If a and b are integers and m is a positive integer, then a is congruent to b modulo m if m divides a − b▪notation a ≡ b (mod m)▪We say that a ≡ b (mod m) is a congruence ▪m is its modulus (plural moduli)▪If a and b are not congruent modulo m⟹a≢b (mod m)13