Relatively prime numbers

Two whole numbers are said to be relatively prime when they have no common divisor apart from one. For example, 5 and 12 are relatively prime, but not 12 and 15, which have 3 as a common divisor.

In an equivalent way, two whole numbers a and b are relatively prime when the fraction a/b is irreducible.

Euclid's algorithm can be used to determine the highest common divisor of two whole numbers, and thus to test whether they are relatively prime.