Two positive integers and are said to be relatively prime or coprime if they share no common divisors greater than 1. That is, their greatest common divisor is . Equivalently, and must have no prime divisors in common. The positive integers and are relatively prime if and only if is in lowest terms.
Relatively prime numbers show up frequently in number theory formulas and derivations:
Euler's totient function determines the number of positive integers less than any given positive integer that is relatively prime to that number.
Consecutive positive integers are always relatively prime, since, if a prime divides both and , then it must divide their difference , which is impossible since .
Two integers and are relatively prime if and only if there exist some such that (a special case of Bezout's Lemma). The Euclidean Algorithm can be used to compute the coefficients .
For two relatively prime numbers, their least common multiple is their product. This pops up in Chinese Remainder Theorem.
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