Re: round4 bug

[Karim BELABAS <Karim.Belabas@math.u-psud.fr>]

On Mon, 14 Apr 2003, Igor Schein wrote:
> ? nfdisc(x^8-18523911338877795571979541015892615821899371353007053617321226050166675063444487117209600000002*x^6+66046032206948280681505445107777407158857150778666621415782489010349723670806137312162405167710337882006595358980644793230404795173825438833351539869976139417951596742423912542341086717592928772515031154397064403303886209889075200000016*x^4-611715422443088666756911217616327714005525597151441388699040484267060775757052196963817130148238195987896644874882570815334090128515884018736389189598104883097830045803551177719524888951453810182790127699384164247291566458717718841875962598408993568359068747730396358706617053542591613414581062482029464867475302414234419200000000*x^2+1090519592570312394824742937139044995002346676910200234065413483369651285433328371966513980373672720867274391022554420298936550273060002685473689170154581627128631733496643010662055728590929714586890390760249491371247258364903201681264280750249866551730999657777761480702970381696097426333212384757160277240403200637394323973803072310856025374382648070080573188733759065218101926564609509283848815844890208490116444737131659677353548170380498496976490332160000000000000000,1);
>   ***   bug in GP (Segmentation Fault), please report
>
> This bug goes back to version 2.2.4.  Increasing primelimit from default 5e5 hides it.

Problem arising when using composites instead of bona fide primes. In most
cases, this is detected by an "impossible inverse" error (which is trapped).
Here, the errror occurs in a different place, and does not exhibit a factor.
So I added some checks to detect this kind of things.

It turns out that special casing the trivial case f(0) = 0 in factormod
enables this specific example to run to completion, without triggering the
new check [ and is a cheap optimization in any case! ].

    Karim.
-- 
Karim Belabas                     Tel: (+33) (0)1 69 15 57 48
Dép. de Mathématiques, Bât. 425   Fax: (+33) (0)1 69 15 60 19
Université Paris-Sud              http://www.math.u-psud.fr/~belabas/
F-91405 Orsay (France)            http://www.parigp-home.de/  [PARI/GP]