nLab cotopology


Formally, a cotopology of a topological space (X,𝒯)(X, \mathcal{T}) is a coarser topology 𝒯 *\mathcal{T}^* on the same set XX that does not forget too many sets. It can be used to generalize the Baire category theorem and to characterize topological completeness. The pair of both topologies, the original and the coarser one, constitute an example of a bitopological space.



Let (X,𝒯)(X, \mathcal{T}) be a topological space. A topology 𝒯 *\mathcal{T}^* on XX is called a cotopology of 𝒯\mathcal{T}—and (X,𝒯 *)(X, \mathcal{T}^*) is called a cospace of (X,𝒯)(X, \mathcal{T})—if

  1. 𝒯 *\mathcal{T}^* is coarser than 𝒯\mathcal{T}, i.e., 𝒯 *𝒯\mathcal{T}^* \subseteq \mathcal{T};

  2. for each point xx and each 𝒯\mathcal{T}-closed 𝒯\mathcal{T}-neighborhood VV of xx in XX there exists a 𝒯 *\mathcal{T}^*-closed 𝒯\mathcal{T}-neighborhood UU of xx in XX such that UU is contained in VV.

If 𝒯\mathcal{T} is regular, the last condition can be replaced by other conditions, see this proposition.


A topological space XX is called cocompact if there is a cotopology 𝒯 *\mathcal{T}^* on XX which is compact.


Every cocompact regular space is a Baire space. A metrisable space is topologically complete if and only if it is cocompact.

A space that admits only Hausdorff cospaces is equivalently an H-closed space, i.e. it (is Hausdorff and) is not a proper dense subspace of another space.


The concept principally appeared in

  • J. D. Weston, On the comparison of topologies 1956, Journal of the London Mathematical Society, vol. s1-32 no. 3, pp. 342-354.

De Groot made cotopologies popular by giving a unifying and generalizing version of the Baire category theorem

  • de Groot, Subcompactness and the Baire category theorem 1963, Nederl. Akad. Wetensch. Proc., Ser. A66 = Indag. Math., vol. 25, pp. 761-767;

unfortunately, his proof contained a gap that was later closed by

  • Isidore Fleischer, On ‘Subcompactness and the Baire category theorem’ 1979, Nederl. Akad. Wetensch. Proc. Ser. A 82 = Indag. Math. 41, pp. 9-11.

In this context some

is mentioned. Further developments include

  • Aarts, de Groot, McDowell, Cotopology for metrizable spaces 1970, Duke Mathematical Journal vol. 37.

  • George Strecker and G. Viglino, Cotopology and Minimal Hausdorff Spaces 1969, Proceedings of the American Mathematical Society Vol. 21 No. 3.

Last revised on March 18, 2019 at 13:26:36. See the history of this page for a list of all contributions to it.