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| A topological space $X$ is said to be \emph{normal} (or $T_4$) if for all disjoint closed sets $D,F\subseteq X$ there exist disjoint open sets $U$ and $V$ such that $C\subseteq U$ and $D\subseteq V$. |
A topological space $X$ is said to be \emph{normal} (or $T_4$) if for all disjoint closed sets $D,F\subseteq X$ there exist disjoint open sets $U$ and $V$ such that $C\subseteq U$ and $D\subseteq V$. |
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| Some authors also require singletons to be closed as part of the definition (i.e. they require the space to be $T_1$ as well). |
Some authors also require singletons to be closed as part of the definition (i.e. they require the space to be $T_1$ as well). |
