Karim BELABAS on Mon, 8 Sep 2003 13:57:58 +0200 (MEST)

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Re: nfeltpowmodpr segfault on degree-1 ideals

On Mon, 8 Sep 2003, Bill Allombert wrote:

> Dear PARI dev
> It seems that nfeltpowmodpr SEGV on degree 1 ideal:
> ? K=nfinit(x^2+1);pr=nfmodprinit(K,idealprimedec(K,5)[1]);
> ? nfeltpowmodpr(K,x,2,pr);
>   ***   bug in GP (Segmentation Fault), please report
> For some reason to_ff_init return NULL for the polynomial,
> but FpXQ_pow will not handle it.

All FpXQxxx(..., T,p) routines [ computations with lifted elements of
Fp[X]/(T) ] were supposed to handle the case of prime fields [ T = NULL ].
For some reason, this one did not...



P.S: the nf*modpr routines are obsolete and simple GP wrappers around more
useful library routines, retained for backward compatibility since there's no
GP interface to work efficiently in finite fields.

Basically, it's a waste to work in O_K / pr [ "dimension n" ], rather than
directly in the isomorphic finite field [ "dimension f(pr/p) ].

nf_to_ff / ff_to_nf provide the required conversions routines, which should
only be used once [ the wrappers use them in the obvious way... ]

P.S2: there was some problems with the precise specifications of the
"modular" routines, e.g

* are t_INT allowed as representants for elements in polynomial quotient rings
(currently "often". Fqxxx routines should be used instead but...).

* does omitting an argument (p = NULL or T = NULL) corresponds to cancelling
the reduction (mod p or mod T), or does T = NULL correspond to the more
specific case of a prime field.  The latter is more useful since one can
suppress the reduction mod p to work in char. 0 quotient rings R[X] / (T),
whereas there's no reason to use a FpXQ routine and to suppress reduction
mod T.

Since the routines are not yet documented, it would be a good idea to decide
once and for all.
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]