$X$ is locally compact, Hausdorff, and has a countable family of compact subsets covering $X$, then $X$ is second-countable

general-topology

Suppose $X$ is locally compact, Hausdorff, and has a countable family of compact subsets whose union is all of $X$. I want to show that $X$ is second-countable.
Suppose the family of countable compact subsets are labelled $C_i$, $i \in \Bbb N$,
Then there are a couple of ideas that come to mind but I am having trouble putting it all together.

1) Taking the complements of the $C_i$ (problem: what if an element is in all of them, also the complements might not satisfy the requirements of a basis)

2) Using local compactness to say that every point has a compact set containing it with an open neighborhood (also containing it), but how will this relate to the $C_i$ ?

Any hints appreciated.

Best Answer

It is not even true that compact Hausdorff spaces are second-countable. (Of course, a compact Hausdorff space is locally compact and can be covered by a countable family of compact subsets.)

An example is the ordinal space $[ 0 , \omega_1 ]$ (where $\omega_1$ denotes the least uncountable ordinal).

  • As a linearly-ordered space, $[0 , \omega_1 ]$ is Hausdorff.
  • Since $[0 , \omega_1]$ has a greatest element, it is compact (this is essentially because there are no infinite strictly decreasing sequences of ordinals).
  • It is not second-countable because any base must include the singletons $\{ \alpha \}$ where $\alpha < \omega_1$ is a successor ordinal, and there are uncountably many of these.

More information about this space can be found on the following post on Dan Ma's Topology Blog:

(If you want a non-compact example of a locally compact σ-compact Hausdorff space that is not second-countable you can just take a disjoint union of countably-infinite copies of $[0,\omega_1]$.)