φ(n)

Certainly! The function φ(n), called Euler's totient function, counts the number of positive integers up to n that are relatively prime to n—that is, numbers less than or equal to n that share no common factors with n other than 1. For example, if n is 12, φ(12) equals 4 because the numbers 1, 5, 7, and 11 are all less than 12 and have no common factors with 12 other than 1. This function is fundamental in number theory and plays a key role in applications like cryptography, particularly in algorithms such as RSA.