Two integers are relatively prime (or coprime) if they have no common
factors other than 1. For example, 21 = 3 × 7 and 8 = 2 × 2 × 2 are relatively prime because
they have no common factors other than 1. By defi nition –1 and 1 are relatively prime to every
integer, and they are the only numbers relatively prime to 0.