Cover (topology)

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In mathematics, particularly topology, a cover of a set is a collection of sets whose union includes as a subset. Formally, if is an indexed family of sets then is a cover of if

Cover in topologyEdit

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


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   there exists some neighborhood N(x) of x such that the set


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.


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

  is a refinement of   if for all   there exists   such that  

In other words, there is a refinement map   satisfying   for every   This map is used, for instance, in the Čech cohomology of  .[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  .

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   being  ), 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.


A simple way to get a subcover is to omit the sets contained in another set in the cover. Consider specifically open covers. Let   be a topological basis of   and   be an open cover of   First take   Then   is a refinement of  . Next, for each   we select a   containing   (requiring the axiom of choice). Then   is a subcover of   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.


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

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

For some more variations see the above articles.

Covering dimensionEdit

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.

See alsoEdit


  1. ^ Bott, Tu (1982). Differential Forms in Algebraic Topology. p. 111.
  2. ^ Munkres, James (1999). Topology (2nd ed.). Prentice Hall. ISBN 0-13-181629-2.


  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 linksEdit