# Commutator

In mathematics, the **commutator** gives an indication of the extent to which a certain binary operation fails to be commutative. There are different definitions used in group theory and ring theory.

## Group theory

The **commutator** of two elements, g and h, of a group G, is the element

- [
*g*,*h*] =*g*^{−1}*h*^{−1}*gh*

and is equal to the group's identity if and only if g and h commute (that is, if and only if *gh* = *hg*). The set of all commutators of a group is not in general closed under the group operation, but the subgroup of *G* generated by all commutators is closed and is called the *derived group* or the *commutator subgroup* of *G*. Commutators are used to define nilpotent and solvable groups and the largest abelian quotient group.

The definition of the commutator above is used throughout this article, but many other group theorists define the commutator as

### Identities (group theory)

Commutator identities are an important tool in group theory.[3] The expression *a ^{x}* denotes the conjugate of a by x, defined as

*x*

^{−1}

*ax*.

- and
- and
- and

Identity (5) is also known as the *Hall–Witt identity*, after Philip Hall and Ernst Witt. It is a group-theoretic analogue of the Jacobi identity for the ring-theoretic commutator (see next section).

N.B., the above definition of the conjugate of a by x is used by some group theorists.[4] Many other group theorists define the conjugate of a by x as *xax ^{−1}*.[5] This is often written . Similar identities hold for these conventions.

Many identities are used that are true modulo certain subgroups. These can be particularly useful in the study of solvable groups and nilpotent groups. For instance, in any group, second powers behave well:

If the derived subgroup is central, then

## Ring theory

The **commutator** of two elements *a* and *b* of a ring (including any associative algebra) is defined by

It is zero if and only if *a* and *b* commute. In linear algebra, if two endomorphisms of a space are represented by commuting matrices in terms of one basis, then they are so represented in terms of every basis. By using the commutator as a Lie bracket, every associative algebra can be turned into a Lie algebra.

The **anticommutator** of two elements a and b of a ring or an associative algebra is defined by

Sometimes is used to denote anticommutator, while is then used for commutator.[6] The anticommutator is used less often, but can be used to define Clifford algebras and Jordan algebras, and in the derivation of the Dirac equation in particle physics.

The commutator of two operators acting on a Hilbert space is a central concept in quantum mechanics, since it quantifies how well the two observables described by these operators can be measured simultaneously. The uncertainty principle is ultimately a theorem about such commutators, by virtue of the Robertson–Schrödinger relation.[7] In phase space, equivalent commutators of function star-products are called Moyal brackets, and are completely isomorphic to the Hilbert-space commutator structures mentioned.

### Identities (ring theory)

The commutator has the following properties:

#### Additional identities

If A is a fixed element of a ring *R*, identity (1) can be interpreted as a Leibniz rule for the map given by . In other words, the map ad_{A} defines a derivation on the ring *R*. Identities (2), (3) represent Leibniz rules for more than two factors, and are valid for any derivation. Identities (4)–(6) can also be interpreted as Leibniz rules. Identities (7), (8) express **Z**-bilinearity.

Some of the above identities can be extended to the anticommutator using the above ± subscript notation.[8] For example:

#### Exponential identities

Consider a ring or algebra in which the exponential can be meaningfully defined, such as a Banach algebra, a ring of formal power series, or the universal enveloping algebra of a Lie algebra.

In such a ring, Hadamard's lemma applied to nested commutators gives:

(For the last expression, see *Adjoint derivation* below.) This formula underlies the Baker–Campbell–Hausdorff expansion of log(exp(*A*) exp(*B*)).
A similar expansion expresses the group commutator of expressions (analogous to elements of a Lie group) in terms of a series of nested commutators (Lie brackets),

## Graded rings and algebras

When dealing with graded algebras, the commutator is usually replaced by the **graded commutator**, defined in homogeneous components as

## Adjoint derivation

Especially if one deals with multiple commutators in a ring *R*, another notation turns out to be useful. For an element , we define the adjoint mapping by:

This mapping is a derivation on the ring *R*:

- .

By the Jacobi identity, it is also a derivation over the commutation operation:

- .

Composing such mappings, we get for example and

We may consider itself as a mapping, , where is the ring of mappings from *R* to itself with composition as the multiplication operation. Then is a Lie algebra homomorphism, preserving the commutator:

By contrast, it is **not** always a ring homomorphism: usually .

### General Leibniz rule

The general Leibniz rule, expanding repeated derivatives of a product, can be written abstractly using the adjoint representation:

Replacing *x* by the differentiation operator , and *y* by the multiplication operator , we get , and applying both sides to a function *g*, the identity becomes the usual Leibniz rule for the *n*^{th} derivative .

## See also

## Notes

- Fraleigh (1976, p. 108)
- Herstein (1975, p. 65)
- McKay (2000, p. 4)
- Herstein (1975, p. 83)
- Fraleigh (1976, p. 128)
- McMahon (2008)
- Liboff (2003, pp. 140–142)
- Lavrov, P.M. "Jacobi -type identities in algebras and superalgebras" (PDF).

## References

- Fraleigh, John B. (1976),
*A First Course In Abstract Algebra*(2nd ed.), Reading: Addison-Wesley, ISBN 0-201-01984-1 - Griffiths, David J. (2004),
*Introduction to Quantum Mechanics*(2nd ed.), Prentice Hall, ISBN 0-13-805326-X - Herstein, I. N. (1975),
*Topics In Algebra*(2nd ed.), John Wiley & Sons - Liboff, Richard L. (2003),
*Introductory Quantum Mechanics*(4th ed.), Addison-Wesley, ISBN 0-8053-8714-5 - McKay, Susan (2000),
*Finite p-groups*, Queen Mary Maths Notes,**18**, University of London, ISBN 978-0-902480-17-9, MR 1802994 - McMahon, D. (2008),
*Quantum Field Theory*, USA: McGraw Hill, ISBN 978-0-07-154382-8

## Further reading

- McKenzie, R.; Snow, J. (2005), "Congruence modular varieties: commutator theory", in Kudryavtsev, V. B.; Rosenberg, I. G. (eds.),
*Structural Theory of Automata, Semigroups, and Universal Algebra*(PDF), Springer, pp. 273–329

## External links

- Hazewinkel, Michiel, ed. (2001) [1994], "Commutator",
*Encyclopedia of Mathematics*, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4