Re: bug in ellglobalred()

["Karim Belabas" <belabas@math.u-bordeaux1.fr>]

* John Cremona [2005-11-02 18:02]:
> The following is wrong:
> 
>          GP/PARI CALCULATOR Version 2.2.12 (development CHANGES-1.1375)
>             i686 running linux (ix86/GMP-4.1.4 kernel) 32-bit version
>      compiled: Nov  2 2005, gcc-3.4.3 20041212 (Red Hat 3.4.3-9.EL4)
>                (readline v4.3 enabled, extended help available)
> 
> 
> (16:29) gp > ellglobalred(ellinit( [0,0,0,-13980,313328]))
> %1 = [42816, [1, 0, 0, 0], 24]
> 
> -- the conductor should be 128448, i.e. the exponent of 3 should be 2
> and not 1.
> 
> A fix would bemuch appreciated -- someone must have an idea of which
> recent changes might have caused this.

I'm responsible for this. Some "nice" formula for the number of roots of
a cubic polynomial X^3 + aX^2 + bX + c mod 3 turns out to be invalid when
operands are interpreted as unsigned integers (used to be signed integers,
before a cleanup of mine).

Do you have a list of examples handy for all types of reduction mod 2 and 3 ?
I'd like to build a regression suite for this function...

Thanks!

    Karim.
-- 
Karim Belabas                  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-bordeaux.fr/~belabas/
F-33405 Talence (France)       http://pari.math.u-bordeaux.fr/  [PARI/GP]