Description:
Let $$$M=\{1,2,3,4,5,6,7\}$$$ be a set with $$$7$$$ elements. Find the number of surjective functions $$$f:M\to M$$$ such that $$$f(f(a))\neq a$$$ and $$$f(f(f(a)))\neq a$$$ for all $$$a\in M$$$.
Note: A function $$$f:A \to B$$$ is called surjective if for each $$$b \in B$$$ there exists $$$a\in A$$$ such that $$$f(a)=b$$$.
Input Format:
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Output Format:
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Note:
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