# Closed Set of Uncountable Finite Complement Topology is not G-Delta

## Theorem

Let $T = \struct {S, \tau}$ be a finite complement topology on an uncountable set $S$.

Let $V \in \tau$ be a closed set of $T$.

Then $V$ is not a $G_\delta$ set.

## Proof

Let $V$ be a closed set of $T$.

Aiming for a contradiction, suppose $V$ is $G_\delta$ set.

$S \setminus V$ is an $F_\sigma$ set.

By definition of closed set, $S \setminus V$ is an open set of $T$.

$S \setminus V$ is not an $F_\sigma$ set.

It follows by Proof by Contradiction that $V$ is not a $G_\delta$ set.

$\blacksquare$