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Legendre's formula gives the exponent of a prime p in n! by summing how many multiples of p, p^2, p^3, ... are ≤ n.
Prime factorization expresses any integer greater than 1 as a product of primes raised to powers, uniquely up to ordering.
The Sieve of Eratosthenes marks multiples of each prime to find all primes up to n in O(n log log n) time.