For prime p and integer a with gcd(a,p)=1: a^(p-1) ≡ 1 (mod p). Generalised by Euler to a^φ(n) ≡ 1 (mod n). Basis of primality testing.
For prime p and integer a with gcd(a,p)=1: a^(p-1) ≡ 1 (mod p). Generalised by Euler to a^φ(n) ≡ 1 (mod n). Basis of primality testing.