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Given a group and a subset of its underlying set, the centralizer subgroup (also: the commutant) of in is the subgroup
of all elements which commute with the elements of .
Equivalently, the centralizer is the joint fixed point subgroup of the inner automorphisms on given by conjugation with the elements .
Notice the similarity with but the difference to the concept of normalizer subgroup, cf. Prop. .
Given a subset of a group , the centralizer subgroup of (Def. ) is a subgroup of the normalizer subgroup:
Since an element which fixes each element separately already fixes the entire subset as such:
(centralizers in -groups are closed)
If is a -topological group, then all its centralizer subgroups are closed subgroups.
First consider a singleton set . By definition, the centralizer of a single element is the preimage of itself under the function
(the adjoint action of on itself).
Noticing here that:
this is continuous function, by the axioms on a topological group;
is a closed subset, by the assumption that is a -space (by this Prop.)
it follows that is the continuous preimage of a closed subset and hence is itself closed (by this Prop.).
Now for a general set , its centralizer is clearly the intersection of the centralizers of (the singleton sets on) its elements:
Since each of the factors on the right isclosed, by the previous argument, the general centralizer subgroup is an intersection of closed subsets and hence itself a closed subset.
See also
Last revised on September 22, 2024 at 06:17:50. See the history of this page for a list of all contributions to it.