centralizers in algebra

1 Abstract definitions and properties

Definition 1.

Let S be a set with a binary operationMathworldPlanetmath *. Let T be a subset of S. Then define the centralizerMathworldPlanetmathPlanetmathPlanetmath in S of T as the subset

CS(T)={sS:s*t=t*s, for all tT}.

The center of S is defined as CS(S). This is commonly denoted Z(S) where Z is derived from the German word zentral. Subsets and elements of the center are called central.

If we regard *:S×SS in the of actions we can perscribe a left action s*t and a right action t*s. The centralizer is thus the set of elements for which the left regularPlanetmathPlanetmathPlanetmath action and the right regular action agree when to T.

It is generally possible to have s*t not lie in T for sCS(T) and tT, and likewise, it is also possible that if s,sCS(T) that s*ss*s. Therefore it should not be presumed that the centralizer is central.

With further axioms on the of operationMathworldPlanetmath we can deduce certain natural for the set CS(T).

Proposition 2.
  1. 1.

    If AB, then CS(B)CS(A). In particular, CS()=S.

  2. 2.

    If S has an identityPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath then CS(T) is non-empty. In particular, in this case Z(S) is non-empty. 11An identity of S is an element eS such that e*s=s*e for all sS.

  3. 3.

    If S is associative and s,sCS(T) then s*sCS(T), we say then that CS(T) is closed to the binary operation of S.

  4. 4.

    If sCS(T) and s has an (strong) inverseMathworldPlanetmathPlanetmath s-1, then s-1CS(T).22We say an inverse is strong if s-1*(s*t)=t=(t*s)*s-1 for all tS. If the operation is associative then this is given for free. There are natural nonassociative operations with this property, such as alternative algebrasMathworldPlanetmath.

  5. 5.

    If S is commutativePlanetmathPlanetmathPlanetmath then CS(T)=S.

  6. 6.

    If T is a subset of the center of S then CS(T)=S.

Note that it is possible for CS(T) be a subset closed to the opertaion without the assumptionPlanetmathPlanetmath of associativity, as for example, when S is commutative.

2 Centralizers in groups

In the category of groups the centralizer in a group G of a subset H can be redefined as:

CG(H)={gG:g-1hg=h, for all hH}.

If one regards conjugationMathworldPlanetmath as a group action hg:=g-1hg then it follows that the centralizer is the same as the pointwise stabilizerMathworldPlanetmath in G of H, where the action is of G on itself by conjugation. Because of this overlap, in some contexts the centralizers is applied to the pointwise stabilizer of a set on which a group acts, though this context no longer refers to the action of conjugation. This is espeically common when there is a need to distinguish between the pointwise stabilizer and the setwise stabilizer.

In this category, the centralizer is always a subgroupMathworldPlanetmathPlanetmath of G. Furthermore, if H is a normal subgroupMathworldPlanetmath of G, then so too is CG(H).

3 Centralizers in rings and algebras

For we treat rings as algebras over and now speak only of algebrasMathworldPlanetmathPlanetmathPlanetmath, which will include nonassociative examples.

In an algebra A there is in fact two binary operations on the set A in question. Thus the abstract definition of the centralizer is ambiguous. However, the additive operation of rings and algebras is always commutative and so any centralizer with respect to this operation is the set A. Thus it is generally accepted practice to assume that centralizers in this context always refer to the multiplicative operation. In this way we have the following properties.

Proposition 3.

Given an algebra A over a commutative unital ring R and a subset B of A, then

  1. 1.

    CA(B) is a submoduleMathworldPlanetmath of A.

  2. 2.

    If A is associative then CA(B) is a subalgebraMathworldPlanetmathPlanetmath.

  3. 3.

    Z(A)CA(B), in particular, if A has a 1 then 1CA(B) and so R embeds in CA(B).


  • A centralizer in an algebra is also called a commutant. This terminology is mostly used in algebras of operators in functional analysis.

  • Let R be a ring (or an algebra). For every ordered pair (a,b) of elements of R, we can define the additive commutator of (a,b) to be the element ab-ba, written [a,b]. With this, one may alternatively define the centralizer of a set SR in a ring R as

    CR(S):={rR[r,s]=0 for all sS}.

    Of course, in this definition, two operations (multiplication and subtraction) are needed instead of one. But the nice aspect about this definition is that one can “measure” commutativity of a ring by the additive commutation operation. For example, one can show that, in a division ring, if every element additively commutes with every additive commutator, then the ring must be a field.

4 Centralizers in Lie algebras

Suppose 𝔤 is a Lie aglebra over a commutative ring of characteristic not 2. Given a subset T of 𝔤, then [s,t]=-[t,s] for s𝔤 and tT from the axioms of a Lie algebra multiplication. Therefore whenever [s,t]=[t,s] it follows that -[t,s]=[t,s] so that [t,s]=0. This motivates the more common redefinition of the centralizer in a Lie algebra:

C𝔤(T)={s𝔤:[s,t]=0, for all tT}.

Despite the incongruety in characteristic 2, this new definition replaces the original definition of centralizers for Lie algebras. The centralizer of a Lie algebra is a subalgebra.

When the Lie multiplication is regarded as a commutatorMathworldPlanetmathPlanetmath, so [a,b]=ab-ba, for example it it universal enveloping algebra, then 0=[a,b]=ab-ba is the same as ab=ba and so the centralizer of the Lie algebra coincides with the centralizer of the associative envelope.

5 Centralizers in other nonassociative algebras

The centralizer need not be a subalgebra on account of the lack of associativity. There are instances of non-associative algebras where the centralizer is however a subalgebra nontheless, for example, Lie algebras as seen above. In travial fashion, if an algebra is commutative then CA(B)=A and so the centralizer is a subalgebra but without any useful properties. There is a suitable additional constraint to add to centralizers to force them to be subalgebras and carry with them more useful in the commutative but nonassociative setting.

We write [a,b] for ab-ba, called the commutator in A of a,bA and also write /a,b,c/ for (ab)c-a(bc) and call it the associatorMathworldPlanetmath in A of a,b,cA.33This notation for associators is non-standard but the standard [a,b,c]=(ab)c-a(bc) is likely confusing given the usual commutator notation used already. Then we can redefine the centralizer in A of a subset T of A as

CA(T)={sA:[s,t]=0,/s,s,t/=/s,t,s/=/t,s,s/=0 for all tT,sA}.

It follows that CA(T) is a subalgebra of A on account of the added associator condition which forces the subset to be closed to the productMathworldPlanetmathPlanetmath.

In alternative algebras, if any one of three associators is 0 then the other three are as well and so the definition reduces to /s,s,t/=0. occur of other nonassociative algebras.

Title centralizers in algebra
Canonical name CentralizersInAlgebra
Date of creation 2013-03-22 17:22:30
Last modified on 2013-03-22 17:22:30
Owner Algeboy (12884)
Last modified by Algeboy (12884)
Numerical id 14
Author Algeboy (12884)
Entry type Definition
Classification msc 17A01
Synonym centraliser
Related topic CentralizerOfASubsetOfAGroup
Related topic LieAlgebra
Defines centralizer
Defines center
Defines central
Defines centraliser
Defines additive commutator