Open and Closed Sets in Indiscrete Topology

Theorem

Let $T = \struct {S, \set {\O, S} }$ be an indiscrete topological space.

Let $H \subseteq S$.

Open Sets in Indiscrete Topology

$H$ is an open set of $T$ if and only if either $H = S$ or $H = \O$.

Closed Sets in Indiscrete Topology

$H$ is a closed set of $T$ if and only if either $H = S$ or $H = \O$.

$F_\sigma$ Sets in Indiscrete Topology

$H$ is an open set of $T$ if and only if either $H = S$ or $H = \O$.

$G_\delta$ Sets in Indiscrete Topology

$H$ is a $G_\delta$ ($G$-delta) set of $T$ if and only if either $H = S$ or $H = \O$.

Sources

• 1978: Lynn Arthur Steen and J. Arthur Seebach, Jr.: Counterexamples in Topology (2nd ed.) ... (previous) ... (next): Part $\text {II}$: Counterexamples: $4$. Indiscrete Topology: $2$