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A Tutorial for Pari/GP

1. Recursive 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 ploth X 2 2 sin X 7 Recursive 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 ploth X 2 2 sin X 7 Recursive 16 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 ploth has some additional enhancements which allow graphing in situations when the calculation of the function takes a lot of time Let us plot C 3 it ploth t 100 110 real zeta 0 5 I t empty 1000 This can take quite some time 1000 is close to the default for many plotting devices we want to specify it explicitly
2. w Vec matid 4 canonical basis v vector w i nfgaloisapply nf s w i M Mat v The vector v contains the images of the integral basis elements as column vectors The last statement concatenates them into a square matrix So M gives the action of s on the integral basis Let s check M 2 That s the identity all right The fixed field of this automorphism is going to be the only non trivial subfield of K 1 seem to recall that polred told us this was the third cyclotomic field Let s check this type nfsubfields nf Indeed there s a quadratic subfield but it s given by T x72 22x x 133 and I don t recognize it But nfisisom T polcyclo 3 indeed tells us that the fields Q z T and Q x x x 1 are isomorphic In fact polred T would tell us the same but does not correspond to a foolproof test polred could have returned some other polynomials We may also check that k matker M 1 is two dimensional then z nfbasistoalg nf k 2 generates the quadratic subfield Notice that 1 z and u are Q linearly dependent and in fact Z linearly as well Exercise how would you check these two assertions in general Answer concat then respectively matrank or matkerint or qf111 z charpoly z z gcd z z and polred z tell us that we found back the same subfield again as we ought to Final check type nfrootsof1 nf Again we find that K contains a cube root of unity since the torsion subgroup of
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 Next step is to create a larger rectangle and plot the Taylor polynomials into the larger rectangle 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 Here comes the central command of this example plotclip 4 What does it do The command plotrecth 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 xlim is 13 the Taylor polynomials take very large values in the interval xlim xlim In particular significant part of the graphs is going to be outside of the rectangle plotclip removes the parts of the plottings which fall off the rectangle boundary plotinit 2 plotscale 2 0 0 1 0 0 0 1 0 plotmove 2 0 5 0 975 plotstring 2 Multiple Taylor Approximations centert vcenter plotdraw 2 0 0 3 0 05 0 05 0 9 12 4 0 05 0 05 relative 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 examples taylor gp for a user function encapsulating the above example and a colormap generator This
4. 3 5 gt which is deduced from the power series expansion of atan x You also know that m cannot be computed from this formula since the convergence is so slow Right Wrong Type p 100 4 x sumalt k 0 1 7k 2 k 1 In a split second we get m to 100 significant digits type Pi to check Similarly try sumpos k 1 k 2 Although once again the convergence is slow the summation is rather fast compare with the exact result Pi72 6 This is less impressive because a bit slower than for alternating sums but still useful Even better sumalt can be used to sum divergent series Type 19 zet s sumalt k 1 1 k 1 k s 1 2 1 8 Then for positive values of s different from 1 zet s is equal to zeta s and the series converges albeit slowly sumalt doesn t care however For negative s the series diverges but zet s still gives the correct result Namely the value of a suitable analytic continuation Try zet 1 zet 2 zet 1 5 and compare with the corresponding values of zeta You should not push the game too far zet 100 for example gives a completely wrong answer Try zet I and compare with zeta I Even some complex values work although the sum is not alternating any more Similarly try sumalt n 1 1 n n I More traditional functions are the numerical integration functions Try intnum t 1 2 1 t and presto you get 100 decimals of log 2 Look at Chapter 3 to see the av
5. 7 Using Numerical Tools o dus mr OE a a a ne n 19 8 Polynomials As e amp Es yb A Bhs me ten eed oe A ag att ate e oad ow he A a 21 9 Power Series il by ee do ae Leal Ne eee ol eH Se Ge eid RS E a 24 10 Working with Elliptic Curves aa a 25 11 Working in Quadratic Number Fields 29 12 Working in General Number Fields 34 12a in O E RON ees RA 34 1252 deals ga 5 428 28 Se an DR et a i Oe A do Bien a nr a ue oe 38 12 3 Class roups and units DAE Sota a a A a A LA eS 41 19 4 Class field theory DAT arto do ke ek A A A A onde Seok DE k 42 12 5 Galois theory over Q wy A ROBE O O et OP Qe ee oe al fe O GE 43 13 Plotting 2 5 2 000260 Gate a Peete Ge Va ae DE See ke ee hae a 4 44 14 GP Prosrammings 1 4 04 soe dia peau RM eee eee tae QUE fe dees Ea eh wed eS 51 This booklet is a guided tour and a tutorial to the 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 User s Manual to PARI GP for detailed explanations This chapter can be read independently for example to get acquainted with the possibilities of gp without having to read the whole manual At this point 1 Greetings So you are sitting in front of your workstation or terminal or PC and you type gp to get the program started or click on the relevant icon or select some menu item It says hello
6. By Fermat s little theorem if n is prime we must have a 1 mod n for all a not divisible by n Hence we could try this with a 2 for example But 2 is a number with approximately 3 1014 digits hence impossible to write down let alone to compute But instead type a Mod 2 n This creates the number 2 considered now as an element of the ring R Z nZ The elements of called intmods can always be represented by numbers smaller than n hence small Fermat s theorem can be rewritten a 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 1ift or centerlift Type a n 1 The result is definitely not equal to Mod 1 n thus proving that n is not a prime If we had obtained Mod 1 n on the other hand it would have given us a hint that n is maybe prime but never 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 to turn on the gp timer then for i 2 ceil sqrt n if nfi 0 print i break This should take less than 5 seconds In general one must use less naive techniques than trial division or be very patient Type fa factor n to let the factoring engine find all prime factors You may stop the timer by typing again Note that as is the case with most prime producing functions the prime factors given by factor are only strong pseudoprimes and
7. these degrees must be prime What about other extensions of degree 2 for instance LO subgrouplist bnr 2 L subgrouplist bnr 2 1 LO resp 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 bnr gen LO has 2 elements associated to the 2 extensions we already know L has 7 elements the 2 from LO and 5 new ones Li setminus Set L Set LO The conductors are vector Li i bnrconductor bnr L1 i among which one sees the identity matrix i e the trivial ideal It is 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 bnr with trivial conductor but we can also use rnfkummer bnr L1 3 pick the subgroup with trivial conductor directly which outputs the unique by Takagi s theorem class field associated to the subgroup L1 3 In fact it is of the form K y e We can check this directly rnfconductor bnf x 2 bnf ful1 12 5 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 a 8th root of unity and K Q The minimal polynomial of is the 8th cyclotomic polynomial namely polcyclo 8 x 1 We issu
8. z 1 is the triangular HNF basis and z 2 is the base change matrix from the canonical basis to the new one with determinant 1 Try matdet z 2 then H z 2 z 1 Fine it works And z 1 indeed looks better than 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 M H gf111 H Its columns give an LLL reduced basis for A qf111 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 log5 and log 15 How to detect it log 15 log 5 log 3 matid 3 m 3 round u 10 25 gf111 m 1 first vector of the LLL reduced basis Vv e lt BE Pretty close In fact lindep automates this kind of search for integer relations try lindep u Let us come back to above and our LLL basis in M Type G M xM Gram matrix m qfminim G norm12 M 1 100 2 This enumerates the vectors in 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 tr
9. 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 4 We have already seen several such as factor Note that factor handles not only integers but also univariate polynomials Type for example factor x 200 1 You can also ask to factor a polynomial modulo a prime p factormod and even in a finite field which is not a prime field factorff Evidently you have functions for computing GCD s gcd extended GCD s bezout solving the Chinese remainder theorem chinese and so on In addition to the factoring facilities you have a few functions related to primality testing such as isprime ispseudoprime precprime and nextprime As previously mentioned only strong pseudoprimes are produced by the latter two they pass the ispseudoprime test the more sophisticated primality tests in isprime being so much slower are not applied by default We also have the usual multiplicative arithmetic functions the Mobius y function moebius the Euler function eulerphi the w and Q functions omega and bigomega the cz functions sigma which compute sums of k th powers of the positive divisors of a given integer etc You can compute continued fractions For example type p 1000 then contfrac exp 1 you obtain the continued fraction of the base of natural logarithms which as you can see obeys a very simple patter
10. No problem Finally type m 2 j k You have an error message since you have typed a row vector while m 2 is a column vector If you type instead m 2 j k it works Type now h mathilbert 20 You get the so called Hilbert matrix whose coefficient of row i and column j is equal to t j 1 1 Incidentally the matrix h takes too much room If you don t want to see it simply type a semi colon at the end of the line 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 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 sizedigit 1 d Or Str 1 4 The length of the character string representing the result Now type hr 1 h do not forget the semicolon we don t want to see the result then dr matdet hr You notice two things First the computation is
11. a fancy calculator able to give me more decimals than I will ever need Not so gp is incredibly more powerful than an ordinary calculator independently of its arbitrary precision possibilities 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 Of course this is going to break all your nice old scripts Well you can either change the compatibility level typing default compatible 3 will send you back to the stone age behavior of good ol version 1 39 15 or rewrite the scripts We really advise you to do the latter if they are not too long since they can now be written much more cleanly than before especially with lexical scoping my and the new control statements break next return Besides it ll be as good a way as any to get used to the new names To know how a specific function was changed just type whatnow 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 0 1 4 and 0 1 4 3 Not only can the number of d
12. also an nfeltnorm function and nfelttrace as well You can also consider u as a principal ideal and just type idealnorm nf u Of course in this way we lose the sign information We will talk about ideals later on If we want all the symmetric functions of u and not only the norm we type charpoly u Note that this gives the characteristic polynomial of u and not in general the minimal polynomial Exercise How does one get the minimal polynomial from this Find a simpler expression for u Now let s work on the field itself The nfinit command already gave us some information The field is totally complex its signature nf sign is 0 2 its discriminant nf disc is 18981 and nf zk is an integral basis The Galois group of its Galois closure can be obtained by typing polgalois A The answer 8 1 1 or 8 1 1 D 4 if the galdata package is installed shows that it is equal to D4 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 non trivial field isomorphism which fixes K exactly one in this case Type nfgaloisconj nf The result tells us that apart from the trivial automorphism the map is the only field automorphism 36 nfgaloisconj nf s Mod 2 A charpoly s and we obtain A once again Let us check that s is of order 2 subst lift s x s It is We may express it as a matrix
13. but you are the sole master on board at this point Break loops are more powerful than we saw look up dbg_down dbg_up to get a chance to inspect local variables in various scopes and dbg err to access all components of an error context To reach grandwizard status you may need to understand the all powerful install function which imports into gp an almost arbitrary function from the PARI library and elsewhere too or how to use the gp2c compiler and its extended types But both are beyond the scope of the present document Have fun ol
14. can check whether this order is maximal with isfundamental D Elements of a quadratic field are of the form a bw where w is chosen as VD 2 if D is even and 1 VD 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 w quadgen D This sets w equal to w as above and is printed w Note however that if several different quadratic orders are used a printed w may have several different meanings For example if you type wi w2 quadgen 23 quadgen 15 29 both will be printed as w but of course they are not equal Hence beware when dealing with several quadratic orders at once 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 For negative discriminants quadratic forms are triples a b c representing the form ax bry cy Such a form will be printed as and can be created by Qfb a b c Such forms can be multiplied divided powered as many PARI objects using the usual op erations and they can also be reduced using the function qfbred 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 qfbnucomp and qfbnupow and this is usually a little faster Finally you have at your disposal the functions gfbclassno which usual
15. 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 ploth t 10000 10004 f t Parametric Splines Points_too 50 How can we compare two graphs of the same function plotted by different methods Docu mentation shows that 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 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 plotinit 2 plotscale 2 0 1 0 1 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 plotmove 2 0 0 plotbox 2 1 1 Suppose we want to draw the graph inside a subrectangle of t
16. e q q Now q may be a torsion point Type ellheight e q which computes the canonical Neron Tate height of q Note that ellheight assumes that e is minimal Which it is This is non zero hence q is not torsion To see this even better type for k 1 20 print ellmul e q k and we see the characteristic parabolic explosion of the size of the points And another proof that q is not torsion assuming Mazur s bound on the size of the rational torsion We could can also type ellorder e q which returns 0 telling us yet again that q is non torsion As a consistency check type ellheight e ellmul e q 20 ellheight e q We indeed find 400 20 as it should be Notice how almost all those e11 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 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 Ok let s try another curve Type E ellinit 0 1 1 0 0 q 0 0 ellheight E q This corresponds to the equation y y x x and an obvious rational point on it Again from the discriminant we see that the conductor is equal to 11 and if you type ellminimalmodel E you will see that the equation for E is minimal This time the height is very close to zero hence q m
17. equal to v 4 Taking any other four points we would in fact always find a dependency Let s find all dependencies Type vp v 11 v 21 v 3 1 m ellheightmatrix e vp matdet m This is now clearly non zero so the first 3 points are linearly independent showing that the rank of the curve is at least equal to 3 It is in fact equal to 3 and e is the curve of smallest conductor having rank 3 We would like to see whether the other points are dependent For this we use the function ellbil Indeed if Q is some point which is dependent on v 1 v 2 and v 3 then matsolve m ellbil e vp Q will by definition give the coefficients of the dependence relation If these coefficients are close to integers then there is a dependency otherwise not This is much safer than using the matker function Thus type w vector v k matsolve m ellbil e vp v k 27 wr round w amp err We see that the coefficients are all very close to integers and we quantified it with the last instruction wr is the vector expressing all the components of v on its first 3 and err gives an upper bound on the maximum distance to an integer We are thus led to strongly believe that the curve has rank exactly 3 with generators v 1 v 2 and v 3 and this can be proved to be the case Two remarks 1 Using the height pairing to find dependence relations as we have done only in fact finds relations modulo torsion but in this example the cur
18. expressed in terms of the variable y to avoid later problems with variable priorities A word of warning both the bnf and all results obtained from it are conditional on a Riemann Hypothesis at this point the bnf must be certified before the following statements become actual theorems Member functions are still available for bnf structures So let s try them bnf pol gives A bnf sign bnf disc bnf zk ok nothing really exciting In fact an nf is included in the bnf structure bnf nf should be identical to NF Thus all functions which took an nf as first argument will equally accept a bnf and a bnr as well which contains even more data Anyway new members are available now bnf no tells us the class number is 4 bnf cyc that it is cyclic of order 4 but that we already knew bnf gen that it is generated by the ideal g bnf gen 1 If you idealfactor bnf g you recognize P 2 You may also play in the other direction with idealhnf The regulator bnf reg is equal to 3 794 bnf tu tells us that the roots of unity in K are exactly the sixth roots of 1 and gives a primitive root 7 a 5a 8a 56 which we have seen already Finally bnf fu gives us a fundamental unit e 3 a 50 a 28 which must be linked to the units u and u2 found above since the unit rank is 1 To find these relations type bnfisunit bnf u bnfisunit bnf u2 Lo and behold u 62e and u2 tel Note Since the fundamenta
19. 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 plotscale describes a range of x and y in the rectangle The limit values of x and y in the coordinate system are coordinates 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 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 Ev
20. graphs will not be exactly the same thus outputting the rectangles may be tricky Thus during the second plotting we ask plotrecth to use the coordinate system of the first plotting Let us add another plotting with fewer points too 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 47 plotrecth 0 t 100 110 f t Parametric Splines Points_toolno_Rescale 20 plotdraw 0 0 05 0 05 relative 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 plotrect With these frills the script becomes plotinit 0 0 9 0 9 relative plotpointtype 1 0 set color of graph points plotpointsize 0 0 4 use tiny markers if available plotrecth 0 t 100 110 f t Parametricino Lines 300 plotpointsize 0 1 normal size markers plotlinetype 1 1 set color of graph lines plotpointtype 1 1 set color of graph points opts Parametric Splines Points_too no_Rescale no_Frame no_
21. if you don t want this a 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 bnfisprincipal so that it only returns the position of pr in the class group 12 4 Class field theory bnr We now survey quickly some class field theoretic routines We must first initialize a Buchmann Number Ray or bnr structure associated to a bnf base field and a modulus Let s keep K and try a finite modulus f 7Z g 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 associated 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 bnf subst bnf x y Then type bnr bnrinit bnf 7 1 bnr cyc tells us the ray class group modulo f is isomorphic to Z 24Z x Z 6Z x Z 2Z The associated generators are bnr gen Just as a bnf contained an nf a bnr contains a bnf hence an nf namely bnr bnf Here bnr clgp refers to the ray class group while bnr bnf clgp refers to the class group rnfkummer bnr 2 rnfkummer bnr 3 42 outputs defining polynomials for the 2 abelian extensions of K of degree 2 resp 3 whose conductor is exactly equal to f the modulus used to define bnr In the current implementation of rnfkummer
22. in its particular manner and then waits for you after its prompt initially or something like gp gt Type 2 2 What happens Maybe not what you expect First of all of course you should tell gp that your input is finished and this is done by hitting the Return or Newline or 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 before hitting Return since this is how it is done on those systems In that case you will simply see 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 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 27 37 Wow even multiplication works Actually maybe those spaces are not necessary after all Let s try 27 37 Seems to be ok We will still insert them in this document since it makes things easier to read but as 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 gp you can use quit or q backslash q the q being of course for quit Try it Now you ve done it You re out of gp so how do you want to contin
23. is to extract them from larger ones using vecextract For instance if h is the 20 x 20 Hilbert matrix as above h mathilbert 20 vecextract h 11 20 11 20 is its lower right quadrant 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 n 10 15 3 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 factor which does exactly this So type factor n 200000 The last argument tells 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 11 as we made factor believe n would be prime Or is that a contradiction More seriously n is not divisible by any prime up to 200000 We could now trial divide further or cheat and call the PARI function factor without the optional second argument but before we do this let us see how to get an answer ourselves
24. much faster than in the rational case If your computer is too fast for you to notice try again with 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 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 norm12 1 h 1 hr the function norm12 gives the square of the L norm i e the sum of the squares of the coefficients The result is larger than 10 showing that some coefficients of 1 hr are wrong by as much as 10 To obtain the correct result after rounding for the inverse we have to use a default precision of 57 digits try it 10 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 as sume that you want a vector whose i th coordinate is equal to i No problem type for example vector 10 i 172 if you want a vector of length 10 Similarly if you type matrix 5 5 i j 1 i j 1 you will get the Hilbert m
25. norm of rc rr is around 2 10737 reasonable enough when we work with 38 significant digits Note that the function norm12 contrary to what its name implies does not give the L norm but its square hence we must take the square root Well this is not absolutely necessary in the present case In fact round itself already provides a built in rough approximation of the error rr round rc amp e Now e contains the number of error bits when rounding rc to rr in other words the sup norm of rc rr is bounded by 27 Now type pol x 5 x74 2 x73 2xx 72 4 x 3 factor pol factor poldisc pol fun p factorpadic pol p 10 The polynomial 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 fun p which factors pol over Q to p adic precision 10 Type fun 5 22 fun 11 fun 23 fun 37 to see different splittings Similarly type 1f p lift factormod pol p 1f 2 1f 11 1f 23 1f 37 which show the different factorizations this time over F In fact even better type successively t t we want t to be a free variable T ffinit 3 3 t pol2 subst T t x fq factorff pol2 3 T centerlift lift fq T which is actually t73 t 2 t 2 with intmod coefficients is defined above to be an irreducible polynomial of degree 3 over F3 This code snippet factors the polynomial po1
26. not proven primes Use isprime fal 1 to rigorously prove primality of the factors The latter command applies isprime to all entries in the first column of 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 factor Baillie Pomerance Selfridge Wagstaff pseudoprimes without small divisors are true primes at least up to 254 and no explicit counter example is known 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 0 p n or simply O p if n 1 where p is the prime and n the p adic precision Note that you have to explicitly type in 372 for instance 9 will not do Unless you want to cheat gp into believing that 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 func tions For example type n 569 0 778 then s sqrt n you obtain one of the square roots of n to check this type s 2 n Type now s log n then e exp s If you know about p adic logarithms you will no
27. of all this at work and learn some little known number theory at the same time First of all type d 3 3299 qfbclassno d We see that the class number is 3 We know in advance that it must be divisible by 3 from the d 3299 case above and Scholz s mirror theorem Let us create a form by typing f qfbred qfbprimeform d 2 2 the last 2 tells qfbred to ignore the Archimedean component 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 f Type g f for i 1 20 g gfbred g 3 print g this time the 3 means to do a single reduction step still not using Shanks s distance We see that we come back to the form f without having the principal form starting with 1 in the cycle so the ideal corresponding to f is not principal Since the class number is equal to 3 we know however that 73 will be a principal ideal aZx How do we find a For this type f3 qfbpowraw f 3 This computes the cube of f without reducing it Hence it corresponds to an ideal of norm equal to 8 23 so we already know that the norm of is equal to 8 We need more information and this will be given by the fourth component of the form Reduce your form until you reach the unit form you will have to type qfbred 1 exactly 6 times then extract the Archimedean com
28. plotcopy The following script puts 4 different graphs on one plot using 2 rectangles only A and T A 2 accumulator T 3 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 48 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 plotlinetype A 3 plotmove A 0 0 plotbox A 1 1 plotdraw A 0 0 XV psdraw A 0 0 relative if hard copy is needed The rectangle A plays the role of accumulator rectangle T is used as a target to plotrecth only Immediately after plotting into rectangle T the contents is copied to accumulator Let us explain numbers which appear in this example we want to create 4 int
29. so that the result does not depend on the output device Try the recursive plot ploth t 100 110 real zeta 0 5 Ixt Recursive It takes approximately the same time Now try specifying fewer points but make PARI approximate the data by a smooth curve ploth t 100 110 real zeta 0 5 I t Splines 100 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 on the same graph f t my z zeta 0 5 1 t real z imag z ploth t 100 110 f t 1000 45 Note the use of the temporary variable z 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 f t To avoid calculation of 5 it twice for the same value of t Similarly we can plot parametric graphs ploth t 100 110 f t Parametric 1000 In that case parametric plot of the real and imaginary parts of a complex function we can also use directly ploth t 100 110 zeta 0 5 I t Complex 1000 ploth t 100 110 zeta 0 5 1 t Complex Splines 100 If your plotting device supports it you may ask PARI to show the points in which it calculated your function ploth t 100 110 f t Parametric Splines Points_ too 100 As you can see the points are very dense on the graph To see some crude graph one can even
30. w then b a 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 lift b Another example type p x 2 w x 5xw 7 then norm p We thus obtain the quartic equation over Q corresponding to the relative quadratic extension over Q w defined by p On the other hand if you type wr sqrt w 2 do not expect to get back w Instead you get the numerical value the function sqrt being considered as a transcendental function even though it is algebraic Type algdep wr 2 this looks for algebraic relations involving the powers of w up to degree 2 This is one way to get w back Similarly type algdep sqrt 3x w 5 4 See the user s manual for the function algdep 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 pZ 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 w quadgen 163 you can type w Mod x quadpoly 163 Then exactly as in the quadratic case you can type w 10 norm 3 4 w 1 4 w a Mod 1 23 w b a 264 obtaining of course the same results Type 1ift if you don t want to see th
31. 2 over the finite field F3 t T This is of course a form of the field F27 We know that Gal F27 F3 is cyclic of order 3 generated by the Frobenius homomorphism u gt u and the roots that we have found give the action of the powers of the Frobenius on t If you do not know what I am talking about please try some examples it s not so hard to figure out We took pain above to factor a polynomial in the variable x over a finite field defined by a polynomial in t even though they were apparently one and the same There is a crucial rule in all routines involving relative extensions the variable associated to the base field is required to have lower priority than the variables of polynomials whose coefficients are taken in that base field Have a look at the section on Variable priorities in the user s manual see The GP programming language Similarly type pol3 x 4 4x x 2 16 fn lift factornf pol3 t 2 1 and we get the factorization of the polynomial po13 over the number field defined by t 1 i e over Q i Without the lift the result would involve number field elements as t_POLMODs of the form Mod 1 t t72 1 which are more explicit but much less readable To summarize in addition to being able to factor integers you can factor polynomials over C and R using polroots over Fp using factormod over Fp using factorff over Qp using factorpadic over Q using factor and over number fields using factornf or
32. A Tutorial for PARI GP version 2 6 1 The PARI Group Institut de Math matiques de Bordeaux UMR 5251 du CNRS Universit Bordeaux 1 351 Cours de la Lib ration F 33405 TALENCE Cedex FRANCE e mail pari math u bordeaux fr Home Page http pari math u bordeaux fr Copyright 2000 2013 The PARI Group Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies Permission is granted to copy and distribute modified versions or translations of this manual under the conditions for verbatim copying provided also that the entire resulting derived work is distributed under the terms of a permission notice identical to this one PARI GP is Copyright 2000 2013 The PARI Group PARI GP is free software you can redistribute it and or modify it under the terms of the GNU General Public License as published by the Free Software Foundation It is distributed in the hope that it will be useful but WITHOUT ANY WARRANTY WHATSOEVER Table of Contents 1 Greetings ly esca dti a Ve ae dd ne a NE Ae a nn ed ah at 4 2 Warming Up eys isna oe Ge ue ER AUS ee ter A Re Se mn ie AUS SS 7 3 The Remaining PARI Types 9 4 Elementary Arithmetic Functions a 14 5 Performing Linear Algebra 15 6 Using Transcendental Functions 17
33. Windows system Mac OS X Microsoft Windows or a 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 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 9 right 2 We also decrease drastically the default precision p 9 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 1038 x 10 pixels 44 Start with a simple plot ploth X 2 2 sin X 7 You can see the limitations of the straightforward mode of plotting while the first several cycles of 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 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 ploth X 2 2 sin X 7
34. X_axis no_Y_axis plotrecth 0 t 100 110 f t opts 30 plotlinetype 1 2 set color of graph lines plotpointtype 1 2 set color of graph points plotrecth 0 t 100 110 f t opts 20 plotdraw 0 0 05 0 05 relative Plotting axes on the second and third 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 makes quite a noticeable difference Keep in mind that plotlinetype plotpointtype plotpointsize may do nothing on some terminals Additionally one can ask PARI to output a plot into a PS file just use the command psdraw instead of plotdraw in the above examples or psploth instead of ploth See psfile argument to default for how to change the output file for this operation Keep in mind that the precision of PARI PS output will be the same as for the current output device 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 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
35. a multiple of 77 But how can we be sure that it is exactly 77 and not a proper multiple Well type sqrt 10007 Pi prodeuler p 2 500 1 1 kronecker 10007 p p This is nothing else than an approximation to the Dirichlet class number formula The function kronecker is the Kronecker symbol in this case simply the Legendre symbol Note also that we have written 1 1 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 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 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 f qfbprimeform 3299 3 2 is not the norm of a prime ideal but 3 is and we see that 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 f we see that qfbprimeform 3299 5 is not on the list 30 although its cube is as it must This i
36. a representation in 0 we use the function modreverse on this polmod C Try it The result has a big denominator 1029 essentially because our initial polynomial T was so bad By the way to get that 1029 you should type denominator content C Trying 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 From now on we forget about T and use only the polynomial A defining a and the components of the vector nf which gives information on our number field K Type u Mod x 3 5xx 2 8 x 56 A 7 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 nf zk not 39 on the power basis 1 x x 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 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 nfinit which accepts either a polynomial or an nf It will be true for ideals congruence su
37. ailable 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 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 1 h z 2in Jo h z dz where C is the circle of radius r centered at the origin The function we want to integrate is fun z my u exp z u 1 u z Here u is a local variable to the function f whenever a function is called 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 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 zero r r 2 Pi intnum t 0 2 Pi real fun r exp I t exp Ixt The function zero r will count the number of zeros of fun whose modulus is less than 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 intcirc to integrate over a circle yielding the much simpler zero2 r intcirc z 0 r fun z This is a little faster than the previous implem
38. ant of the polynomial is 37 7 19 37 If we only want a 7 maximal order we simply type nfbasis T 7 37 Of course nfbasis T 2 3 5 7 would return an order which is maximal at 2 3 5 7 A variant offers a nice generalization nfbasis T 1075 will return an order which is maximal at all primes less than 10 3 Building on the previous points if the field discriminant is y smooth never mind the polynomial discriminant up to a few big primes known to addprimes then bas nfbasis T y returns a basis for the maximal order We can then input the resulting basis to nfinit as nfinit T bas Better the T listP format can be directly used with nfinit where 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 unconditionnally certified independently of the listP parameter T polcompositum x 7 2 polcyclo 5 1 K nfinit T 2 5 7 nfcertify K 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 unconditionnal nfcertify nf 12 2 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
39. arming up Another thing you better get used to pretty fast is error messages Try typing 1 0 Could not be clearer But why has the prompt become funny turning from to break gt When an error occurs we enter a so called 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 break as instructed in the error message last line Comment You can enter the break loop at any time using 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 Now back to our favourite example in precision 38 type floor exp 100 floor is the mathematician s integer part not to be confused with truncate which is the computer scientist s floor 3 4 is equal to 4 whereas 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 gp is unable to compute the integer part of exp 100 given only 38 decimals of accuracy since it has 44 digits Some error messages are more cryptic and sometimes no
40. ation 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 poly nomial pol polcyclo 15 which is of degree y 15 8 Now type r polroots pol We obtain the 8 complex roots of pol given to 38 significant digits To see them better type b they are given as pairs of complex conjugate roots in a random order The only ordering done by the function polroots concerns the real roots which are given first and in increasing order The roots of pol are by definition the primitive 15 th roots of unity To check this simply type rc r 15 Why we get an error message Fair enough vectors cannot be multiplied even less raised to a power However type rc r 15 7 without forgetting the at the end Now it works because powering to a non integer exponent is a transcendental function and hence is applied termwise Note that the fact that 15 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 rr round rc sqrt norml2 rc rr We see that rr is indeed all 1 s and that the L
41. atrix of order 5 hence the mathilbert function is in fact redundant The i and 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 matrix 10 20 Warning The letter I is reserved for the complex number equal to the square root of 1 Hence 1t is forbidden to use it as a variable Try typing vector 10 1 172 the error message that you get clearly indicates that gp does not consider I as a variable There are other reserved variable names Pi Euler and Catalan All function names are forbidden as well On the other hand there is nothing special about i pi euler or catalan When creating vectors or matrices it is often useful to use Boolean operators and the if O statement Indeed an if expression has a value which is of course equal to the evaluated part of the if So for example you can type matrix 8 8 i j if i j 2 1 0 to get a checkerboard matrix of 1 and 0 Note however that a vector or matrix must be created first before being used For example it is possible to write v vector 5 for i 1 5 v i 1 1 but this would fail if the vector v had not been created beforehand Another useful way to create vectors and matrices
42. belian however we can still put a name on the underlying abstract group Use galoisidentify G which return 18 3 By looking at the GAP4 classification we find that 18 3 is S3 x Z 3Z This time the subgroups of G are not obvious fortunately we can ask PARI galoissubgroups G Let us look for a polynomial Q with the property that K is the splitting field of Q x For that purpose let us take o G gen 3 We check that G gen 3 2 is the identity so is of order 2 We now compute the fixed field K7 and the relative factorisation of P over K7 F galoisfixedfield G G gen 3 2 So K is a quadratic extension of K defined by the polynomial R F 3 1 It is well known that K is also defined by x D where D is the discriminant of R over K To compute D we issue D poldisc F 3 1 Mod 1 subst F 1 x y Note that since y in F 3 1 denotes a root of F 1 we have to use subst x y Now we hope that D generate K and compute Q charpoly D We check that Q x9 270x 123932 19683 is irreducible with polisirreducible Q Were it not the case we would multiply D by a random square So D is a generator of K and VD isa generator of K The result is that K is the splitting field of Q x We can check that with nfisisom P subst Q x x 2 13 Plotting PARI supports high and low level graphing functions on a variety of output devices a spe cial purpose window under standard graphical environments the X
43. bgroups 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 lift and Mod will do the job Next from polmods polynomials to column vectors type v nfalgtobasis nf u Sou a a a 8 right Wrong The coordinates of u are given with respect to the integral basis not the power basis 1 a a and they don t coincide type nf zk if you forgot what the integral basis looked like As a polynomial in a we simply have u 7 a 5a 8a 56 which is trivially deduced from the original polmod representation Of course v nfalgtobasis nf lift u would work equally well Indeed we don t need the polmod information since nf already provides the defining polynomial To go back to polmod representation use nfbasistoalg nf v Notice that u is an algebraic integer since v has integer coordinates try 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 u 3 Try it Or we can type 1 u Better yet if we want to know the norm from K to Q of u we type norm u what else 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 nf But we could use nfeltpow nf u 3 nfeltdiv nf 1 u or nfeltpow nf u 1 which would work whatever representation was chosen Of course there is
44. bling the question mark to check whether it s the case on yours Pi In fact the gp header already gave you that information if it was the case just before the copyright message As well if it says something like readline enabled then you should have a look at the 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 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 p 50 then try 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 log Pi 2 immediately after the preceding computation means the result of the last computed expression More generally the results are numbered 1 2 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 r yn not only is not an integer or a rational number but is even a transcendental number when n is a non zero rational number So gp is just
45. c is the global Tamagawa number product of the local cp for primes p dividing the conductor III is the order of the Tate Shafarevich group and Etors is the torsion group of E Now we know many of these quantities Q is equal to E omega 1 if there had been 3 real roots instead of 1 in E roots Q would be equal to 2 E omega 1 The Tamagawa number c is given as the last component of ellglobalred E and here is equal to 1 We already know that the torsion subgroup of E contains a point of order 5 and typing elltors E shows that it is of order exactly 5 So type ls 25 E omegal1 This shows that IIT must be the trivial group For more detailed information on the local reduction of an elliptic curve at a specific prime p use the function elllocalred E p the second component gives the Kodaira symbol in an encoded form See the manual or online help for details 11 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 non square integer D congruent to 0 or 1 modulo 4 is the discriminant of a specific order in a quadratic field We
46. coeff r 0 polcoeff r 1 There is an important technicality in the above why did we need to substitute NF to nf The reason is that data related to nf is given in terms of the variable x seen modulo nf pol but we need x as a free variable for our polynomial divisions Hence the substitution of x by y in our nf data The most natural method is to try directly nffactor nf y 2 u Except that this won t work for the same technical reason as above the main variable of the polynomial to be factored must have higher priority than the number field variable This won t be possible here since nf was defined using the variable x which has the highest possible priority So we need to substitute variables around nffactor NF x 2 nfbasistoalg NF subst lift u x y Of course with better planning we would just have defined nf in terms of the y variable to avoid all these substitutions A much simpler approach is to consider the above as instances of a 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 CI X and Z to products of simpler groups like Z or Z dZ We shall do exactly that in the next section Before that let us mention another famous but in fact simpler discrete logarithm problem namely the one associated to the invertible elements modulo an ideal Zx 1 Just us
47. could not be used on the output of quadclassunit it needs much more data To extract sensible information from such complicated objects you must use one of the many member functions remember to get a complete list In this case bnf clgp which extracts the class group structure This is much better Type no to check that this leading 27 is indeed what we think it is and not some stupid technical parameter Note that bnf clgp no would work just as well or even bnf no As a last check we can request a relative equation for the Hilbert class field of Q 3299 type quadhilbert 3299 It is indeed of degree 27 so everything fits together Working in real quadratic fields instead of complex ones i e with D gt 0 is not very different The same 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 will be represented as a quadruplet a b c d where the quadratic form itself is ax bry cy with a b and c integral and d R is a distance component as defined by Shanks and Lenstra To create such forms one uses the same function as for definite ones but you can add a fourth optional argument to initialize the distance q Qfb a b c d If the discriminant of poldisc q is negative d is silently discarde
48. ct 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 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 gp session on 64 bit machines Type e exp 1 We get the number e 2 718 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 p 100 Then type e then exp 1 once again This last value is the correct value of the mathematical constant e to 100 decimals while the variable e shows the value that was computed to 38 decimals Clearly they coincide to exactly 29 significant digits So 38 digits are printed but how many significant digits are actually contained in the variable e Type tte which indicates we have exactly 2 mantissa words Since 21n 2 In 10 38 5 we see that we have 38 or 39 significant digits on 64 bit machines Come back to 38 decimals p 38 If we type exp 1 you can check that we also obtain 38 decimals However type f exp 1 1E 40 Although the defa
49. d If you omit it this component is set to 0 i e a real zero to the current precision Again these forms can be multiplied divided powered and they can be reduced using qfbred This function is in fact a succession of elementary reduction steps corresponding essentially to a 31 continued fraction expansion and a single one of these steps can be achieved by adding an optional flag to the arguments of qfbred Since handling the fourth component d usually involves computing expensive logarithms the same flag may be used to ignore the fourth component Finally it is sometimes useful to operate on forms of positive discriminant without performing any reduction this is useless in the negative case the functions qfbcompraw and qfbpowraw do exactly that Again the function 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 qfbred Finally you still have at your disposal the function qfbclassno which gives the class number this time guaranteed correct quadregulator which gives the regulator and the much more sophisticated quadclassunit giving the class group s structure and its generators as well as the regulator The qfbclassno and quadregulator functions use an algorithm which is O VD hence become very slow for discriminants of more than 10 digits quadclassunit can be used on a much larger range Let us see examples
50. d e0 The first component 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 e is ellchangecurve e0 gr 2 You can for the moment ignore the third component gr 3 For the impatient reader this is the product of the local Tamagawa numbers c Type q0 2 2 ellisoncurve e0 q0 q ellchangepoint q0 gr 2 ellisoncurve e q The point q0 is on the curve as checked by ellisoncurve and we transferred it onto the minimal model e using ellchangepoint and the change of variable computed above Note that ellchange point is unusual among the elliptic curve functions in that it does not take an e11 structure as its first argument in 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 gr 2 Also the point at infinity is represented as 0 on all elliptic curves this is the identity for the group law Here q 0 0 obviously lies on e which has equation y y z x Let us now play a little with points on e The group law on an elliptic curve is implemented with the functions elladd for addition ellsub for subtraction and ellmul for multiplication by an integer For example 25 the negative of q is ellsub e 0 q and the double is obtained either as ellmul e q 2 or as elladd
51. d using the theory of modular forms Note that we initialize the product to 1 O x 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 x N 1 and above are irrelevant anyhow if we stop the product at n N While we are on the subject of modular forms which together with Taylor series expansions are another great source of power series type ps 122 shortcut for default seriesprecision 122 d x eta x 24 This gives the first 122 terms of the Fourier series expansion of the modular discriminant function of Ramanujan Its coefficients give by definition the Ramanujan 7 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 d42 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 lift Mod 1 2 d centerlift Mod 1 3 d 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 24 10 Working with Elliptic Curves Now we are getting to more complicated objects Just as with number fields w
52. d we see that this is equal to the modular form f found above the quote tells gp to take the derivative of the expression with respect to its main variable The functions u and v considered on the upper half plane with x e 77 are in fact modular functions for To 5077 The function ellan e 30 gives the first 30 coefficients a of the L series of e One can also ask for a single coefficient the millionth is ellak e 1076 Note however that calling el lan e 100000 is much faster than the equivalent vector 100000 k ellak e k For a prime p ellap e p is equivalent to ellak e p this is the integer a such that the number of points on e over F is 1 p a With the standard PARI distribution ellap is the only way to obtain the 28 order of an elliptic curve over F in gp The external package ellsea provides much more efficient routines Finally let us come back to the curve E defined above by E ellinit 0 1 1 0 0 which had an obvious rational 5 torsion point The sign of the functional equation given by 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 E at 1 ls elllseries E 1 is definitely non zero so E has rank 0 According to the Birch and Swinnerton Dyer conjecture which is proved for this curve 1s is given by the following formula in this case Q c MI L E 1 sl where Q is the real period of E
53. ded ideal pr3 together with 1 trivial factorization The new extended ideal rO is equal to the old one in the sense that the products t A are the same It contains a reduced ideal equivalent to pr3 modulo the principal ideals and a generator of the principal ideal that was factored out Now just for fun type r r0 for i 1 3 r idealred nf r 1 5 print r The ideals in the third r and initial rO are equal say t A tg A this means we have found a unit to t in our field and it is easy to extract this unit given the extended component to r0 2 t r 2 u nfeltdiv nf tO t u nfbasistoalg nf u The last line recovers the unit as an algebraic number Type ch charpoly u and we obtain the characteristic polynomial 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 F factor subst ch x x 2 We see that ch x 2 is a product of 2 polynomials of degree 4 hence u is a square Why We now want to find its square root simple method is as follows 39 NF subst nf x y r F 1 1 x 2 nfbasistoalg NF u to find the remainder of the characteristic polynomial of u2 divided by x 2 u This is a poly nomial of degree 1 in x with polmod coefficients and we know that u2 being a root of both polynomials is the root of r hence can be obtained by typing u2 pol
54. e idealstar this is an init function and ideallog Many more functions on ideals are available We mention here the complete list referring to Chapter 3 for detailed explanations idealadd idealaddtoone idealappr idealchinese idealcoprime idealdiv idealfac tor idealhnf idealintersect idealinv ideallist ideallog idealmin idealmul ideal norm idealpow idealprimedec idealred idealstar idealtwoelt idealval nfisideal We suggest you play with these to get a feel for the algebraic number theory package Remem ber that when a matrix usually in HNF is output it is always a Z basis of the result expressed on the integral basis nf zk of the number field which is usually not a power basis 40 12 3 Class groups and units bnf Apart from the above functions you have at your disposal the powerful function bnfinit which initializes a 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 setrand 1 this resets the random seed to make sure we and you get the exact same results Now type bnf bnfinit NF where 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 NF is now
55. e overshot since the distance is now 3173 02 Now we can refine our initial estimate and believe that we should be close to the correct distance if we raise f to the power 621 x 2641 5 3173 which is close to 517 Now if we compute 7517 we hit the principal form right on the dot Note that this is not a lucky accident we will always land extremely close to the correct target using this method and usually at most one reduction correction step is necessary Of course only the distance component can tell us where we are along the cycle 33 Up to now we have only worked to low precision The goal was to obtain this unknown integer 517 Note that this number has absolutely 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 However when PARI makes its computations 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 Hence we now start again by setting the precision to for example 500 we retype the definition of u why is this necessary and then qfbred u 1 7517 Of course we know in advance that we land on the unit form and the fourth component gives us the regulator to 500 decimal places with no effort at all In a similar way we could obtain the so called compac
56. e polynomial x72 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 w quadgen 163 W Mod x quadpoly 163 a 2 w A 2 VW b 3 WwW B 3 VW for i 1 1075 a b for i 1 1075 A B for i 1 1075 axb for i 1 1075 AxB for i 1 1075 a b for i 1 1075 A B Don t retype everything use the arrow keys There is however a slight difference in behavior Keeping our polmod w type 1 w As you can see the result is not the same Type 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 13 of w in C More generally if 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 a Mod x x73 x 1 defining a cubic extension We can for example ask for b a 5 Now assume we want to express a as a polynomial in b This is possible since b is also a generator of the same field No problem type modreverse b This gives a new defining polynomial for the same field i e the characteristic polynomial of b and expresses a in terms of this new polmod i e in terms of a We will see this in more detail in the number field section 4
57. e the command G galoisinit x 4 1 to compute G Gal K Q The command galoisisabelian G returns 2 0 0 2 so G is an abelian group isomorphic to Z 2Z generated by o G gen 1 and 7 G gen 21 These automorphisms are given by their actions on the roots of x 1 in a suitable p adic extension To get the explicit action on we use galoispermtopol G G gen i for i 1 2 and get o and T 3 The last non trivial automorphism is oTr G gen 1 G gen 2 and we have o7 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 7 with galoisfixedfield G G gen 2 1 and get x 2 Now we want the factorisation of xt 1 over that subfield Of course we could use nffactor but here we have a much simpler option galoisfixedfield G G gen 1 2 outputs x72 2 Mod x 3 x x74 1 x 2 y x 1 x72 y x 1 which means that zt 1 x ax 1 x ax 1 where a is a root of x 2 and more precisely a So we recover the well known factorisation 43 ge 1 x Y 22 D a V 2x 1 For our second example let us take the field K defined by the polynomial P x718 3xx 15 115 x712 104 x 9 511xx 6 196 x73 343 G galoisinit P Since galoisinit succeeds the extension K Q is Galois This time galoisisabelian G return 0 so the extension is not a
58. ecimal places of real numbers be large but the number of digits of integers also Try 1000 It is never necessary to tell 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 Xp of course 4 Come back to 38 digits of precision Ap 38 and type exp 100 As you can see the result is printed in exponential format This is because 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 gp had dutifully printed out 44 digits the last few digits would have been wrong Hence gp wants to print only 38 significant digits but to do so it has to print in exponential format 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 or n where 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 gp knows that your number has only 38 digits of accuracy but an integral part with 44 digits So
59. en 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 50 These sizes are expressed as numbers of pixels To inspect these numbers one may use the function plothsizes The units of length are currently used to calculate right and top gaps near graph rectangle of ploth and gaps for 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 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 relative from the above examples which will make size and offset arguments interpreted this way After querying sizes with plothsizes one can fine tune sizes and locations of subrectangles to the details of an arbitrary plotting de
60. entation and no less accurate We may type p 9 since we know that the result is a small integer but the computations should be instantaneous even at p 100 or so then zero 1 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 lt z lt 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 0 implies after elementary transformations that e z y and that e cos y x Hence y We x and hence e cos ye t x2 x Therefore type fun x my u exp x u cos sqrt u 2 x 2 x 20 Then fun 0 is positive while fun 1 is negative Come back to precision 38 and type x0 solve x 0 1 fun x z x0 Ixsqrt exp 2 x0 x072 which is the required zero As a check type 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 The example above also shows the use of the solve function To use 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 solve t 14 15 real zeta 1 2 I t but this does not wor
61. ernal 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 nota tion the last argument of 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 poly nomials 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 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 t to make sure t is a free variable farray x returns a vector of Taylor polynomials for f x which we store in 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 49 plotinit 3
62. for example 1 2 3 4 This gives the row vector whose coordinates are 1 2 3 and 4 If you want a column vector type 1 2 3 4 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 lo and behold the column vector appears as a proper vertical thingy now The b command is used mainly for this purpose The length of a vector is given by well length of course The shorthand cardinal notation v for length v is also available for instance v v is the last element of v Typem 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 rows and the rows are separated by semicolons The matrix is printed naturally in a rectangle shape If you want it printed horizontally just as you typed it type Na or if you want this type of printing to be the permanent default type default output 0 Type default output 1 if you want to come back to the original output mode Now type m 1 2 m 1 m 2 Are explanations necessary In an expression such as mlj kl the j always refers to the row number and the k to the column number and the first index is always 1 never 0 This default cannot be changed Even better type 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 m 1 15 17 8
63. h power must be principal Type pr4 idealpow nf pr 4 v bnfisprincipal bnf pr4 The first component gives the factorization of the ideal in the class group Here 0 means that it is up to equivalence equal to the 0 th power of the generator g given in bnf gen in other words that it is a principal ideal The second component gives us the algebraic number a such that pr4 aZx a being as usual expressed on the integral basis Type alpha v 2 Let us check that the result is correct first type idealnorm bnf alpha Note that we can use a bnf with all the nf functions but not the other way round of course It is indeed equal to 74 2401 which is the norm of pr4 This is only a first check The complete check is obtained by computing the HNF of the principal ideal generated by alpha To do this type idealhnf bnf alpha pr4 Since the equality is true alpha is correct not that there was any doubt But bnfisprin cipal also gives us information for non principal ideals For example type v bnfisprincipal bnf pr The component v 1 is now equal to 3 and tells us that pr is ideal equivalent to the cube of the generator g Of course we already knew this since the product of P 3 and P 4 was principal generated by al as well as the product of all the P i generated by 7 and we noticed that P 2 was equal to g which has order 4 The second component v 2 gives us a on the integral basis such that pr ag Note that
64. here 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 seriesprecision and simply type Ser 1 x 3 Now try 1 x 7 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 1 x Pi as another example This gives us still another solution to our preceding exercise we can type 1 x 3 Since 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 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 large number of functions exist which handle these types but in this tutorial we will only look at a few 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 n 3 Oxx then n is not the integer 3 but a constant polynomial equal to 3x Check it with type n However it looks
65. hic 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 nextprime 2 B if we know that the determinant is less than 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 back slash up to the first newline character is ignored If you want comments which span many lines you can brace them between pairs Everything in between will be ignored as well For instance as a header for the script above you could insert the following Homomorphic imaging scheme to compute the determinant of a classical integral matrix TODO Look up the explicit formula I hope you did not waste your time copying this nonsense did you In addition linear algebra over Z i e work on lattices can also be performed Let us now consider the lattice A generated by the columns of H in Z Cc R Since the determinant is 15 non zero we have in fact a basis What is the structure of the finite abelian group Z A Type matsnf H Wow 20 cyclic factors There is a triangular basis for A triangular when expressed in the canonical basis perhaps it looks better than our initial one Type mathnf H Hum what if I also want the unimodular transformation matrix Simple z mathnf H 1
66. hich 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 ellinit So type e0 ellinit 6 3 9 16 14 This computes a number of things about the elliptic curve defined by the affine equation y 6ry 9y x 3a 16x 14 4 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 member functions Type to get a complete list Whenever ell appears in the right hand side we can apply the corresponding function to an object output by ellinit I m sure you know how the other init functions will be called now don t you Oh by the way neither clgpinit nor pridinit exist Let s try it The discriminant 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 12th power free it is not in standard reduced form so type e ellminimalmodel e0 which gives the e11 structure associated with the standard model exactly as if we had used ellinit on a reduced equation For some related data type gr ellglobalre
67. his 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 x 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 plotinit 3 0 88 0 88 relative plotrecth 3 X 2 2 sin X 7 Recursive 46 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 plotdraw 2 0 0 3 0 1 0 02 relative The output misses something comparing to the output of ploth there are no coordinates of the corners of the internal rectangle Tf your output device supports mouse operations only 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 plotmove 2 0 1 0 1 plotstring 2 2 000 left top vgap 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 it is easy to guess Fortunately plotrecth returns the zx and y l
68. 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 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 idealprimedec and all ideal related functions will accept them On the other hand the function idealtwoelt can be used to find a two element Zx basis of a given ideal as aZx bZx where a and b belong to K but this is not a valid representation for an ideal under 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 idealhnf nf id for valid ideals or ideal hnf 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 nf does not contain enough data for this particular task We ll need a Buchmann Number Field or 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 Let us keep our number field K as above and its nf structure Type P idealprimedec nf 7 This gives the decomposition of the prime number 7 into pr
69. ime ideals We have chosen 7 because it divides 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 idealnorm nf P 1 Let us take one of these ideals say the first so type 38 pr P 1 We obtain its inertia and residue degree as pr e and pr f and its two generators as pr gen One of them is pr p 7 and the other is guaranteed to have valuation 1 at pr What is the Hermite Normal Form of this ideal No problem idealhnf nf pr and we have the desired HNF Let s now perform ideal operations For example type idealmul nf pr idealmul nf pr pr or more simply pr3 idealpow nf pr 3 to get the cube of the ideal pr Since the norm of this ideal is equal to 343 7 to check that it is really the cube of pr and not of other ideals above 7 we can type for i 1 P print idealval nf pr3 P i and we see that the valuation at pr is equal to 3 while the others are equal to zero We could see this as well from idealfactor nf pr3 Let us now work in the class group by hand we shall see simpler ways later We shall work with extended ideals an extended ideal is a pair A t where A is an ordinary ideal as above and t a number field element this pair represents the ideal A id3 pr3 1 r0 idealred nf id3 The input id3 is an exten
70. imits Here is the complete program plotinit 3 0 88 0 88 relative lims plotrecth 3 X 2 2 sin X 7 Recursive Xp 3 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 bottomtvgaptright 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 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 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
71. is is also given by sqrtint poldisc nf pol nf disc Anyway that s 3087 we don t want to work with such a badly skewed polynomial So we type P polred T We see from the third component that the polynomial x 21x 17x 133 defines the same field with much smaller coefficients so type A P 3 The 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 polred tells us that 34 the field defined by z 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 nfsubfields we shall do that later on Type poldisc A this is much better but maybe not optimal yet the index is still 7 Type polredabs A the abs stands for absolute Since it seems that we won t get anything better we ll stick with A note however that 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 nfinit T 3 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 NF nfinit nf 3 The output is a two compo
72. its unit group has order 6 The given generator happens to be equal to u 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 polred or nfinit T is to compute an integral basis i e a Z basis of the maximal order Zg 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 addprimes to inform PARI to keep on eye for these prime numbers Do it only for big primes Say larger than 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 nf The function nfbasis specifically computes the integral basis and is not much quicker than nfinit so is not very useful in its standard use But we can optionnally 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 discrimin
73. k because the real part is positive for t 14 and 15 As it happens the imaginary part works Type solve t 14 15 imag zeta 1 2 I t and this now works We could also narrow the search interval and type for instance solve t 14 14 2 real zeta 1 2 I t which would also work 8 Polynomials First a word of warning it is essential to understand the difference between exact and inexact objects Try gcd x Pi x 2 6 zeta 2 We return a trivial GCD because the notion of GCD for non exact polynomials doesn t make much sense A better quantitative approach is to use polresultant x Pi x 2 6 zeta 2 A result close to zero shows that the GCD is non trivial 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 The simplest way to input a polynomial is to simply write it down or use an explicit formula for the coefficients and the function sum T 1 x72 27xx 10 T sum i 1 100 itl x7i but it is in much more efficient to create a vector of coefficients then convert it to a polynomial using Pol or Polrev Pol 1 2 is x 2 Polrev 1 2 is 2x 1 T Polrev vector 100 i i for i 1 10 4 Polrev vector 100 i i time 60ms for i 1 10 4 sum i 1 100 i 1 x i A time 1 74ms 21 The reason for the discrepancy is that the explicit summ
74. l unit obtained depends on the random seed you could have obtained another unit than e had you not reset the random seed before the computation This was the purpose of the initial setrand instruction which was otherwise unnecessary We are now ready to perform operations in the class group First and foremost let us certify the result type bnfcertify bnf The output is 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 bnfinit function The answer is that as the regulator gets bigger this process gets increasingly difficult and becomes soon impractical while bnfinit still happily spits out results So it makes sense to dissociate the two you can always check afterwards 41 if the result is interesting enough Looking at the tentative regulator you know in advance whether the certification can possibly succeed if bnf reg is large don t waste your time Now that we feel safe about the bnf output let s do some real work For example let us take again our prime ideal pr above 7 Since we know that the class group is of order 4 we deduce that pr raised to the fourt
75. like an integer without being one and this may cause some confusion in programs which actually expect integers Hence if you try to factor n you obtain an empty factorization Because once considered as a polynomial n is a unit in Q z If you try to apply more general arithmetic functions say the Euler totient function known as eulerphi to 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 simplify to the result This is done automatically at the end of a gp command but not in intermediate expressions Hence n above is not an integer but the final result stored in the output history is So if you type type instead of type n the answer is t_INT adding to the confusion 3 As already stated power series expansions are always implicitly around x 0 When we wanted them around x a we replaced x by z a in the function we wanted to expand For complicated functions it may be simpler to use the substitution function subst For example if p 1 x74 3xx73 5 x 2 6 x 7 you may not want to retype this replacing x by z a s
76. lor expansion of any function that can be expanded around 0 and incidentally this explains why we weren t able to do anything with log x which is not defined at 0 In fact gp knows about Laurent series but log x is not meromorphic either at 0 If we try log 1 x then it works 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 log x 2 to have the expansion of log around x 2 As exercises you can try cos x cos x 2 sin x 72 exp cos x gamma 1 x exp exp x 1 1 tan x for different values of serieslength change it using ps newvalue Let s try something else type 1 x 73 No 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 non zero terms Let s try Type 1 x 7 3 the parentheses around 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 1 7 just gave you 1 7 the result being exact PARI doesn t see any reason to make it non exact But I still want that power series To obtain it you can do as in the 1 7 example and type 1 x 7 3 0 x 16 1 x 7 3 1 0 x716 1 x 0 x716 23 Not on this example but t
77. ly gives the class number the function qfbhclassno which gives the Hurwitz class number and the much more sophisticated quadclassunit function which gives the class number and class group structure Let us see examples of all this at work Type qfbclassno 10007 gp tells us that the result is 77 However you may have noticed in the explanation above that the result is only 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 quadclassunit program which s 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 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 77 We get a form starting with 1 i e the identity Raising f to the powers 11 and 7 does not give the identity thus we now know that the order of f is exactly 77 hence the class number is
78. mplies 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 quadclassunit function by typing for example quadclassunit 3299 Note that this function cheats a little and could still give a wrong answer even assuming GRH the forms given could generate a strict subgroup of the class group If we want to use proven bounds under GRH we have to type quadclassunit 3299 1 6 The double comma is not a typo it means we omit an optional second argument As we want to use the optional third argument we have to indicate to gp we skipped this one 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 bnfinit which handles general number fields and gives more complicated results we could certify this result i e remove the GRH assumption Let s do it type bnf bnfinit x 2 3299 bnfcertify bnf A non zero result here 1 means that everything is ok Good but what did we certify after all Let s have a look at this bnf just type it Enlightening isn t it Recall that the init functions we ve already seen 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 bnfcertify
79. n Can you prove it In many cases one wants to perform some task only when an arithmetic condition is satisfied gp gives you the following functions isprime as mentioned above Z_issquare isfundamental to test whether an integer is a fundamental discriminant i e 1 or the discriminant of a quadratic field and the forprime fordiv and sumdiv loops Assume for example that we want to compute the product of all the divisors of a positive integer n The easiest way is to write p 1 fordiv n d p d p There is a simple formula for this product in terms of n and the number of its divisors find and prove it The notation p d is just a shorthand for 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 forprime p 3 1000 if znprimroot p 2 print p Note that this assumes that znprimroot returns the smallest primitive root and this is indeed the case Had we not known about this we could have written forprime p 3 1000 if znorder Mod 2 p p 1 print p 14 which is actually faster since we only compute the order of 2 in Z pZ instead of looking for a generator by trying successive elements whose orders have to be computed as well Arithmetic functions related to quadratic fields binary quadratic forms and general number fields will be seen in the next sections 5 Performing Linear Algebra The standard linear algebra routi
80. nent vector The first component is the new nf type nf NF 1 If you type nf pol you notice that gp indeed replaced your bad polynomial T by a much better one which happens to be A Small wonder nfinit internally called polredabs The second component enables you to switch conveniently to our new polynomial Namely call 9 a root of our initial polynomial T and a a root of the one that polred has found namely A These are algebraic numbers and as already mentioned are represented as polmods For example in our special case 0 and a are equal to the polmods THETA Mod x x74 24 x 2 585 x 1791 ALPHA Mod x x74 x73 21 x72 17 x 133 respectively Here we are considering only the algebraic aspect and hence ignore completely which root 0 or a 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 0 Since we are now going to work with a instead it is necessary to convert these numbers to a representation using a This is what the second component of NF is for type C NF 2 you get Mod 10 7 x73 43 7xx 2 73 7 x 64 x74 x73 21 x72 17 x 133 meaning that 0 2 a a Ba 64 and hence the conversion from a polynomial in 0 to one in a is easy using subst We could get this polynomial from polred as well try polred T 2 If we want the reverse i e to go back from a representation in a to
81. nes are available matdet mateigen eigenvectors matker matimage matrank matsolve to solve a linear system charpoly characteristic polynomial to name a few Bilinear algebra over R is also there qfgaussred Gauss reduction qfsign signature You may also type 8 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 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 h mathilbert 20 we may compute matdet h Mod 1 101 matdet h 1 0 1017100 in Z 101Z and the p adic ring Z101 to 100 words of accuracy respectively Let H 1 h the inverse of h H 1 h integral L vecextract primes 10000 50 MAN extract the last 50 elements v vector L i matdet H Mod 1 L i centerlift chinese v returns the determinant of H Assuming it is an integer less than half the product of elements of L in absolute value which it is In fact we computed an homomorp
82. nffactor Note however that factor itself will try to guess intelligently over which ring you want to factor try pol x 2 1 factor pol factor pol 1 factor pol 1 O I factor pol 1 0 I factor pol Mod 1 2 23 factor pol Mod 1 Mod 1 3 t 2 1 In the present version 2 6 1 it is not possible to factor over other rings than the ones mentioned above For example gp cannot directly factor multivariate polynomials although it is not too hard to write a simple minded Kronecker s substitution to reduce to the univariate case Exercise Other functions related to factoring are padicappr polrootsmod polrootspadic polsturm Play with them a little Finally type polsym pol3 20 where pol3 was defined above This gives the sum of the k th powers of the roots of pol3 up to k 20 of course computed using Newton s formula and not using 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 non zero 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 non rational algebraic numbers 9 Power series Now let s play with power series as we have done at the beginning Type N 39 8 x prod n 1 N if n 4 1 x n 1 1 OCx N 1 78 Apparently only even powers of x appear This is surprising but can be prove
83. o you can write subst p x zta look up the exact description of the subst function Now type subst 1 O x x z 1 Do you understand the error message 4 The valuation at x 0 for a power series p is obtained as valuation p x 3 The Remaining PARI Types Let s talk some more about the basic PARI types Type p x exp x As expected you get the power series expansion to 16 terms if you have not changed the default Now type pr serreverse p You are asking here for the reversion of the power series p in other words the inverse function This is possible only for power series whose first non zero coefficient is that of xt To check the correctness of the result you can type subst p x pr or subst pr x p and you should get back x 0 x717 Now the coefficients of pr obey a very simple formula First we would like to multiply the coefficient of x n by n in the case of the exponential function this would simplify things con siderably The PARI function serlaplace does just that So type ps serlaplace pr The coefficients now become integers which can be immediately recognized by inspection The coeff cient of x is now equal to n 7 In other words we have n 1 pr gt xX n gt 1 Do you know how to prove this The proof is difficult 9 Of course PARI knows about vectors rows and columns are distinguished even though math ematically there is no difference and matrices Type
84. ponent say c By definition of this distance we know that Q 2c y where o denotes real conjugation in our quadratic field This can be automated q f3 while abs component q 1 1 print q q qfbred q 1 c component q 4 Thus if we type 32 a sqrt 8 exp 2 c sa 8 a we know that up to sign a and sa are numerical approximations of a and o a Of course can always be chosen to be positive and a quick numerical check shows that the difference of a and sa is close to an integer and not the sum so that in fact the norm of a is equal to 8 and the numerical approximation to o a is sa Thus we type p x 2 round a sa x 8 and this is the characteristic polynomial of a We can check that the discriminant of this polynomial is a square multiple of d so is indeed in our field More precisely solving for a and using the numerical approximation that we have to resolve the sign ambiguity in the square root we get explicitly a 15221 153V4 2 Note that this can also be done automatically using the functions polred and 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 a check that it is indeed a generator of the ideal corresponding to the form 3 Let us now play a little with cycles Type D 10 7 4 quadclassunit D We get as a result a 5 component vector which tells
85. s z2 zeta t2 if norm z2 lt norm zi t3 t1 tl t2 t2 t3 zl z2 t2 t1 t2 2 print t1 z1 Don t forget the braces they tell 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 gp in the r command it is supplied by 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 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 a exp 7 0 7710 log a All seems in order b log 5 0 7710 exp b Is something wrong We don t recover the number we started with This is normal type exp b teichmuller 5 0 7710 18 and we indeed recover our initial number The Teichm ller character teichmuller x on Z is the unique p 1 st root of unity which is congruent to x modulo p assuming that x is a p adic unit Let us come back to real numbers for the moment Type agm 1 sqrt 2 This gives the arithmetic geometric mean of 1 and V2 and is the basic method for computing complete elliptic integrals In fact type Pi 2 intn
86. se value depends on a technical reason if your machine supports 64 bit integers the standard C library can handle integers up to 264 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 Pm getting bored why don t we get on with some more exciting stuff Well try exp 1 Presto comes out the value of e to 38 digits Try 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 pi The answer is not that enlightening Pi Ah This works better But let s remember that gp distinguishes between uppercase and lowercase letters pi was as meaningless to it as stupid garbage would have been in both cases gp will just create a variable with that funny unknown name you just used Try it Note that it is actually equivalent to type stupidgarbage all spaces are suppressed from the input In the 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 gp scripts More about this later By the way you can ask gp about any identifier you think it might know about just type it prepending a question mark Try Pi and pi for instance On most systems an extended online help should be available try dou
87. st be large Let s try and put some order into this Note that we work only with the integral points but in general rational points should also be considered Type v Vec v convert to a vector hv ellheight e v We convert the list to a vector because t_LISTs are not accepted as input by most mathematical functions They must be converted to a standard vector type first This gives the vector of canonical heights Let us order the points according to their height For this type perm vecsort vector ttv i i a b gt hvlal gt hv b v vecextract v perm Note how we input a non obvious comparison function to vecsort using an inline anonymous function we sort the integers between 1 and the number of points we have declaring that a gt b if and only if the height of the a th point is larger than the height of the b th The result is a permutation which we apply to v using vecextract 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 m ellheightmatrix e vecextract v 1 2 3 4 matdet m 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 m So type matker m we indeed get a non trivial kernel and the coefficients are close to integers Typing elladd e v 11 v 3 does indeed show that it is
88. t be surprised that e is not equal to n Type n e 6 e is in fact equal to n times the p 1 st root of unity teichmuller n Incidentally if you want to get back the integer 569 from the p adic number n type lift n or truncate n 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 12 of the type complex To start we must specify which quadratic field we want to work in For this we use the function quadgen applied to the discriminant d as opposed to the radicand of the quadratic field This returns a number always printed as w equal to d a 2 where a is equal to 0 or 1 according to whether d is even or odd The behavior of quadgen is a little special although its result is always printed as w the variable w itself is not set to that value Hence it is necessary to write systematically w quadgen d using the variable name w or wi etc if you have several quadratic fields otherwise things will get confusing So type w quadgen 163 then charpoly w which asks for the characteristic polynomial of w The result shows what w will represent You may ask for 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 163 Type for example w 10 norm 3 4 w 1 4 w More interesting type a Mod 1 23
89. t representation of the fundamental unit itself or p adic regulators 1 leave this as exercises for the interested reader You can try the 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 x 3299 it should take about 2 seconds Time for a coffee break 12 Working in General Number Fields 12 1 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 quadgen For example assume that we want to work in the number field K defined by one of the roots of the equation x 24x 585x 1791 0 This is done by typing T x74 24 x72 585 x 1791 nf nfinit T We get an nf structure but thanks to member functions we do not need to know anything about it If you type nf pol you will get the polynomial T which you just input nf sign yields the signature 71 172 of the field nf disc the field discriminant nf zk an integral basis etc The integral basis is expressed in terms of a generic root x of 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 poldisc T Ooops small wonder we had such denominators the index of the power order Z x T in the maximal order Zx is well nf index Note that th
90. t so easy to understand For instance try log x It simply tells you that gp does not understand what log x is although it does know the log function as 10g will readily tell us Now let s try sqrt 1 to see what error message we get now Haha gp even knows about complex numbers so impossible to trick it that way Similarly try typing log 2 exp IxPi I I 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 I and i are not the same thing Compare 172 with i 2 for instance Just for fun let s try 6 zeta 2 Pi 2 Pretty close no Now gp didn t seem to know what log x was although it did know how to compute numerical values of log This is annoying Maybe it knows the exponential function Let s give it a try T Type exp x What s this If you had any experience with other computer algebra systems the answer should have simply been exp x again But here the answer is the Taylor expansion of the function around x 0 to 16 terms 16 is the default seriesprecision which can be changed by typing ps n or default seriesprecision n where n is the number of terms that you want in your power series Note the 0 x 16 which ends the series and which is trademark of this type of object in gp It is the familiar big oh notation of analysis You thus automatically get the Tay
91. ue 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 gp has to say about this Type 1 7 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 771 but why complicate matters Seriously the basic point here is that PARI hence 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 4 Pree eB NN IN NNNWN 0 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 0 1 or 7 which are imprecise real numbers hence 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 preci
92. ult precision is still 38 you can check using the method above that we have in fact 96 significant digits The reason is that 1 1E 40 is computed according to the PARI philosophy i e to the best possible precision Since 1E 40 has 39 significant digits and 1 has infinite precision the number 1 1E 30 will have 79 39 40 significant digits hence f also Now type cos 1E 19 The result is printed as 1 0000 but is of course not exactly equal to 1 Using 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 hence the precision is going to be approximately the same as when computing this quantity here 1 0 5 x 1079 where 0 5 x 1073 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 cos 0 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 gp gives you 1 000 with as many mantissa word as the current precision Let s see some more transcendental functions at work Type gamma 10 No problem type 9 to check T
93. um t 0 Pi 2 1 sqrt 1 sin t 72 and the result is the same The elementary transformation x sin t gives the mathematical equality T 1 dz E Vi xaf 2AGM 1 V2 which was one of Gauss s remarkable discoveries in his youth Now type 2 agm 1 1 1 1 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 agm 1 1 7 0 7710 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 agm a b can be computed only when a b is congruent to 1 modulo 16 and not 8 as could be expected Now type 3 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 7 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 7 4 1
94. us that under GRH the class number is equal to 1 and the regulator is approximately equal to 2641 5 You may certify this with bnf bnfinit x72 D 1 insist on finding fundamental unit bnfcertify bnf although it s a little inefficient Indeed bnfcertify needs the fundamental unit which is so large that bnfinit will have a hard time computing it it needs about R log 10 1147 digits of precision So that it would have given up had we not inserted the flag 1 See bnf fu On the other hand you can try quadunit D Impressive isn t it You can check that its logarithm is indeed equal to the regulator Now just as an example let s assume that we want the regulator to 500 decimals say Without cheating and computing the fundamental unit exactly first I claim that simply from the crude approximation above this can be computed with no effort This time we want to start with the unit form Type u gfbred qfbprimeform D 1 2 We use the function qgfbred with no distance since we want the initial distance to be equal to 0 Now type f gfbred 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 of f is around 4 253 Very crudely since we want a distance of 2641 5 this should be encountered approximately at 2641 5 4 253 621 times the distance of f Hence as a first try we type 621 Oops w
95. ust be a torsion point Indeed typing for k 1 5 print ellmul E q k ellorder E q simpler we see in two different ways that q is a point of order 5 Moreover typing elltors E shows that q generates all the torsion of E which is cyclic of order 5 Let s try still another curve y y x 7x 6 e ellinit 0 0 1 7 6 ellglobalred e 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 elltors e shows that the torsion subgroup is trivial so we don t have to worry about torsion points Next the member e roots gives us the 3 roots of the minimal equation over C i e Y X3 7X 25 4 set X Y x y 1 2 so if x y is a real point on the curve x must be at least equal to the smallest root i e x gt 3 Finally if x y is on the curve its opposite is clearly x y 1 Type 26 v ListO for x 3 1000 s ellordinate e x if Hs listput v x s 111 orif Hs 0 v If cardinality of s is non zero insert the first point with given x We thus get a large number 18 of integral points Together with their opposites and the point at infinity this makes a total of 37 integral points which 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 mu
96. ve has trivial torsion as we checked earlier 2 In the above calculation we were lucky that all the v j were Z linear combinations of v 1 v 2 and v 3 and not just Q linear combinations in general the result of matsolve m ellbil e vp Q might have given a vector of rationals if k gt 2 is minimal such that kQ is in the subgroup generated by v 1 v 2 and v 3 then the entries of matsolve m ellbil e vp Q will be rationals with common denominator k Let us explore a few more elliptic curve related functions Keep our curve e of rank 3 and type vi 1 0 z1 ellpointtoz e vi v2 2 0 z2 ellpointtoz e v2 We thus get the complex parametrization of the curve To add the points vi and v2 we should of course type elladd e v1 v2 but we can also type ellztopoint e z1 z2 which has the disadvantage of giving complex numbers but illustrates how the group law on e is obtained from the addition law on C Type f x Ser ellan e 30 This gives a power series which is the Fourier expansion of a modular form of weight 2 for P0 5077 This has been proved directly before Wiles s general result In fact to find the modular parametrization of the curve type modul elltaniyama e u modul 1 v modul 2 We can check in various ways that this indeed parametrizes the curve v72 v u73 7 u 6 is close to O for instance or simply that ellisoncurve e modul returns 1 Now type x u 2xv 1 an
97. vice 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 plot term dumb of gnuplot 14 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 gp in particular how to define functions It s possible to define member function as well but we trust you to find them in the manual We have seen most control statements the missing ones while break next return and various 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 print to screen write to a file Input via read from file input querying user or extern from an external auxiliary program Perhaps the most useful simple command we haven t seen yet is allocatemem which increase the size of gp s scratch space If you regularly see PARI stack overflows messages think about this one To customize gp e g increase the default stack space or load your private script libraries on startup look up The preferences file section in the User s manual 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 my One is usually better off avoiding global variables altogether
98. ying to remove it should produce an error in this case There are m 1 6 such vectors and m 3 gives half of them 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 m qfminim G norm12 M 4 100 2 The flag 2 instructs 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 matrank m 3 says the 100 we have are all included in a 2 dimensional space Let us try m qfminim G norml2 M 4 1 100000 2 This time we find 50886 vectors of the requested length spanning a 4 dimensional space which is actually generated by M 11 M 2 M 3 and M 5 16 6 Using Transcendental Functions All the elementary transcendental functions and several higher transcendental functions are provided T function incomplete function error function exponential integral Bessel functions H H I J K N confluent hypergeometric functions Riemann 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 exa
99. you haven t improved things by increasing the precision Or have you What if we retype exp 100 now that we have 50 digits Try it Now we no longer have an exponential format 6 What if I forget what the current precision is and I don t feel like counting all the decimals Well you can type p by itself You may also learn about gp internal variables and change them using default Type default realprecision then default realprecision 38 Huh In fact this last command is strictly equivalent to Xp 38 Admittedly more cumbersome to type 6 There are more defaults than just format and realprecision type default by itself now they are all there 7 Note that the default command reacts differently according to the number of input argu ments This is not an uncommon behavior for gp functions You can see this from the online help or the complete description in Chapter 3 any argument surrounded by braces in the function prototype is optional which really means that a default argument will be supplied by gp You can then check out from the text what effect a given value will have and in particular the default one 8 Try the following starting in precision 38 type first default format e0 100 then exp 1 Where are my 100 significant digits Well default format only changes the output format but not the default precision On the other hand the Xp command changes both the precision and the output format 2 W
100. ype 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 gamma 100 has 156 decimal digits type p 170 to be on the safe side then gamma 100 once again Another one is to compute 99 directly 17 Try gamma 1 2 10 1I No problem we have the complex T function Now type t 1000 z gamma 1 Ixt t7 1 2 exp Pi 2 t sqrt 2 xPi norm z The latter is very close to 1 in accordance with the complex Stirling formula Let s play now with the Riemann zeta function First turn on the timer type Type zeta 2 then Pi 2 6 This seems correct Type zeta 3 All this takes essentially no time at all However type 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 p1000 This is because PARI uses special formulas to compute zeta n when n is an integer Type zeta 1 1 This also works Now for fun let us compute in a naive way the first complex zero of zeta We know that it is of the form 1 2 i xt 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 zeta gp then type r zeta gp under gp to read it in ti 1 2 14 t2 1 2 15 I eps 1E 50 zi zeta ti until norm z2 lt ep

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