Limit Points in Closed Extension Space/Subset

Theorem

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

Let $T^*_p = \struct {S^*_p, \tau^*_p}$ be the closed extension space of $T$.

Let $U \subseteq S^*_p$ such that $p \in U$.

Let $x \in S$.

Then $x$ is a limit point of $U$.

Proof

Every open set of $T^*_p = \struct {S^*_p, \tau^*_p}$ except $\O$ contains the point $p$ by definition.

So every open set $U \in \tau^*_p$ such that $x \in U$ contains $p$.

So by definition of the limit point of a set, $x$ is a limit point of $U$.

$\blacksquare$

Sources

• 1978: Lynn Arthur Steen and J. Arthur Seebach, Jr.: Counterexamples in Topology (2nd ed.) ... (previous) ... (next): Part $\text {II}$: Counterexamples: $12$. Closed Extension Topology: $21$