Euler's Totient Function

Euler's Totient Function $\phi(n)$ counts the numbers less than $n$ that are co-prime with $n$. For a prime number $p$, $\phi(p) = p-1$, for the product of two primes $p$ and $q$, $\phi(pq) = (p-1)(q-1)$. For example: $\phi(14) = \phi(2 × 7) = (2-1)(7-1) = 6$

Use in the RSA calculation, as part of the private key calculation.