Assume we are solving $\min\{f(x) \ | \ x \in S \}$.
If $f: \mathbb{R}^n \mapsto \mathbb{R}$ is a proper closed convex function, and $S$ is a non-empty closed convex set, does this imply that the above minimization problem has a non-empty solution set? Does an optimal solution always exist in such a setting?
What I know is that (by Wikipedia) a proper convex function is closed if and only if it is lower semi-continuous. Moreover, if I know $f$ is proper closed convex, then this implies the function is lower semi-continuous. By the extension of extreme value theorem to semi-continuous functions, we know that the above minimization has a non-empty solution set since $f$ is lower semi-continuous. So maybe this is it...
Edit: Based on the answer to this question below, I see that such an optimal value always exists. However, in Amir Beck's first-order methods book, there is the following set of assumptions:
Then don't (A,B,C) imply (D) anyways? Why are we also assuming (D) at all?