# gmp prime testing

**Niels Möller
**
nisse@lysator.liu.se

*06 Nov 2002 18:01:02 +0100*

Torbjorn Granlund <tege@swox.com> writes:
>* > > ul mpz_trial_div(mpz_srcptr N , ul start , ul stop)
*>* >
*>* > Sounds fair to me, if Torbjorn likes it.
*>*
*>* Might make sense. What is it supposed to return?
*>* Any factor, or 1 (or 0?) if no factor is found.
*
If any reasonable implementation will compute a factor, I'd say return
that factor or 0 if none was found. I think the trivial factors, i.e.
1 and N, should never be returned.
If it's easy to make some additional guarantees (say, that if ul == 2,
then the returned factor will be a multiple of the smallest prime
factor) then that would be nice too, but I don't know if a
smallest-primefactor-of function is really useful.
/Niels