In mathematics, particularly topology, the homeomorphism group of a topological space is the group consisting of all homeomorphisms from the space to itself with function composition as the group operation. Homeomorphism groups are very important in the theory of topological spaces and in general are examples of automorphism groups. Homeomorphism groups are topological invariants in the sense that the homeomorphism groups of homeomorphic topological spaces are isomorphic as groups.
There is a natural group action of the homeomorphism group of a space on that space. Let be a topological space and denote the homeomorphism group of by . The action is defined as follows:
This is a group action since for all ,
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As with other sets of maps between topological spaces, the homeomorphism group can be given a topology, such as the compact-open topology. In the case of regular, locally compact spaces the group multiplication is then continuous.
If the space is compact and Hausdorff, the inversion is continuous as well and becomes a topological group as one can easily show. If is Hausdorff, locally compact and locally connected this holds as well. However there are locally compact separable metric spaces for which the inversion map is not continuous and therefore not a topological group.
In the category of topological spaces with homeomorphisms, group objects are exactly homeomorphism groups.
In some applications, particularly surfaces, the homeomorphism group is studied via this short exact sequence, and by first studying the mapping class group and group of isotopically trivial homeomorphisms, and then (at times) the extension.