first countable implies compactly generated
Proposition 1.
Any first countable topological space is compactly generated.
Proof.
Suppose is first countable, and has the property that, if is any compact set in , the set is closed in . We want to show tht is closed in . Since is first countable, this is equivalent to showing that any sequence in converging to implies that . Let .
Lemma 1.
is compact.
Proof.
Let be a collection of open sets covering . So for some . Since is open, there is a positive integer such that for all . Now, each for . So is covered by , and , showing that is compact.∎
In addition, as a subspace of , is also first countable. By assumption
, is closed in . Since for all , we see that as well, since is first countable. Hence , and is closed in .∎