Commutator

In algebra, the commutator of two elements of an algebraic structure is a measure of whether the algebraic operation is commutative.

Group theory

In a group, written multiplicatively, the commutator of elements x and y may be defined as

$[x,y]=x^{-1}y^{-1}xy\,$

(although variants on this definition are possible). Elements x and y commute if and only if the commutator [x,y] is equal to the group identity. The subgroup of G generated by all commutators, written [G,G], is normal and indeed characteristic and the quotient G/[G,G] is abelian. A quotient of G by a normal subgroup N is abelian if and only if N contains the commutator subgroup.

Ring theory

In a ring, the commutator of elements x and y may be defined as

$[x,y]=xy-yx.\,$