compact subspace of a Hausdorff space is closed
Let be a Hausdorff space, and be a compact subspace
of . We prove that is open, by finding for every point a neighborhood
disjoint from .
Let . , so by the definition of a Hausdorff space, there exist open neighborhoods of and of such that . Clearly
but since is compact, we can select from these a finite subcover of
Now for every there exists such that . Since and are disjoint, , therefore neither is it in the intersection
A finite intersection of open sets is open, hence is a neighborhood of disjoint from .