# Cover (topology)

In mathematics, and more particularly in set theory, a cover (or covering) of a set $X$ is a family of subsets of $X$ whose union is all of $X$. More formally, if $C=\lbrace U_{\alpha }:\alpha \in A\rbrace$ is an indexed family of subsets $U_{\alpha }\subset X$ (indexed by the set $A$), then $C$ is a cover of $X$ if $\bigcup _{\alpha \in A}U_{\alpha }\supseteq X.$ Thus the collection $\lbrace U_{\alpha }:\alpha \in A\rbrace$ is a cover of $X$ if each element of $X$ belongs to at least one of the subsets $U_{\alpha }$.

## Definition

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 $\{U_{\alpha }\}_{\alpha \in A}$ of $X$ whose union is the whole space $X$. In this case $C$ is said to cover $X$, or that the sets $U_{\alpha }$ cover $X$.
If $Y$ is a (topological) subspace of $X$, then a cover of $Y$ is a collection of subsets $C=\{U_{\alpha }\}_{\alpha \in A}$ of $X$ whose union contains $Y$. That is, $C$ is a cover of $Y$ if $Y\subseteq \bigcup _{\alpha \in A}U_{\alpha }.$ Here, $Y$ may be covered with either sets in $Y$ itself or sets in the parent space $X$.
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, $C=\{U_{\alpha }\}$ is locally finite if, for any $x\in X$, there exists some neighborhood $N(x)$ of $x$ such that the set $\left\{\alpha \in A:U_{\alpha }\cap N(x)\neq \varnothing \right\}$ 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 locally finite, though the converse is not necessarily true.

## Subcover

Let $C$ be a cover of a topological space $X$. A subcover of $C$ is a subset of $C$ that still covers $X$. The cover $C$ is said to be an open cover if each of its members is an open set. That is, each $U_{\alpha }$ is contained in $T$, where $T$ is the topology on X).
A simple way to get a subcover is to omit the sets contained in another set in the cover. Consider specifically open covers. Let ${\mathcal {B}}$ be a topological basis of $X$ and ${\mathcal {O}}$ be an open cover of $X$. First, take ${\mathcal {A}}=\{A\in {\mathcal {B}}:{\text{ there exists }}U\in {\mathcal {O}}{\text{ such that }}A\subseteq U\}$. Then ${\mathcal {A}}$ is a refinement of ${\mathcal {O}}$. Next, for each $A\in {\mathcal {A}},$ one may select a $U_{A}\in {\mathcal {O}}$ containing $A$ (requiring the axiom of choice). Then ${\mathcal {C}}=\{U_{A}\in {\mathcal {O}}:A\in {\mathcal {A}}\}$ is a subcover of ${\mathcal {O}}.$ Hence the cardinality of a subcover of an open cover can be as small as that of any topological basis. Hence, second countability implies space is Lindelöf.

## 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,
    $D=\{V_{\beta }\}_{\beta \in B}$ is a refinement of $C=\{U_{\alpha }\}_{\alpha \in A}$ if for all $\beta \in B$ there exists $\alpha \in A$ such that $V_{\beta }\subseteq U_{\alpha }.$
In other words, there is a refinement map $\phi :B\to A$ satisfying $V_{\beta }\subseteq U_{\phi (\beta )}$ for every $\beta \in B.$ This map is used, for instance, in the Čech cohomology of $X$.
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 on the set of covers of $X$ is transitive and reflexive, i.e. a Preorder. It is never asymmetric for $X\neq \emptyset$.
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 $a_{0}<a_{1}<\cdots <a_{n}$ being $a_{0}<b_{0}<a_{1}<a_{2}<\cdots <a_{n-1}<b_{1}<a_{n}$), 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.

## 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; and
* 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. If no such minimal n exists, the space is said to be of infinite covering dimension.