mpz_prevprime

[Marco Bodrato]

Ciao,

Il 2020-10-03 03:58 Seth Troisi ha scritto:
> I modified the patch a tiny bit. Still hoping to get this in for an

I think that the patch is interesting: a function for searching primes 
backward in the sequence of integers is missing and seems useful.

The proposed interface is the following.

> @deftypefun int mpz_prevprime (mpz_t @var{rop}, const mpz_t @var{op})
> @cindex Previous prime function
> Set @var{rop} to the greatest prime less than @var{op}.

> If a previous prime doesn't exist (i.e. @var{op} < 3), rop is unchanged 
> and
> 0 is returned.

> Return 1 if @var{rop} is a probably prime, and 2 if @var{rop} is 
> definitely
> prime.

I personally do not like the idea that a previous prime can "not exist", 
because in my opinion -2 is a prime, and there are as many negative 
primes as there are positive ones... The function mpz_probab_prime_p in 
our library agrees with my opinion... but... ok, that's my opinion only.

Anyway, the return value is used also to return something more 
interesting: 1 or 2, with the same meaning that the return value has for 
the function mpz_probab_prime_p.
Should we add a return value also to the function mpz_nextprime?

Ĝis,
m