Let $T = (X, \mathcal{T})$ be a D1106: Topological space.

Let $U \subseteq X$ be an D97: Open set in $T$.

Let $F \subseteq X$ be a D98: Closed set in $T$.

Let $U \subseteq X$ be an D97: Open set in $T$.

Let $F \subseteq X$ be a D98: Closed set in $T$.

Then $U \setminus F$ is an D97: Open set in $T$.