Problem #WSP-5316

Problem

Let $ϕ (n)$ be the Euler’s function, namely the amount of numbers from $1$ to $n$ , coprime with $n$ . For two natural numbers $m, n$ such that $GCD (m, n) = 1$ prove that $ϕ (m n) = ϕ (m) ϕ (n)$ .

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