Bitopological space

In mathematics, a bitopological space is a set endowed with two topologies. Typically, if the set is and the topologies are and then the bitopological space is referred to as . The notion was introduced by Kelly in the study of quasimetrics, i.e. distance functions that are not required to be symmetric.


A map from a bitopological space to another bitopological space is called continuous or sometimes pairwise continuous if is continuous both as a map from to and as map from to .

Bitopological variants of topological properties

Corresponding to well-known properties of topological spaces, there are versions for bitopological spaces.

  • A bitopological space is pairwise compact if each cover of with , contains a finite subcover. In this case, must contain at least one member from and at least one member from
  • A bitopological space is pairwise Hausdorff if for any two distinct points there exist disjoint and with and .
  • A bitopological space is pairwise zero-dimensional if opens in which are closed in form a basis for , and opens in which are closed in form a basis for .
  • A bitopological space is called binormal if for every -closed and -closed sets there are -open and -open sets such that , and



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