Please give a a reason, why 1 is a composite

Vincent Lefevre vincent at
Wed Mar 25 19:17:50 UTC 2015

On 2015-03-25 08:45:53 +0100, tg at wrote:
> There are texts (older ones more than modern ones) that define 1 to be
> prime.  The common view today is that it is not prime (and certainly not
> a composite).

I agree, but this is not what the GMP 6.0.0 manual says:

 -- Function: int mpz_probab_prime_p (const mpz_t N, int REPS)
     Determine whether N is prime.  Return 2 if N is definitely prime,
     return 1 if N is probably prime (without being certain), or return
     0 if N is definitely composite.

which implies that a number is either prime or composite. This is wrong!

Vincent Lefèvre <vincent at> - Web: <>
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