Fermat's Little Theorem

Fermat's Little Theorem (so called to distinguish it from Fermat's Last Theorem) is that:

if p is a prime number, then for any integer x, the number x^p − x is an integer multiple of x.

Alternatively, x^p ≡ x (mod p).

Stated another way:

if p is prime and b < p, than b^(p-1) = 1 (mod p).