The number \(a\) is relatively prime to \(b\) if \(a\) has no common factors with \(b\), other than 1.

There are lots of ways of expressing this:

  • \(a\) is prime relative to \(b\).
  • \(a\) is mutually prime to \(b\).
  • \(a\) is coprime to \(b\).
  • \(\mathrm{hcf}(a,b) = 1\).
  • \(a \perp b\).

For example, 14 and 15 are coprime to each other as \(14 = 2 \times 7\), \(15 = 3\times 5\)—that is, they have no common factors.