Locally Compact iff Open Neighborhood contains Compact Set

Theorem
Let $T = \struct{S, \tau}$ be a topological space.

Then:
 * $T$ is locally compact


 * $\forall U \in \tau, x \in U: \exists K \subseteq S: x \in K^\circ \land K \subseteq U \land K$ is compact
 * $\forall U \in \tau, x \in U: \exists K \subseteq S: x \in K^\circ \land K \subseteq U \land K$ is compact

where $K^\circ$ denotes the interior of $K$.

Sufficient Condition
Let $T$ be locally compact.

Let $U \in \tau, x \in U$.

Since $T$ is locally compact there exists a neighborhood basis $\BB$ of $x$ consisting of compact sets.

From Set is Open iff Neighborhood of all its Points, $U$ is a neighborhood of $x$.

By definition of a neighborhood basis:
 * $\exists K \in \BB : K \subseteq U$.

By definition of a neighborhood of $x$:
 * $\exists V \in \tau : x \in V \subseteq K$

By definition of the interior of a subset then:
 * $x \in V \subseteq K^\circ$

Thus
 * $\exists K \subseteq S: x \in K^\circ \land K \subseteq U \land K$ is compact

Necessary Condition
Let
 * $\forall U \in \tau, x \in U: \exists K \subseteq S: x \in K^\circ \land K \subseteq U \land K$ is compact

Let $\BB$ be the set of all compact neighborhoods of $x$.

It is shown that $\BB$ is a neighborhood basis of $x$.

Let $N$ be a neighborhood of $x$.

By definition of a neighborhood of $x$:
 * $\exists U \in \tau : x \in U \subseteq N$

By assumption:
 * $\exists K \subseteq S: x \in K^\circ \land K \subseteq U \land K$ is compact

By definition of the interior of a subset $K^\circ$ is open.

By definition of a neighborhood then $K$ is a neighborhood of $x$.

Hence $K \in \BB$ and $K \subseteq U \subseteq N$.

As $N$ was arbitrary, then $B$ is a neighborhood basis of $x$ consisting of compact sets.