φ(n) is the number of integers k with 1 ≤ k ≤ n and gcd(k,n) = 1. For prime p, φ(p) = p−1; for n = p^a, φ(n) = p^a − p^(a−1); φ is multiplicative.. The key formula is φ(n) = |{k : 1≤k≤n, gcd(k,n)=1}|. This concept is typically introduced in College. This algebraic concept helps students understand relationships and patterns in mathematics.