Karim BELABAS on Mon, 12 Aug 2002 17:48:38 +0200 (MEST)


[Date Prev] [Date Next] [Thread Prev] [Thread Next] [Date Index] [Thread Index]

Re: Bug in elltors


On Mon, 12 Aug 2002, Karim BELABAS wrote:
> On Fri, 19 Jul 2002, Mark J Watkins wrote:
> [directly to me. Forwarded to pari-dev]
>
> > Every version of PARI which I have computes the torsion group for
> > [0,0,0,-1032,-1031] and [0,0,0,-6648,208633] incorrectly,
> > saying they have C2 torsion instead of C6 torsion.
> > [...]
[...]
> I've rechecked the code and could find no other obvious slip. Does anybody
> have an extensive database of curves together with their torsion group that
> we could use to thoroughly check the routine ?

Update: John Cremona has been so kind as to send me his test suite ( 5114
curves, including several examples for all possible torsion groups ).

The current CVS code passes successfully [ gp refuses to answer thrice due to
insufficient precision, but gives the correct result in these 3 cases also,
once precision is increased ].

Unfortunately, so does the old (incorrect) code, which fails on the
aforementioned two curves !

    Karim.
-- 
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/