Metacompact space

From Wikipedia, the free encyclopedia
Jump to: navigation, search

In mathematics, in the field of general topology, a topological space is said to be metacompact if every open cover has a point finite open refinement. That is, given any open cover of the topological space, there is a refinement which is again an open cover with the property that every point is contained only in finitely many sets of the refining cover.

A space is countably metacompact if every countable open cover has a point finite open refinement.

Properties[edit]

The following can be said about metacompactness in relation to other properties of topological spaces:

  • Every paracompact space is metacompact. This implies that every compact space is metacompact, and every metric space is metacompact. The converse does not hold: a counter-example is the Dieudonné plank.
  • Every metacompact space is orthocompact.
  • Every metacompact normal space is a shrinking space
  • The product of a compact space and a metacompact space is metacompact. This follows from the tube lemma.
  • An easy example of a non-metacompact space (but a countably metacompact space) is the Moore plane.
  • In order for a Tychonoff space X to be compact it is necessary and sufficient that X be metacompact and pseudocompact (see Watson).

Covering dimension[edit]

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[edit]

References[edit]