If $\Bbb N$ denotes all positive integers. Then find all functions $f: \mathbb{N} \to \mathbb{N}$ which are strictly increasing and such for all positive integers $n$, we have:
$$f(f(n)) = n+2$$
So far I know that $f(n)$ is greater than $n$ because the function is strictly increasing, but I'm not sure how to use this in order to solve the equation.
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