# Empty Set is Closed/Topological Space

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## Theorem

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

Then $\O$ is closed in $T$.

## Proof

From the definition of closed set, $U$ is open in $T = \struct {S, \tau}$ if and only if $S \setminus U$ is closed in $T$.

By definition of topological space, $S$ is open in $T$.

From Set Difference with Self is Empty Set:

- $S \setminus S = \O$

Hence $\O$ is closed in $T$.

$\blacksquare$

## Sources

- 1975: Bert Mendelson:
*Introduction to Topology*(3rd ed.) ... (previous) ... (next): Chapter $3$: Topological Spaces: $\S 2$: Topological Spaces: Exercise $4$