nLab centralizer

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Definition

Definition

Given a group GG and a subset SGS \,\subset\, G of its underlying set, the centralizer subgroup (also: the commutant) of SS in GG is the subgroup

C G(S){gG|sS(gs=sg)}G C_G(S) \;\coloneqq\; \big\{ g \in G \,\vert\, \underset{s \in S}{\forall} ( g \cdot s \,=\, s \cdot g ) \big\} \;\subset\; G

of all elements cGc \in G which commute with the elements of SS.

Equivalently, the centralizer is the joint fixed point subgroup of the inner automorphisms on GG given by conjugation with the elements sSs \in S.

Notice the similarity with but the difference to the concept of normalizer subgroup, cf. Prop. .

Properties

Proposition

Given a subset SGS \subset G of a group GG, the centralizer subgroup of SS (Def. ) is a subgroup of the normalizer subgroup:

C G(S)N G(S). C_G(S) \; \subset \; N_G(S) \,.

Proof

Since an element gGg \in G which fixes each element sSs \in S separately already fixes the entire subset as such:

sS(gs=sg)(gS=Sg). \underset{s \in S}{\forall} \big( g \cdot s \,=\, s \cdot g \big) \;\;\;\;\; \Rightarrow \;\;\;\;\; \big( g \cdot S \,=\, S \cdot g \big) \,.

Proposition

(centralizers in T 1 T_1 -groups are closed)
If GG is a T 1 T_1 -topological group, then all its centralizer subgroups are closed subgroups.

Proof

First consider a singleton set S={s}S = \{s\}. By definition, the centralizer of a single element sGs \in G is the preimage of itself under the function

G G g gsg 1. \array{ G &\xrightarrow{\;\;}& G \\ g &\mapsto& g \cdot s \cdot g^{-1} \,. }

(the adjoint action of GG on itself).

Noticing here that:

  1. this is continuous function, by the axioms on a topological group;

  2. {s}G\{s\} \subset G is a closed subset, by the assumption that GG is a T 1 T_1 -space (by this Prop.)

it follows that C G({s})GC_G(\{s\}) \subset G is the continuous preimage of a closed subset and hence is itself closed (by this Prop.).

Now for a general set SS, its centralizer is clearly the intersection of the centralizers of (the singleton sets on) its elements:

C G(S)=sSC G({s}). C_G(S) \;=\; \underset{ s \in S }{\cap} C_G\big(\{s\}\big) \,.

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.

References

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.