Show that a function is not a PRFs
Any known plaintext attack instantly reveals the key:
$$f_k(x) - x + 2^n \equiv k^2\mod 2^n$$

On top of that, even without knowing the key or the plaintext, here's a property that no random function should have:

$$f_k(x + 1) \equiv f_k(x) + 1 \mod 2^n$$