Recall that a compact Hausdorff space is second countable if and only if the Banach space $C(X)$ of continuous functions on $X$ is separable. I'm looking for a similar criterion for locally compact Hausdorff spaces, using $C_0(X)$ (the Banach space of continuous functions vanishing at infinity) instead of $C(X)$.
Naive Guess: Suppose $X$ is a second countable locally compact Hausdorff space. Then $C_0(X)$ is separable.
Proposed Proof: Since $X$ is second countable, so is its one-point compactification $\tilde{X}$. By the theorem above $C(\tilde{X})$ is is separable, and thus $C_0(X)$ is separable since subspaces of separable metric spaces are separable (not true for subspaces of arbitrary separable spaces, but OK for separable metric spaces). QED.
Problem: I'm not sure if the first line is true. This seems to be equivalent to the statement that every second countable locally compact Hausdorff space is the countable union of compact subspaces, and I can't find a proof or a counter-example. If this turns out to be false, what IS the right criterion for $C_0(X)$ to be separable?
Best Answer
Your argument is correct, thanks to the following result.
Of course a second countable space is Lindelöf, so you get all of (1)-(4).