# Set is Closed iff Equals Topological Closure/Proof 2

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## Theorem

Let $T$ be a topological space.

Let $H \subseteq T$.

Then $H$ is closed in $T$ if and only if:

- $H = \map \cl H$

## Proof

Let $H^{\complement}$ denote the relative complement of $H$ in $T$.

By definition, we have that $H$ is closed in $T$ if and only if $H^{\complement}$ is open in $T$.

By Set is Open iff Neighborhood of all its Points, this is equivalent to:

- $\forall x \in H^{\complement}: \exists U \in \tau: x \in U \subseteq H^{\complement}$

By Empty Intersection iff Subset of Complement, we have that:

- $U \subseteq H^{\complement} \iff U \cap H = \O$

By Condition for Point being in Closure, it follows that $H^{\complement}$ is open in $T$ if and only if:

- $\forall x \in H^{\complement}: x \notin \map \cl H$

By the Rule of Transposition, this is equivalent to $\map \cl H \subseteq H$.

From Set is Subset of its Topological Closure, we have that $H \subseteq \map \cl H$.

By definition of set equality, this is equivalent to:

- $H = \map \cl H$

$\blacksquare$

## Sources

- 1975: W.A. Sutherland:
*Introduction to Metric and Topological Spaces*... (previous) ... (next): $3.7$: Definitions: Proposition $3.7.15 \ \text{(a)}$