Let $(X,\tau)$ be a [normal topological space](/page/Normal%20Topological%20Space). If $A \subset X$ is closed and $W \in \tau$ is an [open set](/page/Open%20Set) such that $A \subset W$, then there exists an open set $U \in \tau$ such that
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\begin{align*}
A \subset U \subset \overline{U} \subset W.
\end{align*}