initialize
the product to \kbd{1 + O(x\pow (N+1))}, otherwise the whole computation would
be done with polynomials; this would first have been slightly slower and also
totally useless since the coefficients of \kbd{x\pow (N+1)} and above are
irrelevant anyhow if we stop the product at $\kbd{n} = \kbd{N}$.
While we are on the subject of modular forms (which, together with Taylor
series expansions are another great source of power series), type
\bprog
\ps 122 \\@com shortcut for \kbd{default(seriesprecision, 122)}
d = x * eta(x)^24
@eprog\noindent This gives the first 122 terms of the Fourier series
expansion of the modular discriminant function $\Delta$ of Ramanujan. Its
coefficients give by definition the Ramanujan $\tau$ function, which has a
number of marvelous properties (look at any book on modular forms for
explanations). We would like to see its properties modulo 2. Type \kbd{d\%2}.
Hmm, apparently PARI doesn't like to reduce coefficients of power series, or
polynomials for that matter, directly. Can we do it without writing a little
program? No problem. Type instead
\bprog
lift(Mod(1,2) * d)
centerlift(Mod(1,3) * d)
@eprog\noindent and now this works like a charm. The pattern in the first
result is clear; the pattern is less clear in the second result, but
nonetheless there is one. Of course, it now remains to prove it (see Antwerp
III or your resident modular forms guru).
\section{Working with Elliptic Curves}
Now we are getting to more complicated objects. Just as with number fields
which we will meet later on, the first thing to do is to initialize them.
That's because a lot of data will be needed repeatedly, and it's much more
convenient to have it ready once and for all. Here, this is done with the
function \kbd{ellinit}.
So type
\bprog
e0 = ellinit([6,-3,9,-16,-14])
@eprog
This computes a number of things
about the elliptic curve defined by the affine equation
%
$$ y^2+6xy+9y = x^3-3x^2-16x-14\enspace. $$
%
It is not that clear what all these funny numbers mean, except that we
recognize the first few of them as the coefficients we just input. To
retrieve meaningful information from such complicated objects (and number
fields will be much worse), one uses so-called \emph{member
functions}. Type \kbd{?.} to get a complete list. Whenever \kbd{ell} appears
in the right hand side, we can apply the corresponding function to an object
output by \kbd{ellinit}. (I'm sure you know how the other \kbd{init} functions
will be called now, don't you? Oh, by the way, neither \kbd{clgpinit} nor
\kbd{pridinit} exist.)
Let's try it. The discriminant \kbd{e0.disc} is equal to 37, hence
the conductor of the curve is 37. Of course in general it is not so
trivial. In fact, although the equation of the curve is clearly
minimal (since the discriminant is $12$th-power-free), it is not in
standard reduced form, so type
\bprog
e = ellminimalmodel(e0)
@eprog\noindent which
gives the \kbd{ell} structure attached to the standard model,
exactly as if we had used \kbd{ellinit} on a reduced equation. For
some related data, type
\bprog
gr = ellglobalred(e0)
@eprog\noindent The first
component \kbd{gr[1]} tells us that the conductor is 37 as we already
knew. The second component is a 4-component vector which allows us to
get the minimal equation: in fact \kbd{e} is \kbd{ellchangecurve(e0, gr[2])}.
Type
\bprog
q0 = [-2,2]
ellisoncurve(e0, q0)
q = ellchangepoint(q0,gr[2])
ellisoncurve(e, q)
@eprog\noindent
The point \kbd{q0} is on the curve, as checked by \kbd{ellisoncurve}, and we
transferred it onto the minimal model \kbd{e}, using \kbd{ellchangepoint} and
the change of variable computed above. Note that \kbd{ellchangepoint()} is
unusual among the elliptic curve functions in that it does not take an
\kbd{ell} structure as its first argument: in \kbd{gp}, points do not
``know'' which curve they are on, but to move a point from one model to
another we only need to know the coordinates of the point and the
transformation data here stored in \kbd{gr[2]}. Also, the point at infinity
is represented as \kbd{[0]} on all elliptic curves; this is the identity for
the group law.
Here, \kbd{q=[0,0]} obviously lies on \kbd{e}, which has equation
$y^2+y = x^3-x$. Let us now play a little with points on \kbd{e}.
The group law on an elliptic curve is implemented with the functions
\kbd{elladd} for addition, \kbd{ellsub} for subtraction and
\kbd{ellmul} for multiplication by an integer. For
example, the negative of \kbd{q} is \kbd{ellsub(e,[0],q)}, and the
double is obtained either as \kbd{ellmul(e,q,2)} or as
\kbd{elladd(e,q,q)}.
Now \kbd{q} may be a torsion point. Type \kbd{ellheight(e, q)}, which
computes the canonical Neron-Tate height of \kbd{q}. Note that
\kbd{ellheight} does not assume that \kbd{e} is \emph{minimal}! (Although
it is, making things a little faster.)
This is nonzero, hence \kbd{q} is not torsion. To see this even better,
type
\bprog
for(k = 1, 20, print(ellmul(e, q, k)))
@eprog\noindent and we see the characteristic parabolic explosion of the size
of the points. (And another proof that \kbd{q} is not torsion, assuming
Mazur's bound on the size of the rational torsion.) We could can also type
\kbd{ellorder(e, q)} which returns 0, telling us yet again that \kbd{q} is
nontorsion. As a consistency check, type
\bprog
ellheight(e, ellmul(e, q,20)) / ellheight(e, q)
@eprog\noindent
We indeed find $400=20^2$ as it should be.
Notice how (almost) all those \kbd{ell}--prefixed functions take our
elliptic curve as a first argument? This will be true with number
fields as well: whatever object was initialized by an $ob$--\kbd{init}
function will have to be used as a first argument of all the
$ob$--prefixed functions. Conversely, you won't be able to use any
such high-level function before you correctly initialize the relevant
object. \smallskip
Ok, let's try another curve. Type
\bprog
E = ellinit([0,-1,1,0,0])
q = [0,0]; ellheight(E, q)
@eprog\noindent This corresponds to the equation $y^2+y = x^3-x^2$ and an
obvious rational point on it. Again from the discriminant we see that the
conductor is equal to 11, and if you type \kbd{ellminimalmodel(E)} you will
see that the equation for \kbd{E} is minimal. This time the height is
exactly zero, hence \kbd{q} must be a torsion point. Indeed, typing
\bprog
for(k=1, 5, print(ellmul(E, q,k)))
ellorder(E, q) \\@com simpler
@eprog\noindent
we see in two different ways that \kbd{q} is a point of order 5. Moreover,
typing
\bprog
elltors(E)
@eprog\noindent shows that \kbd{q} generates all the torsion of \kbd{E},
which is cyclic of order~$5$. \smallskip
Let's try still another curve, $y^2+y = x^3-7x+6$:
\bprog
e = ellinit([0,0,1,-7,6])
ellglobalred(e)
@eprog\noindent As before, this is a minimal equation; now the conductor is
5077. There are some trivial integral points on this curve, but let's try to
be more systematic. Typing
\bprog
elltors(e)
@eprog\noindent
shows that the torsion subgroup is trivial, so we don't have to worry about
torsion points. Next, the function \kbd{ellratpoints} allows us to find
rational points of small height
\bprog
v = ellratpoints(e,1000)
@eprog\noindent The vector $v$ contains all 130 rational points $(x,y)$ on the
curve whose $x$-coordinate is $n/d$ with $|n|$ and $|d|$ both less than $1000$.
Note that \kbd{ellratpoints(e,10\pow6)} takes less than 1 second, and
produces 344 points. Of course, these are grouped by pairs:
if $(x,y)$ is on the curve, its opposite is $(x,-y-1)$ as
\bprog
ellneg(e, ['x,'y])
@eprog\noindent shows. Note that there is no problem with manipulating points
with formal coordinates. This is large for a curve having such a small
conductor. So we suspect (if we do not know already, since this curve is
quite famous!) that the rank of this curve must be large. Let's try and put
some order into this. First, we eliminate one element in each pair
of opposite points:
\bprog
v = vecsort(v, 1, 8)
@eprog\noindent
The argument $1$ specifies a comparison function: we sort the points by first
coordinate only, in particular two points with the same $x$-coordinate
compare as equal; the $8$ flag eliminates ``duplicates''. The same effect
could be obtained in a more verbose way using an inline anonymous function
\bprog
v = vecsort(v, (P,Q) -> sign(P[1]-Q[1]), 8)
@eprog\noindent We now order the points according to their canonical height:
\bprog
hv = [ ellheight(e,P) | P <- v ];
v = vecextract(v, vecsort(hv,,1)) \\ indirect sort wrt h, then permute
@eprog\noindent
It seems reasonable to take the numbers with smallest height as
possible generators of the Mordell-Weil group. Let's try the first
four: type
\bprog
m = ellheightmatrix(e, v[1..4]); matdet(m)
@eprog\noindent
Since the curve has no torsion, the determinant being close to zero
implies that the first four points are dependent. To find the
dependency, it is enough to find the kernel of the matrix \kbd{m}. So
type \kbd{matker(m)}: we indeed get a nontrivial kernel, and the
coefficients are close to integers. Typing \kbd{elladd(e, v[1],v[3])} does
indeed show that it is equal to \kbd{v[4]}.
Taking any other four points, we seem to always find a dependency.
Let's find all dependencies. Type
\bprog
vp = v[1..3];
m = ellheightmatrix(e,vp);
matdet(m)
@eprog\noindent This is now clearly nonzero so the first 3 points are
linearly independent, showing that the rank of the curve is at least equal to
3. (In fact, \kbd{e} is the curve of smallest conductor having rank 3.)
We would like to see whether the other points are dependent:
if \kbd{Q} is some point which is dependent on \kbd{v[1],v[2]} and \kbd{v[3]}
and
\bprog
c = [ellheight(e, P, Q) | P <- vp]~
@eprog\noindent then \kbd{m\pow(-1) * c} will give the coefficients of the
dependence relation. If these coefficients are not close to integers,
then there is no dependency, otherwise we can round and check rigourously
using a function such as the following:
\bprog
ellcomb(e, P, L) =
{ my (Q = [0]);
for (i = 1, #P, Q = elladd(e, Q, ellmul(e, P[i], L[i])));
return (Q);
}
@eprog\noindent This is safer than using the \kbd{matker} function. Thus, type
\bprog
mi = m^(-1);
w = vector(#v, k, mi * [ellheight(e, P, v[k]) | P <- vp]~)
wr = round(w, &e)
@eprog\noindent
We ``see'' that the coefficients are all very close to integers, and we
quantified it with the last instruction: \kbd{wr} is the vector expressing
all the components of \kbd{v} on its first 3, and $2^{-e}$ gives an upper
bound on the maximum distance to an integer. The rigorous check is positive:
\bprog
for (i=1, #w, if (v[i] != ellcomb(e,vp,wr[i]), error(i)));
@eprog\noindent No error! We must make two technical remarks at this point:
(1) Using the height pairing to find dependence relations as we
have done only finds relations modulo torsion; but in this example, the curve
has trivial torsion, as we checked earlier.
(2) In the above calculation we
were lucky that all the \kbd{v[j]} were $\Z$-linear combinations of
\kbd{v[1]}, \kbd{v[2]} and \kbd{v[3]} and not just $\Q$-linear combinations;
in general the results in $w$ might have given a vector of rationals: if
$k\ge2$ is minimal such that $kQ$ is in the subgroup generated by \kbd{v[1]},
\kbd{v[2]} and \kbd{v[3]}, then the entries of \kbd{matsolve(m, ellbil(e,
vp,Q))} will be rationals with common denominator~$k$. This can be detected
by using \kbd{bestappr} instead of \kbd{round} in the above.
\smallskip
We are thus led to strongly believe that the curve has rank
exactly 3, with generators \kbd{v[1]}, \kbd{v[2]} and \kbd{v[3]}, and this
can be proved to be the case. Indeed, all this (and much more) is automated
in the built-in \kbd{ellrank} function:
\bprog
[r, R, P] = ellrank(e);
Q = ellsaturation(e, P, 100)
@eprog\noindent We find $r = R = 3$ and a vector $P$ containing $3$
rational points. This performs $2$-descent on the curve: the rank belongs
to $[r,R]$ hence is equal to $3$. The points in $P$ are independent
modulo torsion : since $\kbd{\#P} = r = R$, they generate a subgroup
of finite index. The final \kbd{ellsaturation} provides a new vector of
points which is $p$-saturated for all primes $p \leq 100$: the index of the
subgroup they generate is not divisible by any such prime.
In other situations, we could be missing some points and have $\kbd{\#P} <
r$: for instance
\bprog
e = ellinit([-127^2, 0]);
ellrank(e)
@eprog\noindent does not find any point although the rank is $1$; in this
particular case (rank $1$ and ``small'' conductor), \kbd{ellheegner} finds
the missing point, which happens to be a generator.
\smallskip
Let us explore a few more elliptic curve related functions. Keep our
curve \kbd{e} of rank 3, and type
\bprog
v1 = [1,0]; z1 = ellpointtoz(e, v1)
v2 = [2,0]; z2 = ellpointtoz(e, v2)
@eprog\noindent
We thus get the complex parametrization of the curve. To add the points
\kbd{v1} and \kbd{v2}, we should of course type \kbd{elladd(e, v1,v2)},
but we can also type \kbd{ellztopoint(e, z1 + z2)} which has the disadvantage
of giving complex numbers, but illustrates how the group law on \kbd{e} is
obtained from the addition law on $\C$.
Type
\bprog
f = x * Ser(ellan(e, 30))
@eprog\noindent This gives a power series which is the Fourier expansion of a
modular form of weight 2 for $\Gamma_0(5077)$. (This has been proved
directly, before Wiles's general result.) In fact, to find the modular
parametrization of the curve, type
\bprog
modul = elltaniyama(e)
u = modul[1];
v = modul[2];
@eprog\noindent
We can check in various ways that this indeed parametrizes the curve:
\bprog
(v^2 + v) - (u^3 - 7*u + 6)
@eprog\noindent
is close to $0$ for instance, or simply that \kbd{ellisoncurve(e,modul)}
returns~$1$. Now type
\bprog
x * u' / (2*v + 1)
@eprog\noindent and we see that this is equal to the modular form \kbd{f}
found above; the quote \kbd{'} tells \kbd{gp} to take the derivative of the
expression with respect to its main variable. The functions \kbd{u} and
\kbd{v}, considered on the upper half plane with $x=e^{2i\pi\tau}$, are in
fact modular \emph{functions} for $\Gamma_0(5077)$. \smallskip
The function \kbd{ellan(e, 30)} gives the first~$30$ coefficients
$a_n$ of the $L$-series of \kbd{e}. One can also ask for a single
coefficient: the millionth is \kbd{ellak(e, 10\pow 6)}. Note however
that calling \kbd{ellan(e,100000)} is much faster than
the equivalent \kbd{vector(100000,k,ellak(e,k))}. For a
prime~\kbd{p},
\kbd{ellap(e,p)} is equivalent to \kbd{ellak(e,p)}; this is the
integer $a_p$ such that the number of points on \kbd{e} over $\F_p$ is
$1+p-a_p$. (With the standard PARI distribution, \kbd{ellap} is the only way
to obtain the order of an elliptic curve over $\F_p$ in \kbd{gp}. The
external package \kbd{ellsea} provides much more efficient routines.)
Finally, let us come back to the curve \kbd{E} defined above by
\kbd{E = ellinit([0,-1,1,0,0])}, which had an obvious rational $5$-torsion
point. The sign of the functional equation, given by \kbd{ellrootno(E)}, is
$+1$. Assuming the rank parity conjecture, it follows that the Mordell-Weil
group of $E$ has even rank. The value of the L-function of \kbd{E} at 1
\bprog
ls = lfun(E, 1)
@eprog\noindent is definitely nonzero, so \kbd{E} has rank $0$. According to
the Birch and Swinnerton-Dyer conjecture, which is proved for this curve,
\kbd{ls} is given by the following formula (in this case):
%
\def\sha{\hbox{III}}
$$L(E,1)=\dfrac{\Omega\cdot c\cdot|\sha|}{|E_{\text{tors}}|^2}\enspace,$$
%
where $\Omega$ is the real period of $E$, $c$ is the global Tamagawa number,
$|\sha|$ is the order of the Tate-Shafarevich group, and $E_{\text{tors}}$ is the
torsion group of $E$.
Now we know many of these quantities: $\Omega$ is equal to \kbd{E.omega[1]}
The global Tamagawa number $c$ is given by \kbd{elltamagawa(E)} and is $1$
We already know that the torsion subgroup of $E$ contains a point of order 5,
and typing \kbd{elltors(E)} shows that it is of order exactly 5. So type
\bprog
ls * 25/E.omega[1]
@eprog\noindent This shows that $\sha$ must be the trivial group.
A short hand for $\dfrac{\Omega\cdot c}{|E_{\text{tors}}|^2}$ is \kbd{ellbsd(E)},
so the previous line can be written as
\bprog
ls / ellbsd(E)
@eprog
For more detailed information on the local reduction of an elliptic curve at
a specific prime~\kbd{p}, use the function \kbd{elllocalred(E,p)}; the second
component gives the Kodaira symbol in an encoded form. See the manual or
online help for details.
\section{Working in Quadratic Number Fields}
The simplest of all number fields outside $\Q$ are quadratic fields and are
the subject of the present section. We shall deal in the next one with
general number fields (including $\Q$ and quadratic fields!), and one should
be aware that all we will see now has a more powerful, in general easier to
use, equivalent in the general context. But possibly much slower.
Such fields are characterized by their discriminant. Even better, any
nonsquare integer $D$ congruent to 0 or 1 modulo 4 is the discriminant of a
specific order in a quadratic field. We can check whether this order is
maximal with \kbd{isfundamental(D)}. Elements of a quadratic field are of the
form $a+b\omega$, where $\omega$ is chosen as $\sqrt{D}/2$ if $D$ is even and
$(1+\sqrt{D})/2$ if $D$ is odd, and are represented in PARI by quadratic
numbers. To initialize working in a quadratic order, one starts by the
command \kbd{w = quadgen($D$,'w)}.
This sets \kbd{w} equal to $\omega$ as above, and is printed \kbd{w}.
If you need several quadratic orders at the same time, you can
use different variable names:
\bprog
w1 = quadgen(-23,'w1)
w2 = quadgen(-15,'w1)
@eprog\noindent
\smallskip
%
In addition to elements of a quadratic order, we also want to be able to
handle ideals of such orders. In the quadratic case, it is equivalent to
handling binary quadratic forms. Quadratic forms are triples $(a,b,c)$
representing the form $ax^2+bxy+cy^2$. Such a form will be printed as, and
can be created by, \kbd{Qfb($a$,$b$,$c$)}.
Such forms can be multiplied, divided, powered as many PARI objects using
the usual operations, and they can also be reduced using the function
\kbd{qfbred} or \kbd{qfbredsl2} (which also returns the unimodular
transformation matrix). One can also use explicitly \kbd{qfbcomp} /
\kbd{qfbpow} (same as multiplication) and \kbd{qfbcompraw} / \kbd{qfbpowraw}
(composition without reduction) (It is not the purpose of this tutorial to
explain what all these things mean.) In addition, Shanks's NUCOMP algorithm
has been implemented (functions \kbd{qfbnucomp} and \kbd{qfbnupow}), and this
is usually a little faster.
Finally, you have at your disposal the functions \kbd{qfbclassno} which
(\emph{usually}) gives the class number, the function \kbd{qfbhclassno}
which gives the Hurwitz class number, and the much more sophisticated
\kbd{quadclassunit} function which gives the class number and class group
structure.
Let us see examples of all this at work.
Type \kbd{qfbclassno(-10007)}. \kbd{gp} tells us that the result is 77. However,
you may have noticed in the explanation above that the result is only
\emph{usually} correct. This is because the implementers of the algorithm
have been lazy and have not put the complete Shanks algorithm into PARI,
causing it to fail in certain very rare cases. In practice, it is almost
always correct, and the much more powerful \kbd{quadclassunit} program, which
\emph{is} complete (at least for fundamental discriminants) can give
confirmation; but now, under the Generalized Riemann Hypothesis!
So we may be a little suspicious of this class number. Let us check it.
First, we need to find a quadratic form of discriminant $-10007$. Since this
discriminant is congruent to 1 modulo 8, we know that there is an ideal of
norm equal to 2, i.e.~a binary quadratic form $(a,b,c)$ with $a=2$. To
compute it we type \kbd{f = qfbprimeform(-10007, 2)}. OK, now we have a form.
If the class number is correct, the very least is that this form raised to
the power 77 should equal the identity. Type \kbd{f\pow 77}. We get a form
starting with 1, i.e.~the identity. Raising \kbd{f} to the powers 11 and 7
does not give the identity, thus we now know that the order of \kbd{f} is
exactly 77, hence the class number is a multiple of 77. But how can we be
sure that it is exactly 77 and not a proper multiple? Well, type
\bprog
sqrt(10007)/Pi * prodeuler(p=2,500, 1./(1 - kronecker(-10007,p)/p))
@eprog\noindent
This is nothing else than an approximation to the Dirichlet class number
formula. The function \kbd{kronecker} is the Kronecker symbol, in this case
simply the Legendre symbol. Note also that we have written \kbd{1./(1 - \dots)}
with a dot after the first 1. Otherwise, PARI may want to compute the whole
thing as a rational number, which would be terribly long and useless. In fact
PARI does no such thing in this particular case (\kbd{prodeuler} is always
computed as a real number), but you never know. Better safe than sorry!
We find 77.77, pretty close to 77, so things seem in order. Explicit bounds
on the prime limit to be used in the Euler product can be given which make
the above reasoning rigorous.
Let us try the same thing with $D=-3299$. \kbd{qfbclassno} and the Euler
product convince us that the class number must be 27. However, we get stuck
when we try to prove this in the simple-minded way above. Indeed, we type
\kbd{f = qfbprimeform(-3299, 3)} (2 is not the norm of a prime ideal but
3 is), and we see that \kbd{f} raised to the power 9 is equal to the identity.
This is the case for any other quadratic form we choose. So we suspect that the
class group is not cyclic. Indeed, if we list all 9 distinct powers of \kbd{f},
we see that \kbd{qfbprimeform(-3299, 5)} is not on the list, although its cube
is as it must. This implies that the class group is probably equal to a
product of a cyclic group of order 9 by a cyclic group of order 3. The Euler
product plus explicit bounds prove this.
Another way to check it is to use the \kbd{quadclassunit} function by typing
for example
\bprog
quadclassunit(-3299)
@eprog\noindent
Now, if we believe in GRH, the class group is as we thought (see Chapter 3
for a complete description of this function). Note that using the even more
general function \kbd{bnfinit} (which handles general number fields and gives
more complicated results), we could \emph{certify} this result, i.e~remove
the GRH assumption. Let's do it, type
\bprog
bnf = bnfinit(x^2 + 3299); bnfcertify(bnf)
@eprog
A nonzero result (here 1) means that everything is ok. Good, but what did
we certify after all? Let's have a look at this \kbd{bnf} (just type it!).
Enlightening, isn't it? Recall that the \kbd{init} functions (we've already
seen \kbd{ellinit}) store all kind of technical information which you
certainly don't care about, but which will be put to good use by higher level
functions. That's why \kbd{bnfcertify} could not be used on the output of
\kbd{quadclassunit}: it needs much more data.
To extract sensible information from such complicated objects, you must use
one of the many \emph{member functions} (remember: \kbd{?.} to get a complete
list). In this case \kbd{bnf.clgp} which extracts the class group structure.
This is much better. Type \kbd{\%.no} to check that this leading 27 is indeed
what we think it is and not some stupid technical parameter. Note that
\kbd{bnf.clgp.no} would work just as well, or even \kbd{bnf.no}!
As a last check, we can request a relative equation for the Hilbert class
field of $\Q(\sqrt{-3299})$: type \kbd{quadhilbert(-3299)}. It is indeed of
degree 27 so everything fits together.
\medskip
%
Working in real quadratic fields instead of complex ones, i.e.~with $D>0$, is
not very different.
The same \kbd{quadgen} function is used to create elements. Ideals are again
represented by binary quadratic forms $(a,b,c)$, this time indefinite. However,
the Archimedean valuations of the number field start to come into play, hence
in fact quadratic forms with positive discriminant can also
be represented as a pair $[q, d]$ where $q = \kbd{Qfb(a, b, c)}$ is the
indefinite quadratic form $ax^2+bxy+cy^2$, and $d\in \R$ is a ``distance''
component, as defined by Shanks and Lenstra.
The form $q$ can be multiplied, divided, powered, and they can be
reduced using \kbd{qfbred}. To compose extended forms
$[q,d]$ and $[q,d']$, one may use \kbd{qfbcomp} and \kbd{qfbpow} but
ordinary multiplication and powerings also accept pairs.
The \kbd{qfbred} function applies a succession of
elementary reduction steps corresponding essentially to a continued fraction
expansion, and a single one of these steps can be achieved by adding an
(optional) flag to the arguments of \kbd{qfbred}. Again, the function
\kbd{qfbprimeform} gives a prime form, but the form which is given
corresponds to an ideal of prime norm which is usually not reduced. If
desired, it can be reduced using \kbd{qfbred}.
Finally, you still have at your disposal the function \kbd{qfbclassno} which
gives the class number (this time \emph{guaranteed} correct),
\kbd{quadregulator} which gives the regulator, and the much more sophisticated
\kbd{quadclassunit} giving the class group's structure and its generators,
as well as the regulator. The \kbd{qfbclassno} and \kbd{quadregulator}
functions use an algorithm which is $O(\sqrt D)$, hence become very slow for
discriminants of more than 10 digits. \kbd{quadclassunit} can be used on a
much larger range.
Let us see examples of all this at work and learn some little known number
theory at the same time. First of all, type
\bprog
D = 3 * 3299; qfbclassno(D)
@eprog\noindent We see that the class number is 3. (We know
in advance that it must be divisible by 3 from the \kbd{D = -3299} case above
and Scholz's mirror theorem.) Let us create a form by typing
\bprog
f = qfbred(qfbprimeform(D,2))
@eprog\noindent
This gives us a prime ideal of norm equal to 2. Is this ideal principal?
Well, one way to check this, which is not the most efficient but will suffice
for now, is to look at the complete cycle of reduced forms equivalent to
\kbd{f}. Type
\bprog
g = f; until(g == f, g = qfbred(g, 1); print(g))
@eprog\noindent
(the flag $1$ in \kbd{qfbred} means to perform a single reduction step). We
see that we come back to the form \kbd{f} without meeting the principal form
(starting with $\pm1$) in the cycle, so the ideal corresponding to \kbd{f} is
not principal.
Since the class number is equal to 3, we know however that \kbd{f\pow 3} will
be a principal ideal $\alpha\Z_K$. How do we find $\alpha$? For this, type
\bprog
f3 = qfbpowraw(f, 3)
@eprog
This computes the cube of \kbd{f}, without
reducing it. Hence it corresponds to an ideal of norm equal to $8=2^3$, so we
already know that the norm of $\alpha$ is equal to $\pm8$. We need more
information, and this is given by supplementing the form by a \emph{distance}
component $d$, a real number that tracks archimedean information that will
eventually allows to recover $\alpha$: we call such a pair $[f,d]$ an
\emph{extended form}.
\bprog
F3 = [f3, 0.0]; /* initialize */
while(abs(component(F3[1], 1)) != 1, F3 = qfbred(F3, 1); print(F3))
d = F3[2];
@eprog
We reduce the form until we reach the unit form (exactly 6 times),
then extract the Archimedean component, say $d = 8.59\dots$
By definition of this distance, we know that
$${\alpha\over{\sigma(\alpha)}}=\pm e^{2d},$$
where $\sigma$ denotes real conjugation in our quadratic field. Thus, if we type
\bprog
Na = 8; /* |Norm(alpha)| */
a = sqrt(Na * exp(2*d))
sa = Na / a
@eprog\noindent
we know that up to sign, \kbd{a} and \kbd{sa} are
numerical approximations of $\alpha$ and $\sigma(\alpha)$. Of course,
$\alpha$ can always be chosen to be positive, and a quick numerical check
shows that the difference of \kbd{a} and \kbd{sa} is close to an integer, and
not the sum, so that in fact the norm of $\alpha$ is equal to $-8$ and the
numerical approximation to $\sigma(\alpha)$ is \kbd{$-$sa}. Thus we type
\bprog
p = x^2 - round(a-sa)*x - 8
@eprog\noindent
and this ($x^2 - 15221x - 8$) is the characteristic polynomial of $\alpha$.
(A more user-friendly, but much less efficient, way to obtain the same result
is \kbd{algdep(a,2)} or \kbd{algdep(sa,2)}.)
We can check that the discriminant of this polynomial is a square multiple of
\kbd{D} using \kbd{issquare(poldisc(p) / D)}, so $\alpha$ is indeed in our
field. More precisely, solving for $\alpha$ and using the numerical
approximation that we have to resolve the sign ambiguity in the square root,
we get explicitly $\alpha=(15221+153\sqrt D)/2$. Note that this can also be
done automatically using the functions \kbd{polred} and \kbd{modreverse}, as
we will see later in the general number field case. Or by solving a system
of~2 linear equations in 2 variables. (Exercise: now that we have $\alpha$,
check that it is indeed a generator of the ideal corresponding to the form
\kbd{f3}.)
\medskip Let us now play a little with cycles. Type
\bprog
D = 10^7 + 1
quadclassunit(D)
@eprog\noindent
We get as a result a 5-component vector, which tells us that (under GRH) the
class number is equal to 1, and the regulator $R$ is approximately
equal to $2641.5$. You may certify this with
\bprog
bnf = bnfinit(x^2 - D, 1); \\ @com insist on finding fundamental unit
bnfcertify(bnf);
@eprog\noindent
which is fast for small $D$ but costly in general. Indeed \kbd{bnfcertify}
needs the fundamental unit, which is large: we expect coefficients of about
$R/\log(10)\approx 1147$ digits, see \kbd{bnf.fu}! (We inserted the flag $1$
so that \kbd{bnfinit} would at least compute this fundamental unit in
factored form, which allows to reconstruct \kbd{bnf.fu}; \kbd{bnfcertify}
will often fail without it.) On the other hand, you can also try
\kbd{quadunit(D,'w)}. Impressive, isn't it? (You can check that its logarithm
is indeed equal to the regulator.)
Let's now assume that we want the regulator $R$ to 500 decimals, say.
(Without cheating and computing the fundamental unit exactly first!)
This can be computed with no effort, simply from the crude approximation
$2641.5$ above. This time, we start with the unit form. Type:
\bprog
u = [qfbprimeform(D, 1), 0.0];
R = 2641.5; /* rough approximation */
@eprog\noindent
This initializes the distance to $0$.
Now type \kbd{f = qfbred(u, 1)}. This is the
first form encountered along the principal cycle. For the moment, keep the
precision low, for example the initial default precision. The distance from
the identity to \kbd{f} is around 4.253, given by the second component.
The distance function is approximately additive.
Since we want a total distance of $R$, this should be encountered
approximately at $R/f[2] \approx 621$ times the distance of \kbd{f}. Hence,
as a first try, we type \kbd{f \pow 621}. Oops, we overshoot: the distance is
now $3173.018\dots > R$. Now we can refine our initial estimate: we should be
close to the correct distance if we raise \kbd{f} to the power $621*R/3173$
which is a little less than $517$. Now if we compute \kbd{f\pow 517} we
hit the principal form right on the dot. This is not a lucky
accident: we must land extremely close to the correct target using this
method, and usually at most one reduction correction step is necessary.
And the distance component can tell us where we are along the cycle.
Up to now, we have only worked to low precision. The goal was to obtain this
unknown integer $517$. Note that this number has no mathematical
significance: indeed the notion of reduction of a form with positive
discriminant is not well defined since there are usually many reduced forms
equivalent to a given form. (In fact, \kbd{f\pow 516} also yields the
principal form, so you can replace $517$ by $516$ if you wish!)
However, when PARI computes with extended forms, the specific order and
reductions that it performs are dictated entirely by the coefficients of the
quadratic form itself, and not by the distance component, hence the precision
used has no effect.
We now start again by setting the precision to (for example) 500 decimal
digits, we set \kbd{u = precision(u, 500)} (why is this necessary?), and then
\kbd{qfbred(u, 1)\pow 517} (or to the power $516$). We already know that we
land on the unit form, and the distance component gives us the regulator to
500 decimal places with no effort at all.
In a similar way, we could obtain the so-called \emph{compact representation}
of the fundamental unit itself, or $p$-adic regulators. I leave this as
exercises for the interested reader.
You can try the \kbd{quadhilbert} function on that field but, since the class
number is $1$, the result won't be that exciting. If you try it on our
preceding example ($3*3299$) it should take about 2 seconds.
\medskip
Time for a coffee break?
\section{Working in General Number Fields}
\subsec{Elements}
The situation here is of course more difficult. First of all, remembering
what we did with elliptic curves, we need to initialize data linked to our
base number field, with something more serious than \kbd{quadgen}. For
example assume that we want to work in the number field $K$ defined by one of
the roots of the equation $x^4+24x^2+585x+1791=0$. This is done by typing
\bprog
T = x^4 + 24*x^2 + 585*x + 1791
nf = nfinit(T)
@eprog\noindent
We get an \kbd{nf} structure but, thanks to member functions, we do not need
to know anything about it. If you type \kbd{nf.pol}, you will get the
polynomial \kbd{T} which you just input. \kbd{nf.sign} yields the signature
$(r_1,r_2)$ of the field, \kbd{nf.disc} the field discriminant, \kbd{nf.zk}
an integral basis, etc\dots.
The integral basis is expressed in terms of a generic root \kbd{x} of \kbd{T}
and we notice it's very far from being a power integral basis, which is a
little strange for such a small field. Hum, let's check that: \kbd{poldisc(T)}?
Ooops, small wonder we had such denominators, the index of the power order
$\Z[x]/(T)$ in the maximal order $\Z_K$ is, well,
\kbd{nf.index}. Note that this is also given by
\bprog
sqrtint(poldisc(nf.pol) / nf.disc)
@eprog\noindent
Anyway, that's $3087$, we don't want to work with such a badly skewed
polynomial! So, we type
\bprog
P = polred(T)
@eprog\noindent
We see from the third component that the
polynomial $x^4-x^3-21x^2+17x+133$ defines the same field with much smaller
coefficients, so type
\bprog
A = P[3]
@eprog\noindent
The \kbd{polred} function usually gives a simpler polynomial, and also
sometimes some information on the existence of subfields. For example in this
case, the second component of \kbd{polred} tells us that the field defined by
$x^2-x+1=0$, i.e.~the field generated by the cube roots of unity, is a subfield
of~$K$. Note this is incidental information and that the list of subfields
found in this way is usually far from complete. To get the complete list, one
uses \kbd{nfsubfields} (we shall do that later on).
Type \kbd{poldisc(A)}, this is much better, but maybe not optimal yet
(the index is still $7$). Type \kbd{polredabs(A)} (the \kbd{abs} stands for
absolute). Since it seems that we won't get anything better, we'll stick with
\kbd{A} (note however that \kbd{polredabs} finds a smallest generating
polynomial with respect to a bizarre norm which ensures that the index will
be small, but not necessarily minimal). In fact, had you typed
\kbd{nfinit(T, 3)}, \kbd{nfinit} would first have tried to find a good
polynomial defining the same field (i.e.~one with small index) before
proceeding.
It's not too late, let's redefine our number field:
\bprog
NF = nfinit(nf, 3)
@eprog\noindent
The output is a two-component vector. The first component is the new
\kbd{nf}: type
\bprog
nf = NF[1];
@eprog\noindent
If you type \kbd{nf.pol}, you notice that \kbd{gp} indeed replaced your bad
polynomial \kbd{T} by a much better one, which happens to be \kbd{A}. (Small
wonder, \kbd{nfinit} internally called \kbd{polredabs}.) The second component
enables you to switch conveniently to our new polynomial.
Namely, call $\theta$ a root of our initial polynomial \kbd{T}, and $\alpha$
a root of the one that \kbd{polred} has found, namely \kbd{A}. These are
algebraic numbers, and as already mentioned are represented as polmods. For
example, in our special case $\theta$ and $\alpha$ are equal to the polmods
\bprog
THETA = Mod(x, x^4 + 24*x^2 + 585*x + 1791)
ALPHA = Mod(x, x^4 - x^3 - 21*x^2 + 17*x + 133)
@eprog\noindent respectively. Here we are considering only the algebraic
aspect, and hence ignore completely \emph{which} root $\theta$ or $\alpha$ is
chosen.
Now you may have a number of elements of your number field which are
expressed as polmods with respect to your old polynomial, i.e.~as polynomials
in $\theta$. Since we are now going to work with $\alpha$ instead, it is
necessary to convert these numbers to a representation using $\alpha$. This
is what the second component of \kbd{NF} is for: type \kbd{C = NF[2]}, you get
\bprog
Mod(-10/7*x^3 + 43/7*x^2 + 73/7*x - 64, x^4 - x^3 - 21*x^2 + 17*x + 133)
@eprog\noindent
meaning that $\theta =
-\dfrac{10}{7}\alpha^3+\dfrac{43}{7}\alpha^2+\dfrac{73}{7}\alpha-64$, and hence the conversion from a
polynomial in $\theta$ to one in $\alpha$ is easy, using \kbd{subst}. (We
could get this polynomial from \kbd{polred} as well, try \kbd{polred(T, 2)}.)
If we want the reverse, i.e.~to go back from a representation in $\alpha$ to
a representation in $\theta$, we use the function \kbd{modreverse} on this
polmod \kbd{C}. Try it. The result has a big denominator (1029) essentially
because our initial polynomial \kbd{T} was so bad. By the way, to get that
1029,
you should type \kbd{denominator(content(C))}. Trying \kbd{denominator} by
itself would not work since the denominator of a polynomial is defined to be
1 (and its numerator is itself). The reason for this is that we think of a
polynomial as a special case of a rational function.\smallskip
From now on, we forget about \kbd{T}, and use only the polynomial
\kbd{A} defining $\alpha$, and the components of the vector \kbd{nf} which
gives information on our number field $K$. Type
\bprog
u = Mod(x^3 - 5*x^2 - 8*x + 56, A) / 7
@eprog\noindent
This is an element in $K$. There are three equivalent
representations for number field elements: polmod, polynomial, and column
vector giving a decomposition in the integral basis \kbd{nf.zk} (\emph{not} on
the power basis $(1,x,x^2,\dots)$). All three are equally valid when the
number field is understood (is given as first argument to the function).
You will be able to use any one of them as long as the function you call
requires an \kbd{nf} argument as well. However, most PARI functions will
return elements as column vectors. It's an important feature of number
theoretic functions that, although they may have a preferred format for
input, they will accept a wealth of other different formats. We already saw
this for \kbd{nfinit} which accepts either a polynomial or an \kbd{nf}. It
will be true for ideals, congruence subgroups, etc.
Let's stick with elements for the time being. How does one go from one
representation to the other? Between polynomials and polmods, it's easy:
\kbd{lift} and \kbd{Mod} will do the job. Next, from polmods/polynomials to
column vectors: type \kbd{v = nfalgtobasis(nf, u)}. So $\kbd{u} = \alpha^3-
\alpha^2 - \alpha + 8$, right? Wrong! The coordinates of \kbd{u} are given
with respect to the \emph{integral basis}, not the power basis
$(1,\alpha,\alpha^2,\alpha^3)$ (and they don't coincide, type \kbd{nf.zk} if
you forgot what the integral basis looked like). As a polynomial in $\alpha$,
we simply have $\kbd{u} = {1\over7}(\alpha^3 - 5\alpha^2-8\alpha+56)$, which
is trivially deduced from the original polmod representation!
Of course \kbd{v = nfalgtobasis(nf, lift(u))} would work equally well. Indeed
we don't need the polmod information since \kbd{nf} already provides the
defining polynomial. To go back to polmod representation, use
\kbd{nfbasistoalg(nf, v)}. Notice that \kbd{u} is an algebraic integer since
\kbd{v} has integer coordinates (try \kbd{denominator(v) == 1}, which is of
course overkill here, but not so in a program).
Let's try this out. We may for instance compute \kbd{u\pow 3}. Try it. Or, we
can type \kbd{1/u}. Better yet, if we want to know the norm from $K$ to $\Q$
of \kbd{u}, we type \kbd{norm(u)} (what else?); \kbd{trace(u)} works as well.
Notice that none of this would work on polynomials or column vectors since
you don't have the opportunity to supply \kbd{nf}! But we could use
\kbd{nfeltpow(nf,u,3)}, \kbd{nfeltdiv(nf,1,u)} (or \kbd{nfeltpow(nf,u,-1)})
which would work whatever representation was chosen. Of course,
there is also an \kbd{nfeltnorm} function (and \kbd{nfelttrace} as well).
You can also consider $(u)$ as a principal ideal, and just type
\bprog
idealnorm(nf,u)
@eprog\noindent
Of course, in this way, we lose the \emph{sign} information. We will talk
about ideals later on.
If we want all the symmetric functions of \kbd{u} and not only the norm, we
type \kbd{charpoly(u)}. Note that this gives the characteristic polynomial of
\kbd{u}, and not in general the minimal polynomial. We have \kbd{minpoly(u)}
for this.
\misctitle{Exercise} Find a simpler expression for \kbd{u}. \smallskip
Now let's work on the field itself. The \kbd{nfinit} command already
gave us some information. The field is totally complex (its signature
\kbd{nf.sign} is $[0,2]$), its discriminant \kbd{nf.disc} is $18981$ and
\kbd{nf.zk} is an integral basis. The Galois group of its Galois closure
can be obtained by typing \kbd{polgalois(A)}. The answer (\kbd{[8,-1,1]}, or
\kbd{[8,-1,1,"D(4)"]} if the \kbd{galdata} package is installed) shows that
it is equal to $D_4$, the dihedral group with 8 elements, i.e.~the group of
symmetries of a square.
This implies that the field is ``partially Galois'', i.e.~that there exists
at least one nontrivial field isomorphism which fixes $K$, exactly one in
this case. Type \kbd{nfgaloisconj(nf)}. The result tells us that, apart from
the trivial automorphism, the map
$$\alpha \mapsto {1\over7}(-\alpha^3+5\alpha^2+\alpha-49)$$
is the only field automorphism.
\bprog
nfgaloisconj(nf);
s = Mod(%[2], A)
charpoly(s)
@eprog\noindent
and we obtain \kbd{A} once again.
Let us check that \kbd{s} is of order 2: \kbd{subst(lift(s), x, s)}. It is. We
may express it as a matrix:
\bprog
w = Vec( matid(4) ) \\@com canonical basis
v = vector(#w, i, nfgaloisapply(nf, s, w[i]))
M = Mat(v)
@eprog\noindent
The vector \kbd{v} contains the images of the integral basis elements (as
column vectors). The last statement concatenates them into a square matrix.
So, \kbd{M} gives the action of \kbd{s} on the integral basis. Let's check
\kbd{M\pow2}. That's the identity all right.
The fixed field of this automorphism is going to be the only nontrivial
subfield of $K$. I seem to recall that \kbd{polred} told us this was the
third cyclotomic field. Let's check this: type \kbd{nfsubfields(nf)}. Indeed,
there's a quadratic subfield, but it's given by \kbd{T = x\pow 2 + 22*x + 133
} and I don't recognize it. But \kbd{nfisisom(T, polcyclo(3))} indeed tells
us that the fields $\Q[x]/(T)$ and $\Q[x]/(x^2+x+1)$ are isomorphic.
(In fact, \kbd{polred(T)} would tell us the same, but does not correspond to
a foolproof test: \kbd{polred} could have returned some other polynomials.)
We may also check that \kbd{k = matker(M-1)} is two-dimensional, then \kbd{z
= nfbasistoalg(nf, k[,2])} generates the quadratic subfield. Notice that 1,
\kbd{z} and \kbd{u} are $\Q$-linearly dependent, and in fact $\Z$-linearly as
well. Exercise: how would you check these two assertions in general? (Answer:
\kbd{concat}, then respectively \kbd{matrank} or \kbd{matkerint} (or
\kbd{qflll})). \kbd{z = charpoly(z)}, \kbd{z = gcd(z,z')} and \kbd{polred(z)}
tell us that we found back the same subfield again (as we ought to!).
Final check: type \kbd{nfrootsof1(nf)}. Again we find that $K$ contains
a cube root of unity, since the torsion subgroup of its unit group
has order 6. The given generator happens to be equal to \kbd{u}.
\misctitle{Additional comment} (you are no longer supposed to skip this,
but do as you wish):
Before working with ideals, let us note one more thing. The main part of the
work of \kbd{polred} or \kbd{nfinit}$(T)$ is to compute an integral basis,
i.e.~a $\Z$-basis of the maximal order $\Z_K$ of $K$. This implies factoring
the discriminant of the polynomial $T$, which is often not feasible. The
situation may be improved in many ways:
1) First, it is often the case that our number field is of quite a special
type. For example, one may know in advance some prime divisors of the
discriminant. Hence we can ``help'' PARI by giving it that information. More
precisely, we can use the function \kbd{addprimes} to inform PARI to keep on
eye for these prime numbers. Do it only for big primes ! (Say, larger than
\kbd{primelimit}.)
2) The second way in which the situation may be improved is that often we do
not need the complete information on the maximal order, but only require that
the order be $p$-maximal for a certain number of primes $p$ --- but then, we
may not be able to use functions which require a genuine \kbd{nf}. The
function \kbd{nfbasis} specifically computes the integral basis and is not
much quicker than \kbd{nfinit} so is not very useful in its standard use. But
we can optionally provide a list of primes: this returns a basis of an order
which is $p$-maximal at the given primes. For example coming back to our
initial polynomial $T$, the discriminant of the polynomial is
$3^7\cdot7^6\cdot19\cdot37$. If we only want a $7$-maximal order, we simply
type
\bprog
nfbasis([T, [7]])
@eprog\noindent Of course, \kbd{nfbasis([T, [2,3,5,7]])} would return an
order which is maximal at $2,3,5,7$. A variant offers a nice generalization:
\bprog
nfbasis([T, 10^5])
@eprog\noindent will return an order which is maximal at all primes less than
$10^5$.
3) Building on the previous points, \emph{if} the field discriminant is
$y$-smooth (never mind the polynomial discriminant), up to a few big primes
known to \kbd{addprimes}, then \kbd{bas = nfbasis(T, y)} returns a basis for
the maximal order! We can then input the resulting basis to \kbd{nfinit}, as
\kbd{nfinit([T, bas])}. Better: the $[T, \var{listP}]$ format can be
directly used with nfinit, where \var{listP} specifies a finite list of
primes in one of the above ways (explicit list or primes up to some bound),
and the result can be unconditionally certified, independently of the
\var{listP} parameter:
\bprog
T = polcompositum(x^7-2, polcyclo(5))[1];
K = nfinit( [T, [2,5,7]] );
nfcertify(K)
@eprog\noindent The output is a list of composite integers whose complete
factorization must be computed in order to certify the result (which may be
very hard, hence is not done on the spot). When the list is empty, as here,
the result is unconditional
\kbd{nfcertify(nf)}
\subsec{Ideals}
We now want to work with ideals and not only
with elements. An ideal can be represented in many different ways. First, an
element of the field (in any of the various guises seen above) will be
considered as a principal ideal. Then the standard representation is a
square matrix giving the Hermite Normal Form (HNF) of a $\Z$-basis of the
ideal expressed on the integral basis \kbd{nf.zk}. Standard means that most
ideal related functions will use this representation for their output.
Prime ideals can be represented in a special form as well (see
\kbd{idealprimedec}) and all ideal-related functions will accept them. On the
other hand, the function \kbd{idealtwoelt} can be used to find a two-element
$\Z_K$-basis of a given ideal (as $a\Z_K + b\Z_K$, where $a$ and $b$ belong
to $K$), but this is \emph{not} a valid representation for an ideal under
\kbd{gp}, and most functions will choke on it (or worse, take it for
something else and output a meaningless result). To be able to use such an
ideal, you will first have to convert it to HNF form.
Whereas it's very easy to go to HNF form (use \kbd{idealhnf(nf,id)} for valid
ideals, or \kbd{idealhnf(nf,a,b)} for a two-element representation as above),
it's a much more complicated problem to check whether an ideal is principal
and find a generator. In fact an \kbd{nf} does not contain enough data for
this particular task. We'll need a Buchmann Number Field, or \kbd{bnf}, for
that. In particular, we need the class group and fundamental units, at least
in some approximate form. More on this later (which will trivialize the end
of the present section).\smallskip
Let us keep our number field $K$ as above and its \kbd{nf} structure. Type
\bprog
P = idealprimedec(nf,7)
@eprog\noindent
This gives the decomposition of the prime number 7 into prime ideals. We have
chosen 7 because it divides \kbd{nf.index} (in fact, is equal to it), hence
is the most difficult case to treat.
The result is a vector with 4 components, showing that 7 is totally split in
the field $K$ into prime ideals of norm 7 (you can check:
\kbd{idealnorm(nf,P[1])}). Let us take one of these ideals, say the first, so
type
\bprog
pr = P[1]
@eprog
We obtain its inertia and residue degree as \kbd{pr.e}
and \kbd{pr.f}, and its two generators as \kbd{pr.gen}. One of them is
$\kbd{pr.p} = 7$, and the other is guaranteed to have valuation $1$ at
\kbd{pr}. What is the Hermite Normal Form of this ideal? No problem:
\bprog
idealhnf(nf,pr)
@eprog\noindent and we have the desired HNF. Let's now perform ideal
operations. For example type
\bprog
idealmul(nf, pr, idealmul(nf, pr,pr))
@eprog\noindent or more simply
\bprog
pr3 = idealpow(nf, pr,3)
@eprog\noindent
to get the cube of the ideal \kbd{pr}. Since the norm of this ideal is equal
to $343=7^3$, to check that it is really the cube of \kbd{pr} and not of
other ideals above 7, we can type
\bprog
for(i=1, #P, print( idealval(nf, pr3, P[i]) ))
@eprog\noindent
and we see that the valuation at \kbd{pr} is equal to 3, while the others are
equal to zero. We could see this as well from \kbd{idealfactor(nf, pr3)}.
Let us now work in the class group ``by hand'' (we shall see simpler ways
later). We shall work with \emph{extended ideals}: an extended ideal is a pair
\kbd{[A, t]}, where $A$ is an ordinary ideal as above, and $t$ a number field
element; this pair represents the ideal $(t) A$.
\bprog
id3 = [pr3, 1]
r0 = idealred(nf, id3)
@eprog\noindent
The input \kbd{id3} is an extended ideal: pr3 together with 1 (trivial
factorization). The new extended ideal \kbd{r0} is equal to the old one,
in the sense that the products $(t)A$ are the same. It contains a ``reduced''
ideal equivalent to \kbd{pr3} (modulo the principal ideals), and a generator
of the principal ideal that was factored out.
Now, just for fun type
\bprog
r = r0; for(i=1,3, r = idealred(nf,r, [1,5]); print(r))
@eprog\noindent
The ideals in the third \kbd{r} and initial \kbd{r0} are equal, say
$(t) A = (t_0) A$: this means we
have found a unit $(t_0/t)$ in our field, and it is easy to extract this unit
given the extended component:
\bprog
t0 = r0[2]; t = r[2];
u = nfeltdiv(nf, t0, t)
u = nfbasistoalg(nf, u)
@eprog\noindent The last line recovers the unit as an algebraic number.
Type
\bprog
ch = charpoly(u)
@eprog\noindent
and we obtain the characteristic polynomial \kbd{ch} of $u$ again. (Whose
constant coefficient is $1$, hence $u$ is indeed a unit.)
There is of course no reason for $u$ to be a fundamental unit. Let us see if
it is a square. Type
\bprog
F = factor(subst(ch, x, x^2))
@eprog\noindent
We see that \kbd{ch(x\pow2)} is a product of 2 polynomials of degree 4, hence
$u$ is a square. (Why?) We now want to find its square root. A simple method
is as follows:
\bprog
NF = subst(nf,x,y);
r = F[1,1] % (x^2 - nfbasistoalg(NF, u))
@eprog\noindent
to find the remainder of the characteristic polynomial of \kbd{u2} divided by
\kbd{x\pow 2 - $u$}. This is a polynomial of degree 1 in \kbd{x}, with polmod
coefficients, and we know that \kbd{u2}, being a root of both polynomials,
is the root of \kbd{r}, hence can be obtained by typing
\bprog
u2 = -polcoef(r,0) / polcoef(r,1)
@eprog\noindent There is an important technicality in the above: why did we
need to substitute \kbd{NF} to \kbd{nf}? The reason is that data related to
\kbd{nf} is given in terms of the variable \kbd{x}, seen modulo \kbd{nf.pol};
but we need \kbd{x} as a free variable for our polynomial divisions. Hence
the substitution of \kbd{x} by \kbd{y} in our \kbd{nf} data.
The most natural method is to try directly
\bprog
nffactor(nf, y^2 - u)
@eprog\noindent
Except that this won't work for the same technical reason as above: the main
variable of the polynomial to be factored must have \emph{higher} priority
than the number field variable. This won't be possible here since \kbd{nf}
was defined using the variable \kbd{x} which has the highest possible
priority. So we need to substitute variables around:
\bprog
nffactor(NF, x^2 - nfbasistoalg(NF, subst(lift(u),x,y)))
@eprog\noindent
(Of course, with better planning, we would just have defined \kbd{nf} in
terms of the \kbd{'y} variable, to avoid all these substitutions.)
\smallskip
A much simpler approach is to consider the above as instances of a
\emph{discrete logarithm} problem, where we want to express some elements an
abelian group (of finite type) in terms of explicitly given generators, and
transfer all computations from abstract groups like $\text{Cl}(K)$ and
$\Z_K^*$ to products of simpler groups like $\Z^n$ or $\Z/d\Z$. We shall do
exactly that in the next section.
Before that, let us mention another famous (but in fact, simpler)
\emph{discrete logarithm} problem, namely the one attached to the
invertible elements modulo an ideal: $(\Z_K / I)^*$. Just use \kbd{idealstar}
(this is an \kbd{init} function) and \kbd{ideallog}.
Many more functions on ideals are available. We mention here the complete
list, referring to Chapter 3 for detailed explanations:
\kbd{idealadd}, \kbd{idealaddtoone}, \kbd{idealappr}, \kbd{idealchinese},
\kbd{idealcoprime}, \kbd{idealdiv}, \kbd{idealfactor}, \kbd{idealhnf},
\kbd{idealintersect}, \kbd{idealinv}, \kbd{ideallist}, \kbd{ideallog},
\kbd{idealmin}, \kbd{idealmul}, \kbd{idealnorm}, \kbd{idealpow},
\kbd{idealprimedec}, \kbd{idealred},
\kbd{idealstar}, \kbd{idealtwoelt}, \kbd{idealval},
\kbd{nfisideal}.
We suggest you play with these to get a feel for the algebraic number theory
package. Remember that when a matrix (usually in HNF) is output, it is always
a $\Z$-basis of the result expressed on the \emph{integral basis} \kbd{nf.zk}
of the number field, which is usually \emph{not} a power basis.
\subsec{Class Groups and Units, \kbd{bnf}}
Apart from the above functions you have at your disposal the powerful
function \kbd{bnfinit}, which initializes a \kbd{bnf} structure, i.e.~a
number field with all its invariants (including class group and units), and
enough technical data to solve discrete logarithm problems in the class and
unit groups.
First type \kbd{setrand(1)}: this resets the random seed (to make sure we
and you get the exact same results). Now type
\bprog
bnf = bnfinit(NF);
@eprog\noindent
where \kbd{NF} is the same number field as before. You do not want to see the
output clutter a number of screens so don't forget the semi-colon. (Well if
you insist, it is about three screenful in this case, but may require several
Megabytes for larger degrees.) Note that \kbd{NF} is now expressed in terms
of the variable \kbd{y}, to avoid later problems with variable priorities.
A word of warning: both the \kbd{bnf} and all results obtained from it are
\emph{conditional} on a Riemann Hypothesis at this point; the \kbd{bnf} must
be certified before the following statements become actual theorems.
\smallskip
Member functions are still available for \kbd{bnf} structures. So, let's try
them: \kbd{bnf.pol} gives \kbd{A}, \kbd{bnf.sign}, \kbd{bnf.disc},
\kbd{bnf.zk}, ok nothing really exciting. In fact, an \kbd{nf} is included
in the \kbd{bnf} structure: \kbd{bnf.nf} should be identical to
\kbd{NF}. Thus, all functions which took an \kbd{nf} as first argument, will
equally accept a \kbd{bnf} (and a \kbd{bnr} as well which contains even more
data).
Anyway, new members are available now: \kbd{bnf.no} tells us the class number
is 4, \kbd{bnf.cyc} that it is cyclic (of order 4 but that we already knew),
\kbd{bnf.gen} that it is generated by the ideal \kbd{g = bnf.gen[1]}. If you
\kbd{idealfactor(bnf, g)}, you recognize \kbd{P[2]}. (You may also play in
the other direction with \kbd{idealhnf}.) The regulator \kbd{bnf.reg} is
equal to $3.794\dots$. \kbd{bnf.tu} tells us that the roots of unity in $K$
are exactly the sixth roots of 1 and gives a primitive root $\zeta =
{1\over7}(\alpha^3 - 5\alpha^2 - 8\alpha + 56)$, which we have seen already.
Finally \kbd{bnf.fu} gives us a fundamental unit $\epsilon =
{1\over7}(\alpha^3 - 5\alpha^2 - \alpha + 28)$, which must be linked to the
units \kbd{u} and \kbd{u2} found above since the unit rank is~1. To find
these relations, type
\bprog
bnfisunit(bnf, u)
bnfisunit(bnf, u2)
@eprog\noindent
Lo and behold, \kbd{u = $\zeta^2\epsilon^2$} and \kbd{u2 =
$\zeta^{4}\epsilon^1$}.
\misctitle{Note} Since the fundamental unit obtained depends on the random
seed, you could have obtained another unit than $\epsilon$, had you not reset
the random seed before the computation. This was the purpose of the initial
\kbd{setrand} instruction, which was otherwise unnecessary.\medskip
We are now ready to perform operations in the class group. First and
foremost, let us certify the result: type \kbd{bnfcertify(bnf)}. The
output is \kbd{1} if all went well; in fact no other output is possible,
whether the input is correct or not, but you can get an error message (or in
exceedingly rare cases an infinite loop) if it is incorrect.
It means that we now know the class group and fundamental units
unconditionally (no more GRH then!). In this case, the certification process
takes a very short time, and you might wonder why it is not built in as a
final check in the \kbd{bnfinit} function. The answer is that as the
regulator gets bigger this process gets increasingly difficult, and becomes
soon impractical, while \kbd{bnfinit} still happily spits out results. So it
makes sense to dissociate the two: you can always check afterwards, if the
result is interesting enough. Looking at the tentative regulator, you know in
advance whether the certification can possibly succeed: if \kbd{bnf.reg} is
large, don't waste your time.
Now that we feel safe about the \kbd{bnf} output, let's do some real work.
For example, let us take again our prime ideal \kbd{pr} above 7. Since we
know that the class group is of order 4, we deduce that \kbd{pr} raised to
the fourth power must be principal. Type
\bprog
pr4 = idealpow(nf, pr, 4)
v = bnfisprincipal(bnf, pr4)
@eprog\noindent
The first component gives the factorization of the ideal in the class group.
Here, \kbd{[0]} means that it is up to equivalence equal to the 0-th power of
the generator \kbd{g} given in \kbd{bnf.gen}, in other words that it is a
principal ideal. The second component gives us the algebraic number $\alpha$
such that $\kbd{pr4}=\alpha\Z_K$, $\alpha$ being as usual expressed on the
integral basis. Type \kbd{alpha = v[2]}. Let us check that the result is
correct: first, type \kbd{idealnorm(bnf, alpha)}. (Note that we can use a
\kbd{bnf} with all the \kbd{nf} functions; but not the other way round, of
course.) It is indeed equal to $7^4 = 2401$, which is the norm of \kbd{pr4}.
This is only a first check. The complete check is obtained by computing the
HNF of the principal ideal generated by \kbd{alpha}. To do this, type
\kbd{idealhnf(bnf, alpha) == pr4}.
Since the equality is true, \kbd{alpha} is correct (not that there was any
doubt!). But \kbd{bnfisprincipal} also gives us information for nonprincipal
ideals. For example, type
\bprog
v = bnfisprincipal(bnf, pr)
@eprog\noindent
The component \kbd{v[1]} is now equal to \kbd{[3]}, and tells us that \kbd{pr}
is ideal-equivalent to the cube of the generator \kbd{g}. Of course we
already knew this since the product of \kbd{P[3]} and \kbd{P[4]} was
principal (generated by \kbd{al}), as well as the product of all the
\kbd{P[$i$]} (generated by 7), and we noticed that \kbd{P[2]} was equal
to \kbd{g}, which has order 4. The second component \kbd{v[2]} gives us
$\alpha$ on the integral basis such that $\kbd{pr}=\alpha \kbd{g}^3$. Note
that if you \emph{don't} want this $\alpha$, which may be large and whose
computation may take some time, you can just add the flag $1$ (see the online
help) to the arguments of \kbd{bnfisprincipal}, so that it only returns the
position of \kbd{pr} in the class group. \smallskip
\subsec{Class Field Theory, \kbd{bnr}}
We now survey quickly some class field theoretic routines. We must first
initialize a Buchmann Number Ray, or \kbd{bnr}, structure, attached to a
\kbd{bnf} base field and a modulus. Let's keep $K$, and try a finite modulus
${\goth f} = 7\Z_K$. (See the manual for how to include infinite places in
the modulus.) Since $K$ will now become a base field over which we want to
build relative extensions, the attached \kbd{bnf} needs to have variables
of lower priority than the polynomials defining the extensions. Fortunately,
we already took care that, but it would have been easy to deal with the
problem now (as easy as \kbd{bnf = subst(bnf, x, y)}). Then type
\bprog
bnr = bnrinit(bnf, 7, 1);
bnr.cyc
@eprog\noindent
tells us the ray class group modulo ${\goth f}$ is isomorphic to
$\Z/24\Z \times \Z/6\Z \times \Z/2\Z $. The attached generators are
\kbd{bnr.gen}. Just as a \kbd{bnf} contained an \kbd{nf}, a \kbd{bnr}
contains a \kbd{bnf} (hence an \kbd{nf}), namely \kbd{bnr.bnf}. Here
\kbd{bnr.clgp} refers to the ray class group, while \kbd{bnr.bnf.clgp} refers
to the class group.
\bprog
rnfkummer(bnr,, 2)
rnfkummer(bnr,, 3)
@eprog\noindent
outputs defining polynomials for the $2$ abelian extensions of $K$ of degree
$2$ (resp.~$3$), whose conductor is exactly equal to ${\goth f}$ (the modulus
used to define \kbd{bnr}). (In the current implementation of \kbd{rnfkummer},
these degrees must be \emph{prime}.) What about other extensions of degree
$2$ for instance?
\bprog
L0= subgrouplist(bnr, [2])
L = subgrouplist(bnr, [2], 1)
@eprog\noindent
\kbd{L0}, resp.~\kbd{L} is the list of those subgroups of the full ray class
group mod $7$, whose index is $2$, and whose conductor is $7$,
resp.~arbitrary. (Subgroups are given by a matrix of generators, in terms of
\kbd{bnr.gen}.) \kbd{L0} has $2$ elements, attached to the $2$ extensions
we already know. \kbd{L} has $7$ elements, the $2$ from \kbd{L0}, and
$5$ new ones:
\bprog
L1 = setminus(Set(L), Set(L0))
@eprog\noindent
The conductors are
\bprog
vector(#L1, i, bnrconductor(bnr, L1[i]))
@eprog\noindent
among which one sees the identity matrix, i.e. the trivial ideal. (It is
\kbd{L1[3]} in my session, maybe not in yours. Take the right one!) Indeed,
the class group was cyclic of order $4$ and there exists a unique unramified
quadratic extension. We could find it directly by recomputing a \kbd{bnr}
with trivial conductor, but we can also use
\bprog
rnfkummer(bnr, L1[3]) \\ @com pick the subgroup with trivial conductor!
@eprog\noindent
directly which outputs the (unique by Takagi's theorem) class field
attached to the subgroup \kbd{L1[3]}. In fact, it is of the form
$K(\sqrt{-\epsilon})$. We can check this directly:
\bprog
rnfconductor(bnf, x^2 + bnf.fu[1])
@eprog\noindent
\subsec{Galois Theory over $\Q$}
PARI includes a nearly complete set of routines to compute with Galois
extensions of $\Q$. We start with a very simple example.
Let $\zeta$ a $8$th-root of unity and $K=\Q(\zeta)$. The minimal
polynomial of $\zeta$ is the 8$th$ cyclotomic polynomial, namely
\kbd{polcyclo(8)} (=$x^4+1$).
We issue the command
\bprog
G = galoisinit(x^4 + 1);
@eprog\noindent
to compute $G=\text{Gal}(K/\Q)$. The command \kbd{galoisisabelian(G)}
returns \kbd{[2,0;0,2]} so $G$ is an abelian group, isomorphic to $(\Z/2\Z)^2$, generated by
$\sigma$=\kbd{G.gen[1]} and $\tau$=\kbd{G.gen[2]}. These automorphisms are
given by their actions on the roots of $x^4+1$ in a suitable $p$-adic
extension. To get the explicit action on $\zeta$, we use
\kbd{galoispermtopol(G,G.gen[i])} for $i=1,2$ and get $\sigma(\zeta)=-\zeta$
and $\tau(\zeta)=\zeta^3$. The last nontrivial automorphism is
$\sigma\tau$=\kbd{G.gen[1]*G.gen[2]} and we have
$\sigma\tau(\zeta)=-\zeta^3$. (At least in my version, yours may return a
different set of generators, rename accordingly.)
We compute the fixed field of $K$ by the subgroup generated by $\tau$ with
\bprog
galoisfixedfield(G, G.gen[2], 1)
@eprog\noindent
and get $x^2 + 2$. Now we want the factorization of $x^4+1$ over that
subfield. Of course, we could use \kbd{nffactor}, but here we have a much
simpler option: \kbd{galoisfixedfield(G, G.gen[1], 2)} outputs
\bprog
[x^2 + 2, Mod(x^3 + x, x^4 + 1), [x^2 - y*x - 1, x^2 + y*x - 1]]
@eprog\noindent
which means that
$x^4+1=(x^2-\alpha\*x-1)(x^2+\alpha\*x-1)$ where $\alpha$ is a root of $x^2+2$,
and more precisely, $\alpha=\zeta^3+\zeta$. So we recover the well-known
factorization:
$$x^4+1=(x^2-\sqrt{-2}\*x-1)(x^2+\sqrt{-2}\*x-1)$$
For our second example, let us take the field $K$ defined by the polynomial
\bprog
P = x^18 - 3*x^15 + 115*x^12 + 104*x^9 + 511*x^6 + 196*x^3 + 343;
G = galoisinit(P);
@eprog\noindent Since \kbd{galoisinit} succeeds,
the extension $K/\Q$ is Galois. This time \kbd{galoisisabelian(G)} returns
$0$, so the extension is not abelian; however we can still put a name on the
underlying abstract group. Use \kbd{galoisidentify(G)}, which returns $[18,
3]$. By looking at the GAP4 classification we find that $[18, 3]$ is
$S_3\times\Z/3\Z$. This time, the subgroups of $G$ are not obvious,
fortunately we can ask PARI : \kbd{galoissubgroups(G)}.
Let us look for a polynomial $Q$ with the property that $K$ is the splitting
field of $Q(x^2)$. For that purpose, let us take $\sigma$=\kbd{G.gen[3]}. We
check that \kbd{G.gen[3]\^{}2} is the identity, so $\sigma$ is of order $2$. We now compute the fixed field $K^\sigma$ and the relative factorization of $P$ over
$K^\sigma$:
\bprog
F = galoisfixedfield(G, G.gen[3], 2);
@eprog\noindent
So $K$ is a quadratic extension of $K^\sigma$ defined by the polynomial
\kbd{R=F[3][1]}. It is well-known that $K$ is also defined by $x^2-D$,
where $D$ is the discriminant of $R$ (over $K^\sigma$).
To compute $D$, we issue:
\bprog
D = poldisc(F[3][1]) * Mod(1,subst(F[1],x,y));
@eprog\noindent
Note that since \kbd{y} in \kbd{F[3][1]} denotes a root of \kbd{F[1]}, we
have to use \kbd{subst(,x,y)}. Now we hope that $D$ generates $K^\sigma$ and
compute \kbd{Q=charpoly(D)}. We check that $Q=x^9+270\*x^6+12393\*x^3+19683$ is
irreducible with \kbd{polisirreducible(Q)}. (Were it not the case, we would
multiply $D$ by a random square.) So $D$ is a generator of $K^\sigma$ and
$\sqrt{D}$ is a generator of $K$. The result is that $K$ is the splitting
field of $Q(x^2)$. We can check that with
\kbd{nfisisom(P,subst(Q,x,x\^{}2))}.
\subsec{Creating and Listing Number Fields}
Suppose you want to work with cyclic quartic fields (i.e., fields with
Galois group $C_4=\Z/4\Z$ over $\Q$): you could look in a database or in
a book for examples, but PARI provides the simple but powerful \kbd{nflist}
command: in this case you could simply write \kbd{V = nflist("C4")},
which would immediately return a list of polynomials (in the present case
$18$, in some random order) defining cyclic quartic fields. Note that these
polynomials are usually not the simplest, you can for instance do
\kbd{V = apply(polredabs, V)} to reduce them.
Using \kbd{vecmax(apply(z->abs(nfdisc(z)),V))}, you can check that the largest
discriminant is $35152$. Thus, if you only want discriminants up to $10000$
you would write \kbd{V = nflist("C4", [1, 10000])}, which would now output only
$10$ number fields. Or if you want only fields with absolute discriminant
exactly equal to $8000$, you would write \kbd{V = nflist("C4", 8000)}, which
would output the two polynomials $x^4\pm10x^2+20$, which define non-isomorphic
number fields (using \kbd{polsturm}, you can check that one is totally real,
the other totally complex). Finally, if you only want $C_4$-fields with a
given number \kbd{s} of complex places, you would write \kbd{nflist("C4",,s)}
(don't forget the two consecutive commas, since the second argument is the
discriminant range), for instance:
\bprog
for(s = 0, 2, print1([s, #nflist("C4", , s)]," "))
@eprog\noindent
would output \kbd{[0, 10] [1, 0], [2, 10]}, so \kbd{nflist} gives you
$10$ totally real and $10$ totally complex cyclic quartic fields, but none
with mixed signature $(2,1)$, which is a good thing since they do not exist!
Of course you can ask for many more: for instance in a few seconds
\bprog apply(length, #nflist("C4", [1, 10^14], -2))@eprog
\noindent will return
\kbd{[607589, 0, 607609]}, which tells you that there are $607589$ totally
real and $607609$ totally complex cyclic quartic fields with absolute
discriminant up to $10^{14}$: this illustrates the special use of the
parameter $s=-2$, which separates the number fields by increasing number of
complex places (the default is $s=-1$ which ignores the signature altogether).
\smallskip
Of course, \kbd{nflist} applies to many other Galois groups than $C_4$
(where ``Galois group'' is understood as Galois group of the Galois closure,
see below), but before leaving cyclic quartic fields, let us see two more
related instructions. Starting again with \kbd{V = nflist("C4")}, write
\kbd{R = apply(nfresolvent, V)}: this outputs a list of $18$ quadratic
polynomials, which define the unique quadratic subfield of each quartic
field. In the present example, the \kbd{nfsubfields} command
(more precisely \kbd{R = apply(z -> polredabs(nfsubfields(z, 2, 1)[1]), V)})
would have given an identical result, but the purpose of \kbd{nfresolvent}
is different. First, for many Galois groups, it really outputs what one usually
calls the resolvent polynomial, for instance a quadratic polynomial for
an $S_3$ field, a cubic for a non-Galois quartic, etc... Second, as here,
the fields are often constructed as extensions of subfields, and
\kbd{nfresolvent} gives the subfield on which the field has been constructed.
Better, typing \kbd{nfresolvent(V[1], 1)} outputs
\kbd{[x\^{}2 - x - 1, [[5, 2; 0, 1], [1, 1]]]}, which not only gives the
subfield, but the relative conductor as a \kbd{module} (a pair ideal,
list of real places).
The second instruction is \kbd{polsubcyclo}, which has existed for a long time
in PARI. The manual states that \kbd{polsubcyclo(n,d)} gives the subfields
of degree $d$ of $\Q(\zeta_n)$. However, if $n$ is very large, previous
versions would take an inordinate amount of time. The present version
allows us to take $n$ very large, but the price to pay is that the degree
$d$ of the desired subfields must be very small, for instance $d\le30$,
and in addition must be a prime number if $d\ge7$. For instance:
\bprog
? p = 10^16 + 649759; /* A prime chosen so that p = 1 mod 11*19*29*41
? polsubcyclo(p, 11);
time = 42 ms. /* Would have been impossible in previous versions */
? polsubcyclo(p, 19);
time = 348 ms.
? polsubcyclo(p, 29);
time = 10,304 ms. /* Still reasonable */
? polsubcyclo(p, 41);
time = 6min, 43,181 ms. /* Slow but feasible. */
@eprog
\medskip
The most common Galois groups corresponding to number fields of small
degree have been implemented: all solvable groups up to degree $7$
(there are $13$ in degree $6$ for instance), the groups $C_{\ell}$ and
$D_{\ell}$ for $\ell$ prime. In addition, the simple group $A_5$ has also
been included, but by cheating since it is obtained by table lookup
(and available only of you have downloaded and installed the
\kbd{nflistdata} package), in particular because of its relation with
modular forms of weight $1$.
\section{Working with Associative Algebras}
Beyond the realm of number fields, we can perform operations with more
general associative algebras, that need not even be commutative! Of course
things become more complicated. We have two different structures: the first
one allows us to manipulate any associative algebra that is
finite-dimensional over a prime field ($\Q$ or $\F_p$ for some prime~$p$),
and the second one is dedicated to central simple algebras over number
fields, which are some nice algebras that behave a lot like number fields.
Like in other parts of~\kbd{gp}, every function that has to do with
associative algebras begins with the same prefix:~\kbd{alg}.
\subsec{Arbitrary Associative Algebras}
In order to create an associative algebra, you need to tell~\kbd{gp} how to
multiply elements. We do this by providing a \emph{multiplication table} for the
algebra, in the form of the matrix of left multiplication by each basis element,
and use the function~\kbd{algtableinit}.
For instance, let us work in~$\F_3[x]/(x^2)$. Of course, we
could use polmods of intmods to represent elements in this algebra, but let's
introduce the general mechanism with this simple example!
This algebra has a basis with two elements:~$1$
and~$\epsilon$, the image of~$x$ in the quotient. By the way,~\kbd{gp} will
only accept your multiplication table if the first basis vector is~$1$. The
multiplication matrix of~$1$ is the~$2\times 2$ identity matrix, and
since~$\epsilon\cdot 1 = \epsilon$ and~$\epsilon\cdot\epsilon = 0$, the left
multiplication table of~$\epsilon$ is~$\pmatrix{0 & 0 \cr 1 & 0}$. So we
use the following multiplication table:
\bprog
mt1 = [matid(2), [0,0;1,0]]
@eprog\noindent
Since
we want our algebra to be over~$\F_3$, we have to specify the characteristic
and create the algebra with
\bprog
al1 = algtableinit(mt1, 3);
@eprog\noindent
Let's create another one: the algebra of upper-triangular~$2\times 2$
matrices over~$\Q$. This algebra has dimension~$3$, with basis~$1$, $a =
\pmatrix{0 & 1 \cr 0 & 0}$ and~$b = \pmatrix{0 & 0 \cr 0 & 1}$, and these
elements satisfy~$a^2=0$, $ab = a$, $ba=0$ and~$b^2=b$. Watch out, even
though~$a$ and~$b$ are~$2\times 2$ matrices, their left multiplication tables
are~$3\times 3$ matrices! The left multiplication tables of~$a$
and~$b$ are respectively
\bprog
ma = [0,0,0; 1,0,1; 0,0,0];
mb = [0,0,0; 0,0,0; 1,0,1];
@eprog\noindent
The multiplication table of our second algebra is therefore
\bprog
mt2 = [matid(3), ma, mb];
@eprog\noindent
and we can create the algebra with
\bprog
al2 = algtableinit(mt2,0);
@eprog\noindent
In fact, we can omit the second argument and type
\bprog
al2 = algtableinit(mt2);
@eprog\noindent
and the
characteristic of the algebra will implicitly be~$0$. Warning: in
characteristic~$0$, \kbd{algtableinit} expects an integral multiplication table.
In fact, \kbd{gp} does not check that the multiplication table you provided
really defines an associative algebra. You can check it a posteriori
with
\bprog
algisassociative(al2)
@eprog\noindent
or before creating the algebra
with
\bprog
algisassociative(mt1,3)
@eprog\noindent
After creating the algebra, you can get back the
multiplication table that you provided with~\kbd{algmultable(al2)}, the
characteristic with~\kbd{algchar(al1)} and the dimension with~\kbd{algdim(al2)}.
\subsubsec{Elements}
In an associative algebra, we represent elements as column vectors expressing them
on the basis of the algebra. For instance, in~\kbd{al1} $=\F_3[\epsilon]$, the
element~$1-\epsilon$ is represented as~\kbd{[1,-1]\til}. Similarly, in~\kbd{al2}
we can define~$a$, $b$ and~$c = \pmatrix{2 & 1\cr 0 & 1}$ by
\bprog
a = [0,1,0]~
b = [0,0,1]~
c = [2,1,-1]~
@eprog\noindent
We can also draw random elements in a box using
\bprog
algrandom(al2, 10)
@eprog\noindent
You can compute any elementary operation: try various combinations
of~\kbd{algadd}, \kbd{algsub}, \kbd{algneg}, \kbd{algmul}, \kbd{alginv},
\kbd{algsqr}, \kbd{algpow}, using the syntax
\bprog
algmul(al2, b, a)
algpow(al2, c, 10)
@eprog\noindent
and the natural variants. In every algebra we have the left regular
representation, which sends every element~$x$ to the matrix of left
multiplication by~$x$. In~\kbd{gp} we access it by calling
\bprog
algtomatrix(al2, c)
@eprog\noindent
For every element~$x$ in an associative algebra, the trace of that matrix is
called the \emph{trace} of~$x$, the determinant is called the \emph{norm}
of~$x$, and the characteristic polynomial is called the \emph{characteristic
polynomial} of~$x$. We can compute them:
\bprog
algtrace(al2, a)
algnorm(al2, c)
algcharpoly(al2, b)
@eprog
\subsubsec{Properties}
Now let's try to compute some interesting properties of our algebras. Maybe the
simplest one we want to test is whether an algebra is commutative;
\kbd{algiscommutative} does that for us: you can use it to check that~\kbd{al1}
is of course commutative, but~\kbd{al2} is not since for instance~$ab=a\neq 0 =
ba$. More precisely, we can compute a basis of the center of an algebra
with~\kbd{algcenter}. Since~\kbd{al1} is commutative, we obtain the identity
matrix, and
\bprog
algcenter(al2)
@eprog\noindent
tells us that the center of~\kbd{al2} is
one-dimensional and generated by the identity.
An important object in the structure of associative algebras is the
\emph{Jacobson radical}: the set of elements that annihilate every simple left
module. It is a nilpotent two-sided ideal. An algebra is \emph{semisimple} if
its Jacobson radical is zero, and for every algebra~$A$ with radical~$J$, the
quotient~$A/J$ is semisimple. You can compute a basis for the Jacobson radical
using~\kbd{algradical}. For instance, the radical of~\kbd{al1} is generated by
the element~$\epsilon$. Indeed, in a commutative algebra the Jacobson
radical is equal to the set of nilpotent elements. The radical of~\kbd{al2} has
basis~$a$: it is the subspace of strictly upper-triangular~$2\times 2$ matrices.
You can also directly test whether an algebra is semisimple
using~\kbd{algissemisimple}. Let's compute the semisimplification of our
second algebra by quotienting out its radical:
\bprog
al3 = algquotient(al2, algradical(al2));
@eprog\noindent
Check that~\kbd{al3} is indeed semisimple now that you know how to do it!
Group algebras provide interesting examples: when~$G$ is a finite group and~$F$
is a field, the group algebra~$F[G]$ is semisimple if and only if the
characteristic of~$F$ does not divide the order of~$G$. Let's try it!
\bprog
K = nfsplitting(x^3-x+1); \\ Galois closure -> S_3
G = galoisinit(K)
al4 = alggroup(G, 5) \\ F_5[S_3]
algissemisimple(al4)
@eprog\noindent
Check what happens when you change the characteristic of the algebra.
The building blocks of semisimple algebras are \emph{simple} algebras:
algebras with no nontrivial two-sided ideals. Since the Jacobson radical is a
two-sided ideal, every simple algebra is semisimple. You can check whether an
algebra is simple using~\kbd{algissimple}. For instance, you can check that
\bprog
algissimple(al1)
@eprog\noindent
returns~\kbd{0}, but this is not very interesting since~\kbd{al1} is not even
semisimple. Instead we can test whether~\kbd{al3} is simple, and since we
already know that it is semisimple we can prevent~\kbd{gp} from checking it
again by using the optional second argument:
\bprog
algissimple(al3, 1)
@eprog\noindent
We see that~\kbd{al3} is not simple, and this implies that we can decompose it
further. Indeed, every semisimple algebra is isomorphic to a product of simple algebras.
We can obtain this decomposition with
\bprog
dec3 = algsimpledec(al3)[2];
apply(algdim, dec3)
@eprog\noindent
We see that~\kbd{al3} is isomorphic to~$\Q\times\Q$. Similarly, we can
decompose~\kbd{al4}.
\bprog
dec4 = algsimpledec(al4)[2];
apply(algdim, dec4)
@eprog\noindent
We see that~\kbd{al4} is isomorphic to~$\F_5\times\F_5\times A$ where~$A$ is a
mysterious 4-dimensional simple algebra. However every simple algebra over a
finite field is isomorphic to a matrix algebra over a possibly larger finite
field. By computing the center
\bprog
algcenter(dec4[3])
@eprog\noindent
we see that this algebra has center~$\F_5$, so that~\kbd{al4} is isomorphic
to~$\F_5\times\F_5\times M_2(\F_5)$.
\subsec{Central Simple Algebras over Number Fields}
As we saw, simple algebras are building blocks for more complicated algebras.
The center of a simple algebra is always a field, and we say that an
algebra over a field~$F$ is \emph{central} if its center is~$F$. The most
natural noncommutative generalization of a number field is a \emph{division
algebra} over~$\Q$: an algebra in which every nonzero element is invertible.
Since the center of a division algebra over~$\Q$ is a number field, we do not
lose any generality by considering central division algebras over number
fields. However, the tensor product of two central division algebras is not
always a division algebra, but division algebras are always simple and central
simple algebras are closed under tensor products, giving a much nicer global
picture. This is why we choose central simple algebras over number fields as our
noncommutative generalization of number fields.
\subsubsec{Creation}
Let's create our first central simple algebras! A well-known construction is
that of \emph{quaternion algebras}, which proceeds as follows.
Let~$F$ be a number field, and let~$a,b\in F^\times$; the quaternion
algebra~$(a,b)_F$ is the $F$-algebra generated by two elements~$i$ and~$j$
satisfying~$i^2=a$, $j^2=b$ and~$ij=-ji$. Hamilton's quaternions
correspond to the choice~$a=b=-1$, but it is not the only possible
one! Here is our first quaternion algebra:
\bprog
Q = nfinit(y);
al1 = alginit(Q,[-2,-1]); \\ (-2,-1)_Q
@eprog\noindent
Note that we represented the rationals~$\Q$ with an~\kbd{nf} structure and used
the variable~$y$ in that structure. The reason for this variable choice will
be clearer after looking at the next construction. You can see from the
definition of a quaternion algebra that it has dimension~$4$ over its center, a
fact that you can check in our example:
\bprog
algdim(al1)
@eprog\noindent
Cyclic algebras generalize the quaternion algebra construction.
Let~$F$ be a number field and~$K/F$ a cyclic extension of degree~$d$
with~$\sigma\in\text{Gal}(K/F)$ an automorphism of order~$d$, and let~$b\in
F^\times$; the \emph{cyclic algebra}~$(K/F,\sigma,b)$ is the
algebra~$\bigoplus_{i=0}^{d-1}u^iK$ with~$u^d=b$ and~$k u = u \sigma(k)$
for all~$k\in K$. It is a central simple algebra of dimension~$d^2$ over~$F$.
Let's construct one. First, start with a cyclic extension of number fields. A
simple way of obtaining such an extension is to take a cyclotomic field
over~$\Q$.
\bprog
T = polcyclo(5)
K = rnfinit(Q,T);
aut = Mod(x^2,T)
@eprog\noindent
Here the variable of~\kbd{T} must have higher priority than that of~\kbd{Q} to
build the~\kbd{rnf} structure. Now choose an element~$b\in\Q^\times$, say~$3$.
\bprog
b = 3
@eprog\noindent
Now we can create the algebra and check that it has the right dimension:~$16$.
\bprog
al2 = alginit(K, [aut,b]);
algdim(al2)
@eprog\noindent
We can recover the field~\kbd{K}, the automorphism~\kbd{aut} and the
element~\kbd{b} respectively as follows.
\bprog
algsplittingfield(al2)
algaut(al2)
algb(al2)
@eprog\noindent
In order to see how we recover quaternion algebras with this construction, let's
look at
\bprog
algsplittingfield(al1).pol
algaut(al1)
algb(al1)
@eprog\noindent
We see that the quaternion algebra~$(-2,-1)_{\Q}$ constructed by~\kbd{gp} is
represented as a cyclic algebra~$(\Q(\sqrt{-2})/\Q, \sigma, -1)$ by
writing~$\Q+\Q i+\Q j+\Q ij = \Q(i) + j\Q(i)$.
A nice feature of central simple algebras over a given number field is that they
are completely classified up to isomorphism, in terms of certain invariants. We
therefore provide functions to create an algebra directly from its invariants.
The first basic invariant is the dimension of the algebra over its center.
In fact, the
dimension of a central simple algebra is always a square, and we call the square
root of the dimension the \emph{degree} of the algebra. For instance, a
quaternion algebra has degree~$2$. We can access the degree with
\bprog
algdegree(al1)
algdegree(al2)
@eprog\noindent
Let~$F$ be a number field and~$A$ a central simple algebra of degree~$d$
over~$F$. To every place~$v$ of~$F$ we can attach a \emph{Hasse invariant}
$h_v(A)\in(\dfrac{1}{d}\Z)/\Z\subset\Q/\Z$, with
the additional restrictions that the invariant is~$0$ at every complex place and
in~$(\dfrac{1}{2}\Z)/\Z$ at every real place. These
invariants are~$0$ at all but finitely many places. For instance we can compute
the invariants for our algebras:
\bprog
alghassei(al1)
alghassef(al1)
alghassei(al2)
alghassef(al2)
@eprog\noindent
The output of~\kbd{alghassei} (infinite places) is a vector of integers of length the number
of real places of~$F$, and the corresponding Hasse invariants are these integers
divided by~$d$. Here we learn that the invariant of~\kbd{al1} at~$\infty$
is~$1/2$ and that of~\kbd{al2} is~$0$. Similarly, the output of~\kbd{alghassef}
(finite places) is a pair of vectors, one containing primes of the base field,
and a vector of integers of the same length representing the Hasse invariants at
finite places. We learn that~\kbd{al1} has invariant~$1/2$ at~$2$ and~$0$ at
every other finite place, and that~\kbd{al2} has invariant~$-1/3$ at~$3$,
invariant~$1/3$ at~$5$, and~$0$ at every other finite place.
These invariants give a complete classification of central simple
algebras over~$F$:
\item two central simple algebras over~$F$ of the same degree are isomorphic if
and only if they have the same invariants;
\item the sum of the Hasse invariants is~$0$ in $\Q/\Z$;
\item for every degree~$d$ and finite collection of Hasse invariants satisfying
the conditions above, there exists a central simple algebra over~$F$ having
those invariants.
Let's use~\kbd{gp} to construct an algebra from its invariants. First let's
construct a number field and a few places.
\bprog
nf = nfinit(y^2-2);
p3 = idealprimedec(nf,3)[1]
p17 = idealprimedec(nf,17)[1]
@eprog\noindent
Now let's construct an algebra of degree~$6$. The following Hasse invariants
satisfy the correct conditions since~$1/3+1/6+1/2=0$ in~$\Q/\Z$:
\bprog
hf = [[p3,p17],[1/3,1/6]]; hi = [1/2,0]; \\ nf has 2 real places
@eprog\noindent
Finally we can create the algebra:
\bprog
al3 = alginit(nf,[6,hf,hi]);
@eprog\noindent
This will require less than half a minute but a lot of memory:
this is an algebra of dimension~$2\times 6^2 = 72$ over~$\Q$ and we
are doing a nontrivial computation in the initialization! You can check that the
dimension over~$\Q$ is what we expect:
\bprog
algdim(al3,1)
@eprog\noindent
During the initialization, \kbd{gp} computes an integral multiplication table
for the algebra. This allows us to recreate a table version of the algebra:
\bprog
mt3 = algmultable(al3);
al4 = algtableinit(mt3);
@eprog\noindent
We can then check that the algebra is simple as expected:
\bprog
algissimple(al4)
@eprog\noindent
Finally, an important test for a central simple algebra is whether it is a
division algebra.
\bprog
algisdivision(al3)
@eprog
\subsubsec{Elements}
In a cyclic algebra~$(K/F,\sigma,b)$ of degree~$d$, we represent elements as
column vectors of length~$d$, expressed on the basis of the~$K$-vector
space~$\bigoplus_{i=0}^{d-1}u^iK$:
\bprog
a = [x^3-1, -x^2, 3, -1]~*Mod(1,T) \\represents an element in al2
@eprog\noindent
To represent elements in the quaternion algebra~\kbd{al1}, we must view it as a
cyclic algebra, and therefore use the representation~\kbd{al1} $ =
\Q(i)+j\Q(i)$. The standard basis elements~$1,i,j,k=ij=-ji$ become
\bprog
one = [1,0]~
i = [x,0]~
j = [0,1]~
k = [0,-x]~
@eprog\noindent
The expected equalities hold:
\bprog
algsqr(al1,i) == -2*one
algsqr(al1,j) == -1*one
algsqr(al1,k) == -2*one
algmul(al1,i,j) == k
algmul(al1,j,i) == -k
@eprog\noindent
Like~\kbd{nfinit} for number fields, the~\kbd{alginit} function computes an
integral basis of
the algebra being initialized. More precisely, an \emph{order} in
a~$\Q$-algebra~$A$ is a subring~${\cal O}\subset A$ that is finitely generated
as a~$\Z$-module and such that~$\Q{\cal O} = A$. In an algebra
structure
computed with~\kbd{alginit}, we store a basis of an order~${\cal O}_0$, which we
will call the integral basis of the algebra. There is no canonical choice for
such a basis, and not every integral element has integral coordinates with
respect to that basis (the set of integral elements in~$A$ does not form a
ring in general). By default, ${\cal O}_0$ is a maximal order and hence behaves
in a way similar to the ring of integers in a number field. You can disable the
(costly) computation of a maximal order with an optional argument:
\bprog
al5 = alginit(nf,[6,hf,hi],,0);
@eprog\noindent
This command should be faster than the initialization of~\kbd{al3}.
As in number fields, you can represent elements of central simple algebras in
\emph{algebraic form}, which means the cyclic algebra representation we
described above, or in \emph{basis form}, which means as a~$\Q$-linear
combination of the integral basis. You can switch between the two
representations:
\bprog
algalgtobasis(al2, a)
algalgtobasis(al1, j)
algbasistoalg(al3, algrandom(al3,1))
@eprog\noindent
As usual you can compute any elementary operation: try various combinations
of~\kbd{algadd}, \kbd{algsub}, \kbd{algneg}, \kbd{algmul}, \kbd{alginv},
\kbd{algsqr}, \kbd{algpow}.
Every central simple algebra~$A$ over a number field~$F$ admits a \emph{splitting
field}, i.e. an extension~$K/F$ such that~$A\otimes_F K\cong M_d(K)$. We always
store such a splitting in an~\kbd{alginit} structure, and you can access it
using
\bprog
algsplittingfield(al1) \\ K as an rnf structure
algtomatrix(al1, k) \\ image of k by the splitting isomorphism
@eprog\noindent
For every~$x\in A$, the trace (resp. determinant, characteristic polynomial)
of that matrix is in~$K$ (resp.~$K$,~$K[X]$) and is called the
\emph{reduced trace} (resp. \emph{reduced norm}, \emph{reduced characteristic
polynomial}) of~$x$. You can compute them using
\bprog
algtrace(al3, vector(72,i,i==3)~)
algnorm(al2, a)
algcharpoly(al1, -1+i+j+2*k)
@eprog
\section{Plotting}
PARI supports high and low-level graphing functions, on a variety of output
devices: a special purpose window under standard graphical environments (the
\kbd{X Windows} system, Mac OS X, Microsoft Windows), or a \kbd{PostScript}
file ready for the printer. These functions use a multitude of flags, which
are mostly power-of-2. To simplify understanding we first give these flags
symbolic names.
\bprog
/* Relative positioning of graphic objects: */
nw = 0; se = 4; relative = 1;
sw = 2; ne = 6;
/* String positioning: */
/* V */ bottom = 0; /* H */ left = 0; /* Fine tuning */ hgap = 16;
vcenter = 4; center = 1; vgap = 32;
top = 8; right = 2;
@eprog\noindent
We also decrease drastically the default precision.
\bprog
\p 9
@eprog\noindent
This is very important, since plotting involves calculation of functions at
a huge number of points, and a relative precision of 38 significant digits
is an obvious overkill: the output device resolution certainly won't reach
$10^{38} \times 10^{38}$ pixels!
Start with a simple plot:
\bprog
ploth(X = -2, 2, sin(X^7))
@eprog\noindent
You can see the limitations of the ``straightforward'' mode of plotting:
while the first several cycles of \kbd{sin} reach $-1$ and $1$, the cycles
which are closer to the left and right border do not. This is understandable,
since PARI is calculating $\sin(X^7)$ at many (evenly spaced) points, but
these points have no direct relationship to the ``interesting'' points on
the graph of this function. No value close enough to the maxima and minima
are calculated, which leads to wrong turning points in the graph. To fix
this, one may use variable steps which are smaller where the function varies
rapidly:
\bprog
ploth(X = -2, 2, sin(X^7), "Recursive")
@eprog\noindent
The precision near the edges of the graph is much better now.
However, the recursive plotting (named so since PARI subdivides intervals
until the graph becomes almost straight) has its own pitfalls. Try
\bprog
ploth(X = -2, 2, sin(X*7), "Recursive")
@eprog\noindent The graph looks correct far away, but it has a straight
interval near the origin, and some sharp corners as well. This happens
because the graph is symmetric with respect to the origin, thus the middle 3
points calculated during the initial subdivision of $[-2,2]$ are exactly on
the same line. To PARI this indicates that no further subdivision is needed,
and it plots the graph on this subinterval as a straight line.
There are many ways to circumvent this. Say, one can make the right limit
2.1. Or one can ask PARI for an initial subdivision into 16 points instead
of default 15:
\bprog
ploth(X = -2, 2, sin(X*7), "Recursive", 16)
@eprog\noindent
All these arrangements break the symmetry of the initial subdivision, thus
make the problem go away. Eventually PARI will be able to better detect such
pathological cases, but currently some manual intervention may be required.
The function \kbd{ploth} has some additional enhancements which allow
graphing in situations when the calculation of the function takes a lot of
time. Let us plot $\zeta({1\over 2} + it)$:
\bprog
ploth(t = 100, 110, real(zeta(0.5+I*t)), /*empty*/, 1000)
@eprog\noindent
This can take quite some time. (1000 is close to the default for many
plotting devices, we want to specify it explicitly so that the result does
not depend on the output device.) Try the recursive plot:
\bprog
ploth(t = 100, 110, real(zeta(0.5+I*t)), "Recursive")
@eprog\noindent
It takes approximately the same time. Now try specifying fewer points,
but make PARI approximate the data by a smooth curve:
\bprog
ploth(t = 100, 110, real(zeta(0.5+I*t)), "Splines", 100)
@eprog\noindent
This takes much less time, and the output is practically the same. How to
compare these two outputs? We will see it shortly. Right now let us plot
both real and complex parts of $\zeta$ on the same graph:
\bprog
f(t) = my(z = zeta(0.5+I*t)); [real(z),imag(z)];
ploth(t = 100, 110, f(t), , 1000)
@eprog\noindent (Note the use of the temporary variable \kbd{z}; \kbd{my}
declares it local to the function's body.)
Note how one half of the roots of the real and imaginary parts coincide.
Why did we define a function \kbd{f(t)}? To avoid calculation of
$\zeta({1\over2} + it)$ twice for the same value of t. Similarly, we can
plot parametric graphs:
\bprog
ploth(t = 100, 110, f(t), "Parametric", 1000)
@eprog\noindent In that case (parametric plot of the real and imaginary parts
of a complex function), we can also use directly
\bprog
ploth(t = 100, 110, zeta(0.5+I*t), "Complex", 1000)
ploth(t = 100, 110, zeta(0.5+I*t), "Complex|Splines", 100)
@eprog
If your plotting device supports it, you may ask PARI to show the points
in which it calculated your function:
\bprog
ploth(t = 100, 110, f(t), "Parametric|Splines|Points_too", 100)
@eprog
As you can see, the points are very dense on the graph. To see some crude
graph, one can even decrease the number of points to 30. However, if you
decrease the number of points to 20, you can see that the approximation to
the graph now misses zero. Using splines, one can create reasonable graphs
for larger values of t, say with
\bprog
ploth(t = 10000, 10004, f(t), "Parametric|Splines|Points_too", 50)
@eprog
How can we compare two graphs of the same function plotted by different
methods? Documentation shows that \kbd{ploth} does not provide any direct
method to do so. However, it is possible, and even not very complicated.
The solution comes from the other direction. PARI has a power mix of high
level plotting function with low level plotting functions, and these functions
can be combined together to obtain many different effects. Return back
to the graph of $\sin(X^7)$. Suppose we want to create an additional
rectangular frame around our graph. No problem!
First, all low-level graphing work takes place in some virtual drawing
boards (numbered from 0 to 15), called ``rectangles'' (or ``rectwindows'').
So we create an empty ``rectangle'' and name it rectangle 2 (any
number between 0 and 15 would do):
\bprog
plotinit(2)
plotscale(2, 0,1, 0,1)
@eprog
This creates a rectwindow whose size exactly fits the size of the output
device, and makes the coordinate system inside it go from 0 to 1 (for both
$x$ and $y$). Create a rectangular frame along the boundary of this rectangle:
\bprog
plotmove(2, 0,0)
plotbox(2, 1,1)
@eprog
Suppose we want to draw the graph inside a subrectangle of this with upper
and left margins of $0.10$ (so 10\% of the full rectwindow width), and
lower and top margins of $0.02$, just to make it more interesting. That
makes it an $0.88 \times 0.88$ subrectangle; so we create another rectangle
(call it 3) of linear size 0.88 of the size of the initial rectangle and
graph the function in there:
\bprog
plotinit(3, 0.88, 0.88, relative)
plotrecth(3, X = -2, 2, sin(X^7), "Recursive")
@eprog
(nothing is output yet, these commands only fills the virtual drawing
boards with PARI graphic objects). Finally, output rectangles 2 and 3 on
the same plot, with the required offsets (counted from upper-left corner):
\bprog
plotdraw([2, 0,0, 3, 0.1,0.02], relative)
@eprog
\noindent The output misses something comparing to the output of
\kbd{ploth}: there are no coordinates of the corners of the internal
rectangle. If your output device supports mouse operations (only
\kbd{gnuplot} does), you can find coordinates of particular points of the
graph, but it is nice to have something printed on a hard copy too.
However, it is easy to put $x$- and $y$-limits on the graph. In the
coordinate system of the rectangle 2 the corners are $(0.1,0.1)$,
$(0.1,0.98)$, $(0.98,0.1)$, $(0.98,0.98)$. We can mark lower $x$-limit by
doing
\bprog
plotmove(2, 0.1,0.1)
plotstring(2, "-2.000", left+top+vgap)
@eprog\noindent
Computing the minimal and maximal $y$-coordinates might be trickier, since
in principle we do not know the range in advance (though for $\sin(X^7)$ it
is easy to guess!). Fortunately, \kbd{plotrecth} returns the $x$- and
$y$-limits.
Here is the complete program:
\bprog
plotinit(3, 0.88, 0.88, relative)
lims = plotrecth(3, X = -2, 2, sin(X^7), "Recursive")
\p 3 \\ @com $3$ significant digits for the bounding box are enough
plotinit(2); plotscale(2, 0,1, 0,1)
plotmove(2, 0,0); plotbox(2, 1,1)
plotmove(2, 0.1,0.1);
plotstring(2, lims[1], left+top+vgap)
plotstring(2, lims[3], bottom+vgap+right+hgap)
plotmove(2, 0.98,0.1); plotstring(2, lims[2], right+top+vgap)
plotmove(2, 0.1,0.98); plotstring(2, lims[4], right+hgap+top)
plotdraw([2, 0,0, 3, 0.1,0.02], relative)
@eprog
We started with a trivial requirement: have an additional frame around
the graph, and it took some effort to do so. But at least it was possible,
and PARI did the hardest part: creating the actual graph.
Now do a different thing: plot together the ``exact'' graph of
$\zeta({1/2}+it)$ together with one obtained from splines approximation.
We can emit these graphs into two rectangles, say 0 and 1, then put these
two rectangles together on one plot. Or we can emit these graphs into one
rectangle 0.
However, a problem arises: note how we
introduced a coordinate system in rectangle 2 of the above example, but we
did not introduce a coordinate system in rectangle 3. Plotting a
graph into rectangle 3 automatically created a coordinate system
inside this rectangle (you could see this coordinate system in action
if your output device supports mouse operations). If we use two different
methods of graphing, the bounding boxes of the graphs will not be exactly
the same, thus outputting the rectangles may be tricky. Thus during
the second plotting we ask \kbd{plotrecth} to use the coordinate system of
the first plotting. Let us add another plotting with fewer
points too:
\bprog
plotinit(0, 0.9,0.9, relative)
plotrecth(0, t=100,110, f(t), "Parametric",300)
plotrecth(0, t=100,110, f(t), "Parametric|Splines|Points_too|no_Rescale",30);
plotrecth(0, t=100,110, f(t), "Parametric|Splines|Points_too|no_Rescale",20);
plotdraw([0, 0.05,0.05], relative)
@eprog
This achieves what we wanted: we may compare different ways to plot a graph,
but the picture is confusing: which graph is what, and why there are multiple
boxes around the graph? At least with some output devices one can control
how the output curves look like, so we can use this to distinguish different
graphs. And the mystery of multiple boxes is also not that hard to solve:
they are bounding boxes for calculated points on each graph. We can disable
output of bounding boxes with appropriate options for \kbd{plotrect}.
With these frills the script becomes:
\bprog
plotinit(0, 0.9,0.9, relative)
plotrecth(0, t=100,110, f(t), "Parametric|no_Lines", 300)
opts="Parametric|Splines|Points_too|no_Rescale|no_Frame|no_X_axis|no_Y_axis";
plotrecth(0, t=100,110,f(t), opts, 30);
plotdraw([0, 0.05,0.05], relative)
@eprog
\noindent Plotting axes on the second graph would not hurt, but
is not needed either, so we omit them. One can see that the discrepancies
between the exact graph and one based on 30 points exist, but are pretty
small. On the other hand, decreasing the number of points to 20 would make
quite a noticeable difference.
Additionally, one can ask PARI to convert a plot to PS (PostScript)
or SVG (Scalable Vector Graphics) format: just use the command
\kbd{plotexport} instead of \kbd{plotdraw} in the above examples (or
\kbd{plothexport} instead of \kbd{ploth}). This returns a character string
which you can then write to a file using \kbd{write}.
Now suppose we want to join many different small graphs into one picture.
We cannot use one rectangle for all the output as we did in the example
with $\zeta({1/2}+it)$, since the graphs should go into different places.
Rectangles are a scarce commodity in PARI, since only 16 of them are
user-accessible. Does it mean that we cannot have more than 16 graphs on
one picture? Thanks to an additional operation of PARI plotting engine,
there is no such restrictions. This operation is \kbd{plotcopy}.
The following script puts 4 different graphs on one plot using 2 rectangles
only, \kbd{A} and \kbd{T}:
\bprog
A = 2; \\@com accumulator
T = 3; \\@com temporary target
plotinit(A); plotscale(A, 0, 1, 0, 1)
plotinit(T, 0.42, 0.42, relative);
plotrecth(T, x= -5, 5, sin(x), "Recursive")
plotcopy(T, 2, 0.05, 0.05, relative + nw)
plotmove(A, 0.05 + 0.42/2, 1 - 0.05/2)
plotstring(A,"Graph", center + vcenter)
plotinit(T, 0.42, 0.42, relative);
plotrecth(T, x= -5, 5, [sin(x),cos(2*x)], 0)
plotcopy(T, 2, 0.05, 0.05, relative + ne)
plotmove(A, 1 - 0.05 - 0.42/2, 1 - 0.05/2)
plotstring(A,"Multigraph", center + vcenter)
plotinit(T, 0.42, 0.42, relative);
plotrecth(T, x= -5, 5, [sin(3*x), cos(2*x)], "Parametric")
plotcopy(T, 2, 0.05, 0.05, relative + sw)
plotmove(A, 0.05 + 0.42/2, 0.5)
plotstring(A,"Parametric", center + vcenter)
plotinit(T, 0.42, 0.42, relative);
plotrecth(T, x= -5, 5, [sin(x), cos(x), sin(3*x),cos(2*x)], "Parametric")
plotcopy(T, 2, 0.05, 0.05, relative + se)
plotmove(A, 1 - 0.05 - 0.42/2, 0.5)
plotstring(A,"Multiparametric", center + vcenter)
plotmove(A, 0, 0)
plotbox(A, 1, 1)
plotdraw(A)
\\ s = plotexport(A, relative); write("foo.ps", s) \\ @com if hard copy needed
@eprog
The rectangle \kbd{A} plays the role of accumulator, rectangle \kbd{T} is
used as a target to \kbd{plotrecth} only. Immediately after plotting into
rectangle \kbd{T} the contents is copied to accumulator. Let us explain
numbers which appear in this example: we want to create 4 internal rectangles
with a gap 0.06 between them, and the outside gap 0.05 (of the size of the
plot). This leads to the size 0.42 for each rectangle. We also
put captions above each graph, centered in the middle of each gap. There
is no need to have a special rectangle for captions: they go into the
accumulator too.
To simplify positioning of the rectangles, the above example uses relative
``geographic'' notation: the last argument of \kbd{plotcopy} specifies the
corner of the graph (say, northwest) one counts offset from. (Positive
offsets go into the rectangle.)
To demonstrate yet another useful plotting function, design a program to
plot Taylor polynomials for a $\sin x$ about 0. For simplicity, make the
program good for any function, but assume that a function is odd, so only
odd-numbered Taylor series about 0 should be plotted. Start with defining
some useful shortcuts
\bprog
xlim = 13; ordlim = 25; f(x) = sin(x);
default(seriesprecision,ordlim)
farray(t) = vector((ordlim+1)/2, k, truncate( f(1.*t + O(t^(2*k+1)) )));
FARRAY = farray('t); \\@com\kbd{'t} to make sure \kbd{t} is a free variable
@eprog\noindent
\kbd{farray(x)} returns a vector of Taylor polynomials for $f(x)$, which we
store in \kbd{FARRAY}. We want to plot $f(x)$ into a rectangle, then make
the rectangle which is 1.2 times higher, and plot Taylor polynomials into the
larger rectangle. Assume that the larger rectangle takes 0.9 of the final
plot.
First of all, we need to measure the height of the smaller rectangle:
\bprog
plotinit(3, 0.9, 0.9/1.2, 1);
opts = "Recursive | no_X_axis|no_Y_axis|no_Frame";
lims = plotrecth(3, x= -xlim, xlim, f(x), opts,16);
h = lims[4] - lims[3];
@eprog\noindent
Next step is to create a larger rectangle, and plot the Taylor polynomials
into the larger rectangle:
\bprog
plotinit(4, 0.9,0.9, relative);
plotscale(4, lims[1], lims[2], lims[3] - h/10, lims[4] + h/10)
plotrecth(4, x = -xlim, xlim, subst(FARRAY,t,x), "no_Rescale");
@eprog
Here comes the central command of this example:
\bprog
plotclip(4)
@eprog\noindent
What does it do? The command \kbd{plotrecth(\dots, "no\_Rescale")} scales the
graphs according to coordinate system in the rectangle, but it does not pay
any other attention to the size of the rectangle. Since \kbd{xlim} is 13,
the Taylor polynomials take very large values in the interval
\kbd{-xlim...xlim}. In particular, significant part of the graphs is going
to be \emph{outside} of the rectangle. \kbd{plotclip} removes the parts of
the plottings which fall off the rectangle boundary
\bprog
plotinit(2)
plotscale(2, 0.0, 1.0, 0.0, 1.0)
plotmove(2,0.5,0.975)
plotstring(2,"Multiple Taylor Approximations",center+vcenter)
plotdraw([2, 0, 0, 3, 0.05, 0.05 + 0.9/12, 4, 0.05, 0.05], relative)
@eprog\noindent
These commands draw a caption, and combine 3 rectangles (one with the
caption, one with the graph of the function, and one with graph of Taylor
polynomials) together. The plots are not very beautiful with the default
colors. See \kbd{examples/taylor.gp} for a user function encapsulating the
above example, and a colormap generator.
This finishes our survey of PARI plotting functions, but let us add
some remarks. First of all, for a typical output device the picture is
composed of small colored squares (pixels), as a very large checkerboard.
Each output rectangle is a disjoint union of such squares. Each drop
of paint in the rectangle will color a whole square in it. Since the
rectangle has a coordinate system, it is important to know how this
coordinate system is positioned with respect to the boundaries of these
squares.
The command \kbd{plotscale} describes a range of $x$ and $y$ in the
rectangle. The limit values of $x$ and $y$ in the coordinate system are
coordinates \emph{of the centers} of corner squares. In particular,
if ranges of $x$ and $y$ are $[0,1]$, then the segment which connects (0,0)
with (0,1) goes along the \emph{middle} of the left column of the rectangle.
In particular, if we made tiny errors in calculation of endpoints of this
segment, this will not change which squares the segment intersects, thus
the resulting picture will be the same. (It is important to know such details
since many calculations are approximate.)
Another consideration is that all examples we did in this section were
using relative sizes and positions for the rectangles. This is nice, since
different output devices will have very similar pictures, while we
did not need to care about particular resolution of the output device.
On the other hand,
using relative positions does not guarantee that the pictures will be
similar. Why? Even if two output devices have the same resolution,
the picture may be different. The devices may use fonts of different
size, or may have a different ``unit of length''.
The information about the device in PARI is encoded in 6 numbers: resolution,
size of a character cell of the font, and unit of length, all separately
for horizontal and vertical direction. These sizes are expressed as
numbers of pixels. To inspect these numbers one may use the function
\kbd{plothsizes}. The ``units of length'' are currently used to calculate
right and top gaps near graph rectangle of \kbd{ploth}, and gaps for
\kbd{plotstring}. Left and bottom gaps near graph rectangle are calculate
using both units of length, and sizes of character boxes (so that there
is enough place to print limits of the graphs).
What does it show? Using relative sizes during plotting produces
\var{approximately} the same plotting on different devices, but does not
ensure that the plottings ``look the same''. Moreover, ``looking the
same'' is not a desirable target, ``looking tuned for the environment''
will be much better. If you want to produce such fine-tuned plottings,
you need to abandon a relative-size model, and do your plottings in
pixel units. To do this one removes flag \kbd{relative} from the above
examples, which will make size and offset arguments interpreted this way.
After querying sizes with \kbd{plothsizes} one can fine-tune sizes and
locations of subrectangles to the details of an arbitrary plotting
device.
The last two elements of the array returned by \kbd{plothsizes} are the
dimensions of the display, if applicable. If there is no real display, like
in svg or postscript plots, the width and height of display are set to $0$.
To check how good your fine-tuning is, you may test your graphs with a
medium-resolution plotting (as many display output devices are), and
with a low-resolution plotting (say, with \kbd{plotterm("dumb")} of gnuplot).
\section{GP Programming}
Do we really need such a section after all we have learnt so far? We now
know how to write scripts and feed them to \kbd{gp}, in particular how to
define functions. It's possible to define \emph{member} function as well, but
we trust you to find them in the manual. We have seen most control
statements: the missing ones (\kbd{while}, \kbd{break}, \kbd{next},
\kbd{return} and various \kbd{for} loops) should be straightforward. (You
won't forget to look them up in the manual, will you?)
Output is done via variants of the familiar \kbd{print} (to screen),
\kbd{write} (to a file). Input via \kbd{read} (from file), \kbd{input}
(querying user), or \kbd{extern} (from an external auxiliary program).
To customize \kbd{gp}, e.g.~increase the default stack space or load your
private script libraries on startup, look up \kbd{The preferences file}
section in the User's manual. We strongly advise to set \kbd{parisizemax} to
a large nonzero value, about what you believe your machine can stand: this
both limits the amount of memory PARI will use for its computation (thereby
keeping your machine usable), and let PARI increases its stack size (up to
this limit) to accommodate large computations. If you regularly see \kbd{PARI
stack overflows} messages, think about this one!
For clarity, it is advisable to declare local variables in user functions
(and in fact, with the smallest possible scope), as we have done so far with
the keyword \kbd{my}. As usual, one is usually better off avoiding global
variables altogether.
\emph{Break loops} are more powerful than we saw: look up \kbd{dbg\_down} /
\kbd{dbg\_up} (to get a chance to inspect local variables in various scopes)
and \kbd{dbg\_err} (to access all components of an error context).
To reach grandwizard status, you may need to understand the all powerful
\kbd{install} function, which imports into \kbd{gp} an (almost) arbitrary
function from the PARI library (and elsewhere too!), or how to use the
\kbd{gp2c} compiler and its extended types. But both are beyond the scope of
the present document.
Have fun!
\bye