# Cover (topology)

In mathematics, particularly topology, a cover of a set ${\displaystyle X}$ is a collection of sets whose union contains ${\displaystyle X}$ as a subset. Formally, if ${\displaystyle C=\lbrace U_{\alpha }:\alpha \in A\rbrace }$ is an indexed family of sets ${\displaystyle U_{\alpha }}$, then ${\displaystyle C}$ is a cover of ${\displaystyle X}$ if

${\displaystyle X\subseteq \bigcup _{\alpha \in A}U_{\alpha }.}$

## Cover in topology

Covers are commonly used in the context of topology. If the set X is a topological space, then a cover C of X is a collection of subsets Uα of X whose union is the whole space X. In this case we say that C covers X, or that the sets Uα cover X. Also, if Y is a subset of X, then a cover of Y is a collection of subsets of X whose union contains Y, i.e., C is a cover of Y if

${\displaystyle Y\subseteq \bigcup _{\alpha \in C}U_{\alpha }}$

Let C be a cover of a topological space X. A subcover of C is a subset of C that still covers X.

We say that C is an open cover if each of its members is an open set (i.e. each Uα is contained in T, where T is the topology on X).

A cover of X is said to be locally finite if every point of X has a neighborhood that intersects only finitely many sets in the cover. Formally, C = {Uα} is locally finite if for any xX, there exists some neighborhood N(x) of x such that the set

${\displaystyle \left\{\alpha \in A:U_{\alpha }\cap N(x)\neq \varnothing \right\}}$

is finite. A cover of X is said to be point finite if every point of X is contained in only finitely many sets in the cover. A cover is point finite if it is locally finite, though the converse is not necessarily true.

## Refinement

A refinement of a cover C of a topological space X is a new cover D of X such that every set in D is contained in some set in C. Formally,

${\displaystyle D=\{V_{\beta \in B}\}\;{\text{is a refinement of}}\;C=\{U_{\alpha \in A}\}\;{\text{when}}\;\forall \beta \ \exists \alpha \ V_{\beta }\subseteq U_{\alpha }}$.

In other words, there is a refinement map ${\displaystyle \phi :B\rightarrow A}$ satisfying ${\displaystyle V_{\beta }\subseteq U_{\phi (\beta )}}$ for every ${\displaystyle \beta \in B}$. This map is used, for instance, in the Čech cohomology of X.[1]

Every subcover is also a refinement, but the opposite is not always true. A subcover is made from the sets that are in the cover, but omitting some of them; whereas a refinement is made from any sets that are subsets of the sets in the cover.

The refinement relation is a preorder on the set of covers of X.

Generally speaking, a refinement of a given structure is another that in some sense contains it. Examples are to be found when partitioning an interval (one refinement of ${\displaystyle a_{0} being ${\displaystyle a_{0}), considering topologies (the standard topology in euclidean space being a refinement of the trivial topology). When subdividing simplicial complexes (the first barycentric subdivision of a simplicial complex is a refinement), the situation is slightly different: every simplex in the finer complex is a face of some simplex in the coarser one, and both have equal underlying polyhedra.

Yet another notion of refinement is that of star refinement.

## Subcover

A simple way to get a subcover is to omit the sets contained in another set in the cover. Consider specifically open covers. Let ${\displaystyle {\mathcal {B}}}$ be a topological basis of ${\displaystyle X}$ and ${\displaystyle {\mathcal {O}}}$ be an open cover of ${\displaystyle X}$. First take ${\displaystyle {\mathcal {A}}=\{A\in {\mathcal {B}}\mid \exists U\in {\mathcal {O}}:A\subseteq U\}}$. Then ${\displaystyle {\mathcal {A}}}$ is a refinement of ${\displaystyle {\mathcal {O}}}$. Next, for each ${\displaystyle A\in {\mathcal {A}}}$, we select a ${\displaystyle U_{A}\in {\mathcal {O}}}$ containing ${\displaystyle A}$ (requiring the axiom of choice). Then ${\displaystyle {\mathcal {C}}=\{U_{A}\in {\mathcal {O}}\mid A\in {\mathcal {A}}\}}$ is a subcover of ${\displaystyle {\mathcal {O}}}$. Hence the cardinality of a subcover of an open cover can be as small as that of any topological basis. Hence in particular second countability implies a space is Lindelöf.

## Compactness

The language of covers is often used to define several topological properties related to compactness. A topological space X is said to be

Compact
if every open cover has a finite subcover, (or equivalently that every open cover has a finite refinement);
Lindelöf
if every open cover has a countable subcover, (or equivalently that every open cover has a countable refinement);
Metacompact
if every open cover has a point-finite open refinement;
Paracompact
if every open cover admits a locally finite open refinement.

For some more variations see the above articles.

## Covering dimension

A topological space X is said to be of covering dimension n if every open cover of X has a point-finite open refinement such that no point of X is included in more than n+1 sets in the refinement and if n is the minimum value for which this is true.[2] If no such minimal n exists, the space is said to be of infinite covering dimension.