Question 94
Let $f: \mathbb{R} \to \mathbb{Z}$ be a function satisfying
$$
f(x - y) - 2f(x) + f(x + y) \geq -1
$$
for all $x, y \in \mathbb{R}$. Find all possible values of the set $\{f(x) \mid x \in \mathbb{R}\}$.
$$
f(x - y) - 2f(x) + f(x + y) \geq -1
$$
for all $x, y \in \mathbb{R}$. Find all possible values of the set $\{f(x) \mid x \in \mathbb{R}\}$.