Space is Compact iff exists Basis such that Every Cover has Finite Subcover
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Theorem
Let $T = \struct {S, \tau}$ be a topological space.
Then $T = \struct {S, \tau}$ is compact if and only if $\tau$ has a basis $\BB$ such that:
- from every cover of $S$ by elements of $\BB$, a finite subcover of $S$ can be selected.
Proof
Sufficient Condition
Let every open cover for $S$ have a finite subcover.
Let $\BB$ be a basis $\BB$.
Then every cover of $S$ by elements of $\BB$ is an open cover for $S$.
So:
- from every cover of $S$ by elements of $\BB$, a finite subcover of $S$ can be selected.
$\Box$
Necessary Condition
Let $\tau$ have a basis $\BB$ such that:
- from every cover of $S$ by elements of $\BB$, a finite subcover of $S$ can be selected.
Let $\AA$ be an open cover of $S$.
Let $f: \AA \to \powerset B$ be the mapping defined by:
- $\forall A \in \AA : \map f A = \set{B \in \BB : B \subseteq A}$
Since each element of $\AA$ is open:
- $A = \bigcup \map f A$ for each $A \in \AA$.
Let $\AA' = \bigcup f \sqbrk {\AA}$.
Then $\AA'$ is an cover of $S$ by elements of $\BB$
By the premise, $\AA'$ has a finite subset $\FF'$ that covers $S$.
Let $g: \FF' \to \AA$ be the map each element of $\FF'$ to an element of $\AA$ that contains it.
Note that since $\FF'$ is finite, this does not require the Axiom of Choice.
Let $\FF = g \sqbrk {\FF'}$.
Then $\FF$ is a finite subcover of $\AA$.
It follows that every open cover of $S$ has a finite subcover of $S$.
$\blacksquare$