Group representation explained

In the mathematical field of representation theory, group representations describe abstract groups in terms of bijective linear transformations of a vector space to itself (i.e. vector space automorphisms); in particular, they can be used to represent group elements as invertible matrices so that the group operation can be represented by matrix multiplication.

In chemistry, a group representation can relate mathematical group elements to symmetric rotations and reflections of molecules.

Representations of groups allow many group-theoretic problems to be reduced to problems in linear algebra. In physics, they describe how the symmetry group of a physical system affects the solutions of equations describing that system.

The term representation of a group is also used in a more general sense to mean any "description" of a group as a group of transformations of some mathematical object. More formally, a "representation" means a homomorphism from the group to the automorphism group of an object. If the object is a vector space we have a linear representation. Some people use realization for the general notion and reserve the term representation for the special case of linear representations. The bulk of this article describes linear representation theory; see the last section for generalizations.

Branches of group representation theory

The representation theory of groups divides into subtheories depending on the kind of group being represented. The various theories are quite different in detail, though some basic definitions and concepts are similar. The most important divisions are:

Representation theory also depends heavily on the type of vector space on which the group acts. One distinguishes between finite-dimensional representations and infinite-dimensional ones. In the infinite-dimensional case, additional structures are important (e.g. whether or not the space is a Hilbert space, Banach space, etc.).

One must also consider the type of field over which the vector space is defined. The most important case is the field of complex numbers. The other important cases are the field of real numbers, finite fields, and fields of p-adic numbers. In general, algebraically closed fields are easier to handle than non-algebraically closed ones. The characteristic of the field is also significant; many theorems for finite groups depend on the characteristic of the field not dividing the order of the group.

Definitions

A representation of a group G on a vector space V over a field K is a group homomorphism from G to GL(V), the general linear group on V. That is, a representation is a map

\rho\colonG\toGL\left(V\right)

such that

\rho(g1g2)=\rho(g1)\rho(g2),    forallg1,g2\inG.

Here V is called the representation space and the dimension of V is called the dimension or degree of the representation. It is common practice to refer to V itself as the representation when the homomorphism is clear from the context.

In the case where V is of finite dimension n it is common to choose a basis for V and identify GL(V) with, the group of

n x n

invertible matrices on the field K.

\ker\rho=\left\{g\inG\mid\rho(g)=id\right\}.

A faithful representation is one in which the homomorphism is injective; in other words, one whose kernel is the trivial subgroup consisting only of the group's identity element.

\alpha\circ\rho(g)\circ\alpha-1=\pi(g).

Examples

Consider the complex number u = e2πi / 3 which has the property u3 = 1. The set C3 = forms a cyclic group under multiplication. This group has a representation ρ on

C2

given by:

\rho\left(1\right)= \begin{bmatrix} 1&0\\ 0&1\\ \end{bmatrix}    \rho\left(u\right)= \begin{bmatrix} 1&0\\ 0&u\\ \end{bmatrix}    \rho\left(u2\right)= \begin{bmatrix} 1&0\\ 0&u2\\ \end{bmatrix}.

This representation is faithful because ρ is a one-to-one map.

Another representation for C3 on

C2

, isomorphic to the previous one, is σ given by:

\sigma\left(1\right)= \begin{bmatrix} 1&0\\ 0&1\\ \end{bmatrix}    \sigma\left(u\right)= \begin{bmatrix} u&0\\ 0&1\\ \end{bmatrix}    \sigma\left(u2\right)= \begin{bmatrix} u2&0\\ 0&1\\ \end{bmatrix}.

The group C3 may also be faithfully represented on

R2

by τ given by:

\tau\left(1\right)= \begin{bmatrix} 1&0\\ 0&1\\ \end{bmatrix}    \tau\left(u\right)= \begin{bmatrix} a&-b\\ b&a\\ \end{bmatrix}    \tau\left(u2\right)= \begin{bmatrix} a&b\\ -b&a\\ \end{bmatrix}

where

a=Re(u)=-\tfrac{1}{2},    b=Im(u)=\tfrac{\sqrt{3}}{2}.

Another example:

Let

V

be the space of homogeneous degree-3 polynomials over the complex numbers in variables

x1,x2,x3.

Then

S3

acts on

V

by permutation of the three variables.

For instance,

(12)

sends
3
x
1
to
3
x
2
.

Reducibility

See main article: Irreducible representation.

A subspace W of V that is invariant under the group action is called a subrepresentation. If V has exactly two subrepresentations, namely the zero-dimensional subspace and V itself, then the representation is said to be irreducible; if it has a proper subrepresentation of nonzero dimension, the representation is said to be reducible. The representation of dimension zero is considered to be neither reducible nor irreducible, [1] just as the number 1 is considered to be neither composite nor prime.

Under the assumption that the characteristic of the field K does not divide the size of the group, representations of finite groups can be decomposed into a direct sum of irreducible subrepresentations (see Maschke's theorem). This holds in particular for any representation of a finite group over the complex numbers, since the characteristic of the complex numbers is zero, which never divides the size of a group.

In the example above, the first two representations given (ρ and σ) are both decomposable into two 1-dimensional subrepresentations (given by span and span), while the third representation (τ) is irreducible.

Generalizations

Set-theoretical representations

A set-theoretic representation (also known as a group action or permutation representation) of a group G on a set X is given by a function ρ : GXX, the set of functions from X to X, such that for all g1, g2 in G and all x in X:

\rho(1)[x]=x

\rho(g1g2)[x]=\rho(g1)[\rho(g2)[x]],

where

1

is the identity element of G. This condition and the axioms for a group imply that ρ(g) is a bijection (or permutation) for all g in G. Thus we may equivalently define a permutation representation to be a group homomorphism from G to the symmetric group SX of X.

For more information on this topic see the article on group action.

Representations in other categories

Every group G can be viewed as a category with a single object; morphisms in this category are just the elements of G. Given an arbitrary category C, a representation of G in C is a functor from G to C. Such a functor selects an object X in C and a group homomorphism from G to Aut(X), the automorphism group of X.

In the case where C is VectK, the category of vector spaces over a field K, this definition is equivalent to a linear representation. Likewise, a set-theoretic representation is just a representation of G in the category of sets.

When C is Ab, the category of abelian groups, the objects obtained are called G-modules.

For another example consider the category of topological spaces, Top. Representations in Top are homomorphisms from G to the homeomorphism group of a topological space X.

Two types of representations closely related to linear representations are:

See also

References

Notes and References

  1. Web site: 2019-09-04. 1.4: Representations. 2021-06-23. Chemistry LibreTexts. en.