Euler's totient function φ(n) counts the positive integers up to n that are coprime to n. For a prime power p^k, φ(p^k) = p^(k-1)(p-1). Since φ is multiplicative, φ(n) = n∏(1 - 1/p) over primes dividing n.
Compute φ(n) for small values and verify the formula. Recognize the multiplicative structure and its connection to cyclic groups.
Thinking φ(n) requires checking all n integers (use the formula instead). Confusing φ with other arithmetic functions like σ or τ.
The totient function asks: among the integers 1, 2, ..., n, how many share no common factor with n? If n = 12, the integers coprime to 12 are {1, 5, 7, 11} — just those sharing no factor of 2 or 3 — so φ(12) = 4. You already know modular arithmetic, so you know that working "mod n" means working within a set of residues; φ(n) precisely counts the size of the multiplicative group mod n, the residues that have multiplicative inverses. This connection is why the totient function appears everywhere in number theory and cryptography.
The formula φ(n) = n∏(1 − 1/p) over primes p dividing n comes from inclusion-exclusion. Start with n integers; remove the multiples of p₁ (there are n/p₁ of them), then the multiples of p₂, then add back the double-counted multiples of p₁p₂, and so on. Each prime factor p "knocks out" a fraction 1/p of the survivors, leaving a fraction (1 − 1/p). For n = 12 = 2² × 3: φ(12) = 12 × (1 − 1/2) × (1 − 1/3) = 12 × 1/2 × 2/3 = 4. The multiplicativity you studied — that φ(mn) = φ(m)φ(n) whenever gcd(m, n) = 1 — lets you decompose any n into its prime power factors first: φ(p^k) = p^{k−1}(p−1), because the only numbers not coprime to p^k are its p^{k−1} multiples.
The power of φ becomes visible in Euler's theorem: if gcd(a, n) = 1, then a^{φ(n)} ≡ 1 (mod n). This is a direct consequence of the multiplicative group having order φ(n) — every element raised to the group's order returns to the identity. Fermat's little theorem is the special case n = p prime, where φ(p) = p − 1. This interplay between counting (φ), modular arithmetic, and group structure is a microcosm of how number theory works: a simple counting question opens into deep algebraic structure.
The RSA cryptosystem rests entirely on φ. Given n = pq (a product of two large primes), φ(n) = (p−1)(q−1) is easy to compute if you know p and q, but essentially impossible if you only know n (factoring large integers is computationally hard). The encryption and decryption exponents are chosen to be inverses mod φ(n), and the security guarantee — that decryption undoes encryption — follows directly from Euler's theorem. The totient function thus bridges elementary arithmetic counting and the algorithms securing modern internet communication.