The function "φ(n)", known as the Euler's totient function, counts the number of positive integers up to a given integer n that are relatively prime to n. Two numbers are considered relatively prime if their greatest common divisor (GCD) is 1. For example, φ(9) equals 6 because the numbers 1, 2, 4, 5, 7, and 8 are all relatively prime to 9. The value of φ(n) can be calculated using the prime factorization of n. If n is expressed as the product of its prime factors, φ(n) can be determined using the formula: φ(n) = n × (1 - 1/p1) × (1 - 1/p2) ... for each distinct prime factor p. This function is important in number theory and has applications in areas like cryptography.