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Euler's Totient Function Explained — Phi(n) Computation, Sieve Variant in O(N log log N) [Cryptography, Google]
Eulers totient function phi(n) counts integers up to n coprime to n and underpins RSA, modular inverse for composite moduli, and Eulers theorem. Master the prime-factorization formula, the sieve variant in O(N log log N), and the multiplicative-function tricks that make phi indispensable for competitive programming.
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