Closure operator

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This editable Main Article is under development and subject to a disclaimer.

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In mathematics a closure operator is a unary operator or function on subsets of a given set which maps a subset to a containing subset with a particular property.

A closure operator on a set X is a function F on the power set of X, F:𝒫X->𝒫X, satisfying:

A⊆B⇒FA⊆FB;
A⊆FA;
FFA=FA.

A topological closure operator satisfies the further property

F(A∪B)=FA∪FB.

A closed set for F is one of the sets in the image of F

## Closure system[edit]

A closure system is the set of closed sets of a closure operator. A closure system is defined as a family 𝒞 of subsets of a set X which contains X and is closed under taking arbitrary intersections:

𝒮⊆𝒞⇒∩𝒮∈𝒞.

The closure operator F may be recovered from the closure system as

FA=⋂A⊆C∈𝒞C.

## Examples[edit]

In many algebraic structures the set of substructures forms a closure system. The corresponding closure operator is often written ⟨A⟩ and termed the substructure "generated" or "spanned" by A. Notable examples include

* Subsemigroups of a semigroup S. The semigroup generated by a subset A may also be obtained as the set of all finite products of one or more elements of A.
* Subgroups of a group. The subgroup generated by a subset A may also be obtained as the set of all finite products of zero or more elements of A or their inverses.
* Normal subgroups of a group. The normal subgroup generated by a subset A may also be obtained as the subgroup generated by the elements of A together with all their conjugates.
* Submodules of a module (algebra) or subspaces of a vector space. The submodule generated by a subset A may also be obtained as the set of all finite linear combinations of elements of A.

The principal example of a topological closure system is the family of closed sets in a topological space. The corresponding closure operator is denoted A‾. It may also be obtained as the union of the set A with its limit points.