Karim BELABAS on Thu, 12 Sep 2002 03:08:56 +0200 (MEST)

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Re: Bug in ellap() (fwd)

On Wed, 11 Sep 2002, Karim BELABAS wrote:
> Attached is a transcript of a gp session which demonstrates
> that there is a bug in `ellak' for version 2.2.3alpha (using
> version 2.1.4 yields identical results). I inserted newlines
> for added clarity.

Case closed.

It was quite a weird bug, with a silly cause: [P] -> -[P] did not reduce its
result modulo p ("not needed"), which in very rare circumstances could produce
a dummy relation because when adding two points with the same x-coordinate,
the code checks whether the y coordinate are equal (over Z, _not_ mod p), and
decide the result is 0 if not ...

It's fixed in CVS. [both stable and unstable]

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/