Karim BELABAS on Thu, 5 Sep 2002 23:04:33 +0200 (MEST) |
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Re: rnfisnorm |
On Mon, 2 Sep 2002, Martin Bright wrote: > I'm encountering strange behaviour with rnfisnorm in GP/PARI 2.2.3. The > example in the manual works fine, but if I change it a bit: > > ? bnf = bnfinit(y^2-35); > ? p = x^2-42; > ? rnf = rnfequation(bnf,p,1); > ? ext = [p, rnf[2], bnfinit(rnf[1])]; > ? rnfisnorm(bnf,ext,10,0) > *** inconsistent data in inverseimage. > ? rnfisnorm(bnf,ext,17,0) > *** bug in GP (Segmentation Fault), please report > > Am I doing something stupid, or should it work? I had never checked that function. As it stands, rnfisnorm is badly broken [I could even crash gp playing with it]. This is true in all versions of PARI featuring this routine, not only the 2.2.3-alpha... The "ext" paremeter described in the manual does not make sense. One needs to set ext = [p, rnf[3], bnfinit(rnf[1])]; ^^^-------------------- _not_ rnf[2] AND to patch the function [ because rnf[3] is used in quite a different way than (the useless) rnf[2] would ]. I have fixed this in the unstable CVS [and don't want to backport this yet]. Since you are testing an alpha version of PARI, you might as well update from CVS and get an up to date development version. Thanks for spotting this! Karim. P.S: It makes more sense to introduce rnfisnorminit(bnf, p) as in S = rnfisnorminit(bnf, p) \\ black box encapsulating everything we need rnfisnorm(S, x, flag) \\ solve Ny = x for arbitrary RHS Since the function never worked, I'm not breaking backward compatibility by changing the interface, and it's more convenient this way. I will do this shortly. P.S2: There is a special case where the function _would_ work: iff rnf[3] is 0. [ that is iff Res_Y(bnf.pol(Y), p(X,Y)) is squarefree, which is never the case if p does not depend from Y (unless bnf = Q) ] -- Karim Belabas Tel: (+33) (0)1 69 15 57 48 Dép. de Mathematiques, Bat. 425 Fax: (+33) (0)1 69 15 60 19 Université Paris-Sud Email: Karim.Belabas@math.u-psud.fr F-91405 Orsay (France) http://www.math.u-psud.fr/~belabas/ -- PARI/GP Home Page: http://www.parigp-home.de/