Ononot — Find the number of functions f: \{1, 2, 3, 4, 5\} \to \{1, 2

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Combinatoricshard

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Find the number of functions $f: \{1, 2, 3, 4, 5\} \to \{1, 2, 3, 4, 5\}$ such that $f(f(x)) = x$ for all $x$ .