# Definition:Closure Operator

*This page is about Closure Operator. For other uses, see Closure.*

## Definition

### Ordering

Let $\struct {S, \preceq}$ be an ordered set.

A **closure operator** on $S$ is a mapping:

- $\cl: S \to S$

which satisfies the following conditions for all elements $x, y \in S$:

$\cl$ is inflationary | \(\displaystyle x \) | \(\displaystyle \preceq \) | \(\displaystyle \map \cl x \) | |||||

$\cl$ is increasing | \(\displaystyle x \preceq y \) | \(\displaystyle \implies \) | \(\displaystyle \map \cl x \preceq \map \cl y \) | |||||

$\cl$ is idempotent | \(\displaystyle \map \cl {\map \cl x} \) | \(\displaystyle = \) | \(\displaystyle \map \cl x \) |

### Power Set

When the ordering in question is the subset relation on a power set, the definition can be expressed as follows:

Let $S$ be a set.

Let $\powerset S$ denote the power set of $S$.

A **closure operator** on $S$ is a mapping:

- $\cl: \powerset S \to \powerset S$

which satisfies the following conditions for all sets $X, Y \subseteq S$:

\((1)\) | $:$ | $\cl$ is inflationary | \(\displaystyle \forall X \subseteq S:\) | \(\displaystyle X \) | \(\displaystyle \subseteq \) | \(\displaystyle \map \cl X \) | ||

\((2)\) | $:$ | $\cl$ is increasing | \(\displaystyle \forall X, Y \subseteq S:\) | \(\displaystyle X \subseteq Y \) | \(\displaystyle \implies \) | \(\displaystyle \map \cl X \subseteq \map \cl Y \) | ||

\((3)\) | $:$ | $\cl$ is idempotent | \(\displaystyle \forall X \subseteq S:\) | \(\displaystyle \map \cl {\map \cl X} \) | \(\displaystyle = \) | \(\displaystyle \map \cl X \) |

## Notation

The closure operator of $H$ is variously denoted:

- $\map \cl H$
- $\map {\mathrm {Cl} } H$
- $\overline H$
- $H^-$

Of these, it can be argued that $\overline H$ has more problems with ambiguity than the others, as it is also frequently used for the set complement.

$\map \cl H$ and $\map {\mathrm {Cl} } H$ are cumbersome, but they have the advantage of being clear.

$H^-$ is neat and compact, but has the disadvantage of being relatively obscure.

On $\mathsf{Pr} \infty \mathsf{fWiki}$, $H^-$ is notation of choice, although $\map \cl H$ can also be found in places.

## Also see

- Definition:Closed Set under Closure Operator
- Definition:Closed Element under Closure Operator
- Definition:Closure of Set under Closure Operator
- Definition:Closure of Element under Closure Operator

- Closure (Topology), which is demonstrated to be an instance of a
**closure operator**in Topological Closure is Closure Operator.