Alexander's Compactness Theorem

Theorem
Let $T$ be a topological space.

Then $T$ is compact :


 * If $T$ has a sub-basis $\mathcal S$ such that from every cover of $X$ by elements of $\mathcal S$, a finite subcover of $X$ can be selected.

Proof
Suppose that $T$ is compact.

Let $\mathcal S$ be a sub-basis of $X$.

Then by the definition of a compact space, from every cover of $X$ by elements of $\mathcal S$, a finite subcover can be selected.

Now, with a view to obtain a contradiction, suppose that the space $X$ is not compact, yet every cover of $X$ by elements of $\mathcal S$ has a finite subcover.

Use Zorn's Lemma to find an open cover $\mathcal C$ which has no finite subcover that is maximal among such covers.

That means that if $V$ is an open set and $V \notin \mathcal C$, then $\mathcal C \cup \{V\}$ has a finite subcover, necessarily of the form $\mathcal C_0 \cup \{V\}$ for some finite subfamily $\mathcal C_0$ of $\mathcal C$.

Consider $\mathcal C \cap \mathcal S$, that is, the subbasic subfamily of $\mathcal C$.

If it covered $X$, then by hypothesis, it would have a finite subcover.

But $\mathcal C$ does not have such, so $\mathcal C \cap \mathcal S$ does not cover $X$.

Let $x \in X$ that is not covered.

$\mathcal C$ covers $X$, so $\exists U \in \mathcal C: x \in U$.

$\mathcal S$ is a subbasis, so for some $S_1, \ldots, S_n \in \mathcal S$, $x \in S_1 \cap \cdots \cap S_n \subseteq U$.

Since $x$ is uncovered, $S_i \notin \mathcal C$.

As noted above, this means that for each $i$, $S_i$ along with a finite subfamily $\mathcal C_i$ of $\mathcal C$, covers $X$.

But then $U$ and all the $\mathcal C_i$ cover $X$, so $\mathcal C$ has a finite subcover after all.