Set is Closed iff Equals Topological Closure/Proof 1
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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 $H'$ denote the derived set of $H$.
By Closed Set iff Contains all its Limit Points, $H$ is closed in $T$ if and only if $H' \subseteq H$.
By Union with Superset is Superset, $H' \subseteq H$ if and only if $H = H \cup H'$.
The result follows from the definition of closure.
$\blacksquare$
Sources
- 1953: Walter Rudin: Principles of Mathematical Analysis ... (previous) ... (next): $2.27 b$