# Kuratowski's Closure-Complement Problem

## Theorem

Let $T = \struct {S, \tau}$ be a topological space.

Let $A \subseteq S$ be a subset of $T$.

By successive applications of the operations of complement relative to $S$ and the closure, there can be as many as $14$ distinct subsets of $S$ (including $A$ itself).

### Example

Let $\R$ be the real number line with the usual (Euclidean) topology.

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

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

## Proof

That there can be as many as $14$ will be demonstrated by example.

### Complement

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

 $\ds A'$ $=$ $\ds \left({\gets \,.\,.\, 0}\right]$ Definition of Unbounded Closed Real Interval $\ds$  $\, \ds \cup \,$ $\ds \left\{ {1} \right\}$ Definition of Singleton $\ds$  $\, \ds \cup \,$ $\ds \left[{2 \,.\,.\, 3}\right)$ Definition of Half-Open Real Interval $\ds$  $\, \ds \cup \,$ $\ds \left({3 \,.\,.\, 4}\right]$ ... adjacent to Half-Open Real Interval $\ds$  $\, \ds \cup \,$ $\ds \left({\R \setminus \Q \cap \left[{4 \,.\,.\, 5}\right]}\right)$ Irrational Numbers from $4$ to $5$ $\ds$  $\, \ds \cup \,$ $\ds \left[{5 \,.\,.\, \to}\right)$ Definition of Unbounded Closed Real Interval

### Interior

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

 $\ds A^\circ$ $=$ $\ds \openint 0 1 \cup \openint 1 2$ Union of Adjacent Open Intervals

### Closure

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

 $\ds A^-$ $=$ $\ds \closedint 0 2$ Definition of Closed Real Interval $\ds$  $\, \ds \cup \,$ $\ds \set 3$ Definition of Singleton $\ds$  $\, \ds \cup \,$ $\ds \closedint 4 5$ Definition of Closed Real Interval

### Exterior

The exterior of $A$ in $\R$ is given by:

 $\ds A^e$ $=$ $\ds \openint \gets 0$ Definition of Unbounded Open Real Interval $\ds$  $\, \ds \cup \,$ $\ds \openint 2 3 \cup \openint 3 4$ Definition of Union of Adjacent Open Intervals $\ds$  $\, \ds \cup \,$ $\ds \openint 5 \to$ Definition of Unbounded Open Real Interval

### Closure of Complement

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

 $\ds A^{\prime \, -}$ $=$ $\ds \hointl \gets 0$ Definition of Unbounded Closed Real Interval $\ds$  $\, \ds \cup \,$ $\ds \set 1$ Definition of Singleton $\ds$  $\, \ds \cup \,$ $\ds \hointr 2 \to$ Definition of Unbounded Closed Real Interval

### Closure of Interior

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

 $\ds A^{\circ \, -}$ $=$ $\ds \closedint 0 2$ Definition of Closed Real Interval

### Interior of Closure

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

 $\ds A^{- \, \circ}$ $=$ $\ds \openint 0 2$ Definition of Open Real Interval $\ds$  $\, \ds \cup \,$ $\ds \openint 4 5$ Definition of Open Real Interval

### Interior of Closure of Interior

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

 $\ds A^{\circ \, - \, \circ}$ $=$ $\ds \openint 0 2$ Definition of Open Real Interval

### Interior of Complement of Interior

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

 $\ds A^{\circ \, \prime \, \circ}$ $=$ $\ds \openint \gets 0$ Definition of Unbounded Open Real Interval $\ds$  $\, \ds \cup \,$ $\ds \openint 2 \to$ Definition of Unbounded Open Real Interval

### Closure of Interior of Complement

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

 $\ds A^{\prime \, \circ \, -}$ $=$ $\ds \hointl \gets 0$ Definition of Unbounded Closed Real Interval $\ds$  $\, \ds \cup \,$ $\ds \closedint 2 4$ Definition of Closed Real Interval $\ds$  $\, \ds \cup \,$ $\ds \hointr 5 \to$ Definition of Unbounded Closed Real Interval

### Closure of Interior of Closure

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

 $\ds A^{- \, \circ \, -}$ $=$ $\ds \closedint 0 2$ Definition of Closed Real Interval $\ds$  $\, \ds \cup \,$ $\ds \closedint 4 5$ Definition of Closed Real Interval

### Interior of Complement of Interior of Closure

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

 $\ds A^{- \, \circ \, \prime \, \circ}$ $=$ $\ds \left({\gets \,.\,.\, 0}\right)$ Definition of Unbounded Open Real Interval $\ds$  $\, \ds \cup \,$ $\ds \left({2 \,.\,.\, 4}\right)$ Definition of Open Real Interval $\ds$  $\, \ds \cup \,$ $\ds \left({5 \,.\,.\, \to}\right)$ Definition of Unbounded Open Real Interval

### Complement of Interior of Closure of Interior

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

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

### Proof of Maximum

It remains to be shown that there can be no more than $14$.

Consider an arbitrary subset $A$ of a topological space $T = \struct {S, \tau}$.

To simplify the presentation:

let $a$ be used to denote the operation of taking the complement of $A$ relative to $S$: $\map a A = S \setminus A$
let $b$ be used to denote the operation of taking the closure of $A$ in $T$: $\map b A = A^-$
let $I$ be used to denote the identity operation on $A$, that is: $\map I A = A$.
let the parentheses and the reference to $A$ be removed, so as to present, for example:
$\map a {\map b {\map a A} }$
as:
$a b a$
$\map a {\map a A} = A$

or, using the compact notation defined above:

$(1): \quad a a = I$
$\map b {\map b A} = \map b A = A^-$

or, using the compact notation defined above:

$(2): \quad b b = b$

Let $s$ be a finite sequence of elements of $\set {a, b}$.

By successive applications of $(1)$ and $(2)$, it is possible to eliminate all multiple consecutive instances of $a$ and $b$ in $s$, and so reduce $s$ to one of the following forms:

$\text{a)}: \quad a b a b \ldots a$
$\text{b)}: \quad b a b a \ldots a$
$\text{c)}: \quad a b a b \ldots b$
$\text{d)}: \quad b a b a \ldots b$
$b a b$ is regular closed.

By Interior equals Complement of Closure of Complement, the interior of $A$ is:

$a b a$

Recall the definition of regular closed:

a set $A$ is regular closed if and only if it equals the closure of its interior.

And so as $b a b$ is regular closed:

$b a b = b a b a \paren {b a b}$

So, adding an extra $b$ to either of $a b a b a b a$ or $b a b a b a$ will generate a string containing $b a b a b a b$ which can be reduced immediately to $b a b$.

It follows that the possible different subsets of $S$ that can be obtained from $A$ by applying $a$ and $b$ can be generated by none other than:

$I$
$a$
$a b$
$a b a$
$a b a b$
$a b a b a$
$a b a b a b$
$a b a b a b a$
$b$
$b a$
$b a b$
$b a b a$
$b a b a b$
$b a b a b a$

... a total of $14$.

Hence the result.

$\blacksquare$

## Also known as

This result is also known as Kuratowski's Closure-Complement Theorem.

## Source of Name

This entry was named for Kazimierz Kuratowski.