# Kuratowski's Closure-Complement Problem/Complement of Interior of Closure of Interior

## Theorem

Let $\R$ be the real number space under the usual (Euclidean) topology.

Let $A \subseteq \R$ be defined as:

 $\displaystyle A$ $:=$ $\displaystyle \left({0 \,.\,.\, 1}\right) \cup \left({1 \,.\,.\, 2}\right)$ Definition of Union of Adjacent Open Intervals $\displaystyle$  $\, \displaystyle \cup \,$ $\displaystyle \left\{ {3} \right\}$ Definition of Singleton $\displaystyle$  $\, \displaystyle \cup \,$ $\displaystyle \left({\Q \cap \left({4 \,.\,.\, 5}\right)}\right)$ Rational Numbers from $4$ to $5$ (not inclusive)

The complement of the interior of the closure of the interior of $A$ in $\R$ is given by:

 $\displaystyle A^{\circ \, - \, \circ \, \prime}$ $=$ $\displaystyle \left({\gets \,.\,.\, 0}\right]$ Definition of Unbounded Closed Real Interval $\displaystyle$  $\, \displaystyle \cup \,$ $\displaystyle \left[{2 \,.\,.\, \to}\right)$ Definition of Unbounded Closed Real Interval

## Proof

 $\displaystyle A^{\circ \, - \, \circ}$ $=$ $\displaystyle \left({0 \,.\,.\, 2}\right)$ Definition of Open Real Interval

The result follows by inspection.

$\blacksquare$