| John Cremona on Tue, 17 Jun 2014 15:26:10 +0200 |
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| Re: precision problem in nffactor |
Thanks, Karim! Am I right in thinking that there is no issquare() function for number field elements (or a sqrt() function similarly)? John On 17 June 2014 12:33, Karim Belabas <Karim.Belabas@math.u-bordeaux.fr> wrote: > * John Cremona [2014-06-17 10:27]: >> In 2.7.1 I see this: >> >> ? nffactor(nfinit(y^2 - 22), Mod(1, y^2 - 22)*x^2 + >> Mod(926246528884912528275985458927067632*y - >> 4344481316563541186659879867597013188, y^2 - 22)) >> *** at top-level: nffactor(nfinit(y^2- >> *** ^-------------------- >> *** nffactor: the PARI stack overflows ! >> current stack size: 8000000 (7.629 Mbytes) >> [hint] you can increase GP stack with allocatemem() >> >> but increasing the precision makes the problem go away: > [...] >> so it does not seem like a memory issue at all. Surely with exact >> input and output the user should not have to think about internal >> working precision? > > Not a memory issue indeed. The low-level function from the polroots > module that (sharply) estimates the largest modulus of a polynomial > T can't handle the case T(0) = 0 (nor the case where T is constant, > but that's not the issue here...). > > It never occurs in the original context of polroots(), but it can occur > in the context of nfroots / nffactor that call it with T := the image > through the successive complex embeddings of our algebraic polynomial. > > Fixed in master (2.8.*). Thanks for your report ! > > Cheers, > > K.B. > > P.S. >> This arose while computing the torsion subgroup of an elliptic curve >> over Q(sqrt(22)) [...] > > I'll soon commit the code corresponding to this :-) (written by F. Brunault > and B. Banwait during the Atelier, last January) > > -- > Karim Belabas, IMB (UMR 5251) Tel: (+33) (0)5 40 00 26 17 > Universite Bordeaux 1 Fax: (+33) (0)5 40 00 69 50 > 351, cours de la Liberation http://www.math.u-bordeaux1.fr/~kbelabas/ > F-33405 Talence (France) http://pari.math.u-bordeaux1.fr/ [PARI/GP] > `