Interior of Closure is Regular Open
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Theorem
Let $T = \left({S, \tau}\right)$ be a topological space.
Let $H \subseteq S$.
Then $H^{- \circ}$ is regular open.
Proof
We must show that $H^{- \circ - \circ} = H^{- \circ}$.
First we show that $H^{- \circ - \circ} \subseteq H^{- \circ}$.
By the definition of interior:
- $H^{- \circ} \subseteq H^-$
By Topological Closure of Subset is Subset of Topological Closure:
- $H^{- \circ -} \subseteq H^{--} = H^-$
- $H^{- \circ - \circ } \subseteq H^{- \circ}$
$\Box$
Next we show $H^{- \circ - \circ} \supseteq H^{- \circ}$.
Again by the definition of interior:
- $H^{- \circ -} \supseteq H^{- \circ}$
- $H^{- \circ - \circ} \supseteq H^{- \circ \circ} = H^{- \circ}$
$\Box$
Thus by definition of set equality:
- $H^{- \circ - \circ} = H^{- \circ}$
$\blacksquare$
Sources
- 1964: Steven A. Gaal: Point Set Topology: $\text{I}: \ \S 2$: Interior, Exterior, Boundary and Closure