Question 121
Find all functions $f: \mathbb{R} \to \mathbb{R}$ such that
$$ f(f(y) + 2 + x) + f(f(y) - x) = y f(y)(x + 1) $$
holds for all real numbers $x$ and $y$.
$$ f(f(y) + 2 + x) + f(f(y) - x) = y f(y)(x + 1) $$
holds for all real numbers $x$ and $y$.