Definition:Closed Neighborhood
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Definition
Let $\left({S, \tau}\right)$ be a topological space.
Let $A \subseteq S$ be a subset of $S$.
Let $N_A$ be a neighborhood of $A$.
If $\complement_S \left({N_A}\right) \in \tau$, that is if $N_A$ is closed in $S$, then $N_A$ is called a closed neighborhood.
Linguistic Note
The UK English spelling of this is neighbourhood.
