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All functions f over Z such that (f(x + y))^2 = f(x^2) + f(y^2) [PAMO 2018]

Source: Pan African Math Olympiad 2018, Problem 1

June 30, 2018

Problem Statement

Find all functions f:ZZf : \mathbb Z \to \mathbb Z such that (f(x+y))2=f(x2)+f(y2)(f(x + y))^2 = f(x^2) + f(y^2) for all x,yZx, y \in \mathbb Z.