IMO Shortlist 2014 problem A6

Find all functions $f : \mathbb{Z} \to \mathbb{Z}$ such that $n^2 + 4 f(n) = f(f(n))^2$ for all $n \in \mathbb{Z}$ .

(United Kingdom)

Izvor: https://www.imo-official.org/problems/IMO2014SL.pdf

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