Fermat primality test - Wikipedia
https://en.wikipedia.org/wiki/Fermat_primality_testFermat's little theorem states that if p is prime and a is not divisible by p, then If one wants to test whether p is prime, then we can pick random integers a not divisible by p and see whether the equality holds. If the equality does not hold for a value of a, then p is composite. This congruence is unlikely to hold for a random a if p is composite. Therefore, if the equality does hold for one or more values of a, then we say that p is probably prime.