Question 60
Find all functions from the set of the real numbers to the set of the real numbers which satisfy the functional equation
$$
f(f(x + y)) = f(x^2 - y^2) + 4xyf(x + y)
$$
for all real numbers $x$ and $y$.
$$
f(f(x + y)) = f(x^2 - y^2) + 4xyf(x + y)
$$
for all real numbers $x$ and $y$.