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Euler's totient φ(n) counts the integers from 1 to n coprime to n. It is computed as n·Π(1 − 1/p) over the distinct prime factors, and equals n − 1 exactly when n is prime.
Euler's totient
φ(n) = n · Π over distinct primes p | n of (1 − 1/p)
36 = 2² × 3², so φ = 36 × ½ × ⅔ = 12.
RSA encryption relies on Euler's theorem a^φ(n) ≡ 1 (mod n) to construct the decryption exponent.