Cover (topology)

Short description: Subsets whose union equals the whole set

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 $C=\{U_{\alpha }:\alpha \in A\}$ is an indexed family of subsets $U_{\alpha }\subset X$ (indexed by the set $A$), then $C$ is a cover of $X$ if $\bigcup _{\alpha \in A}{U}_{\alpha }=X$. Thus the collection $\{U_{\alpha }:\alpha \in A\}$ is a cover of $X$ if each element of $X$ belongs to at least one of the subsets $U_{\alpha }$.

A subcover of a cover of a set is a subset of the cover that also covers the set. A cover is called an open cover if each of its elements is an open set.

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 $\{U_{\alpha }{\}}_{\alpha \in A}$ of $X$ whose union is the whole space $X$. In this case we say that $C$ covers $X$, or that the sets $U_{\alpha }$ cover $X$.

Also, 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 $Y$, i.e., $C$ is a cover of $Y$ if

$Y\subseteq \bigcup _{\alpha \in A}{U}_{\alpha }.$

That is, we may cover $Y$ with either sets in $Y$ itself or sets in the parent space $X$.

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 U_α 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 = {U_α} is locally finite if for any $x\in X,$ there exists some neighborhood N(x) of x such that the set

$\{\alpha \in A:U_{\alpha }\cap N(x)\ne \varnothing \}$

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,

$D=\{V_{\beta }{\}}_{\beta \in B}$ 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 $\varphi :B\to A$ satisfying $V_{\beta }\subseteq U_{\varphi (\beta )}$ 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, irreflexive, and asymmetric.

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.

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 ${ℬ}$ be a topological basis of $X$ and ${𝒪}$ be an open cover of $X.$ First take ${𝒜}=\{A\in {ℬ}:\text{ there exists }U\in {𝒪}\text{ such that }A\subseteq U\}.$ Then ${𝒜}$ is a refinement of ${𝒪}$. Next, for each $A\in {𝒜},$ we select a $U_A\in {𝒪}$ containing $A$ (requiring the axiom of choice). Then ${𝒞}=\{U_A\in {𝒪}:A\in {𝒜}\}$ is a subcover of ${𝒪}.$ 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. If no such minimal n exists, the space is said to be of infinite covering dimension.

See also

• Atlas (topology) – Set of charts that describes a manifold
• Bornology – Mathematical generalization of boundedness
• Covering space – Type of continuous map in topology
• Grothendieck topology – Mathematical structure
• Partition of a set – Mathematical ways to group elements of a set
• Set cover problem – Classical problem in combinatorics
• Star refinement

References

1. Introduction to Topology, Second Edition, Theodore W. Gamelin & Robert Everist Greene. Dover Publications 1999. ISBN 0-486-40680-6
2. General Topology, John L. Kelley. D. Van Nostrand Company, Inc. Princeton, NJ. 1955.

External links

• Hazewinkel, Michiel, ed. (2001), "Covering (of a set)", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4, https://www.encyclopediaofmath.org/index.php?title=p/c026950