# Group algebra

In functional analysis and related areas of mathematics, **group algebras** are constructions that generalize the concept of group ring to some classes of topological groups with the aim to reduce the theory of representations of topological groups to the theory of representations of topological algebras. There are several nonequivalent definitions of group algebra, each of which is considered convenient in a particular situation.

## Idea: group algebras for finite groups

If is a finite group then for any field the set of all functions possesses a natural structure of (finite-dimensional) Hopf algebra over with (the usual pointwise summation of functions and multiplication them by scalars, and)

- the multiplication , generated by the pointwise multiplication of functions,

- the comultiplication , generated by the operation of multiplication in

- and the antipode generated by the operation of taking the inverse element in

The dual space (of all linear functionals ) is the dual Hopf algebra, where, in particular, the multiplication can be described by the formula

or by the formula

where

are delta-functionals, and

are the expansions of and along the basis in the vector space .

- The algebra (denoted also as ) is called the
*group algebra*(or the*group ring*) of the group over the field .

- A map of a group into a unital associative algebra over is called a representation of the group in the algebra , if it preserves the unit and the multiplication:

**Example**: the operation of passage to the delta-functional

is a representation of in :

There is a natural correspondence between the representations of the group in (unital associative) algebras and the homomorphisms of the unital algebras :

**Theorem (universal property)**.*For any finite group and for any unital associative algebra over the formula*

*establishes a one-to-one correspondence between the representations of the group in and the homomorphisms of the unital associative algebras .*

This observation has a series of important corollaries, which allow to reduce the theory of representations of finite groups to the theory of representations of finite-dimensional algebras[1][2].

The construction of the group algebra can be easily generalized to arbitrary (not necessarily finite) groups (in the purely algebraic sense, without topology) with the same purposes: the generalization is called group ring , and many results are preserved in this way, including the fact that possesses the universal property (and is a Hopf algebra when is a field). But up to the recent time the generalizations to the topological groups faced numerous difficulties because of the lack of the convenient categories of topological vector spaces with dualities. The generalizations were mostly constructed in the category of Banach spaces, but the absence of a suitable duality in this category led to various distortions of the properties of these constructions, in particular, they were not Hopf algebras, and even the correspondence between the representations of groups and the homomorphisms of their group algebras was usually violated (however, this correspondence sometimes could be understood in some special sense).

## Classical group algebras in functional analysis

In classical functional analysis, there are several constructions generalizing the purely algebraic idea of group algebra, but preserving only some part of its main properties.

### The algebra *C*_{c}(*G*) of continuous functions with compact support

*C*(

_{c}*G*) of continuous functions with compact support

If *G* is a locally compact Hausdorff group, *G* carries an essentially unique left-invariant countably additive Borel measure μ called a Haar measure. Using the Haar measure, one can define a convolution operation on the space *C _{c}*(

*G*) of complex-valued continuous functions on

*G*with compact support;

*C*(

_{c}*G*) can then be given any of various norms and the completion will be a group algebra.

To define the convolution operation, let *f* and *g* be two functions in *C _{c}*(

*G*). For

*t*in

*G*, define

The fact that *f* * *g* is continuous is immediate from the dominated convergence theorem. Also

where the dot stands for the product in *G*. *C _{c}*(

*G*) also has a natural involution defined by:

where Δ is the modular function on *G*. With this involution, it is a *-algebra.

Theorem.With the norm:

C(_{c}G) becomes an involutive normed algebra with an approximate identity.

The approximate identity can be indexed on a neighborhood basis of the identity consisting of compact sets. Indeed, if *V* is a compact neighborhood of the identity, let *f _{V}* be a non-negative continuous function supported in

*V*such that

Then {*f _{V}*}

_{V}is an approximate identity. A group algebra has an identity, as opposed to just an approximate identity, if and only if the topology on the group is the discrete topology.

Note that for discrete groups, *C _{c}*(

*G*) is the same thing as the complex group ring

**C**[

*G*].

The importance of the group algebra is that it captures the unitary representation theory of *G* as shown in the following

Theorem.LetGbe a locally compact group. IfUis a strongly continuous unitary representation ofGon a Hilbert spaceH, then

is a non-degenerate bounded *-representation of the normed algebra

C(_{c}G). The map

is a bijection between the set of strongly continuous unitary representations of

Gand non-degenerate bounded *-representations ofC(_{c}G). This bijection respects unitary equivalence and strong containment. In particular, π_{U}is irreducible if and only ifUis irreducible.

Non-degeneracy of a representation π of *C _{c}*(

*G*) on a Hilbert space

*H*

_{π}means that

is dense in *H*_{π}.

### The convolution algebra *L*^{1}(*G*)

*L*

^{1}(

*G*)

It is a standard theorem of measure theory that the completion of *C _{c}*(

*G*) in the

*L*

^{1}(

*G*) norm is isomorphic to the space

*L*

^{1}(

*G*) of equivalence classes of functions which are integrable with respect to the Haar measure, where, as usual, two functions are regarded as equivalent if and only if they differ only on a set of Haar measure zero.

Theorem.L^{1}(G) is a Banach *-algebra with the convolution product and involution defined above and with theL^{1}norm.L^{1}(G) also has a bounded approximate identity.

### The group C*-algebra *C**(*G*)

*C**(

*G*)

Let **C**[*G*] be the group ring of a discrete group *G*.

For a locally compact group *G*, the group C*-algebra *C**(*G*) of *G* is defined to be the C*-enveloping algebra of *L*^{1}(*G*), i.e. the completion of *C _{c}*(

*G*) with respect to the largest C*-norm:

where π ranges over all non-degenerate *-representations of *C _{c}*(

*G*) on Hilbert spaces. When

*G*is discrete, it follows from the triangle inequality that, for any such π, one has:

hence the norm is well-defined.

It follows from the definition that *C**(*G*) has the following universal property: any *-homomorphism from **C**[*G*] to some **B**(*H*) (the C*-algebra of bounded operators on some Hilbert space *H*) factors through the inclusion map:

### The reduced group C*-algebra *C*_{r}*(*G*)

*C*(

_{r}**G*)

The reduced group C*-algebra *C _{r}**(

*G*) is the completion of

*C*(

_{c}*G*) with respect to the norm

where

is the *L*^{2} norm. Since the completion of *C _{c}*(

*G*) with regard to the

*L*

^{2}norm is a Hilbert space, the

*C*norm is the norm of the bounded operator acting on

_{r}**L*

^{2}(

*G*) by convolution with

*f*and thus a C*-norm.

Equivalently, *C _{r}**(

*G*) is the C*-algebra generated by the image of the left regular representation on

*ℓ*

^{2}(

*G*).

In general, *C _{r}**(

*G*) is a quotient of

*C**(

*G*). The reduced group C*-algebra is isomorphic to the non-reduced group C*-algebra defined above if and only if

*G*is amenable.

### von Neumann algebras associated to groups

The group von Neumann algebra *W**(*G*) of *G* is the enveloping von Neumann algebra of *C**(*G*).

For a discrete group *G*, we can consider the Hilbert space ℓ^{2}(*G*) for which *G* is an orthonormal basis. Since *G* operates on ℓ^{2}(*G*) by permuting the basis vectors, we can identify the complex group ring **C**[*G*] with a subalgebra of the algebra of bounded operators on ℓ^{2}(*G*). The weak closure of this subalgebra, *NG*, is a von Neumann algebra.

The center of *NG* can be described in terms of those elements of *G* whose conjugacy class is finite. In particular, if the identity element of *G* is the only group element with that property (that is, *G* has the infinite conjugacy class property), the center of *NG* consists only of complex multiples of the identity.

*NG* is isomorphic to the hyperfinite type II_{1} factor if and only if *G* is countable, amenable, and has the infinite conjugacy class property.

## Stereotype group algebras

The full analogy with the purely algebraic situation appears in the stereotype theory where a series of natural group algebras is constructed including the following four examples.

- On each locally compact group one can consider the algebra of all continuous functions with the pointwise multiplication. Being endowed with the topology of uniform convergence on compact sets , it becomes a stereotype algebra. Its stereotype dual space , which consists of Radon measures with compact support on , is a stereotype algebra with respect to the operation of convolution:[3]

- The algebra is called the
*stereotype group algebra of measures*on the locally compact group .[4]

- On each real Lie group one can consider the algebra of all smooth functions with the pointwise multiplication, and the topology of uniform convergence with all derivatives on compact sets . Again, it is a stereotype algebra. Its stereotype dual space , which consists of distributions with compact support on , is a stereotype algebra with respect to the operation of convolution of distributions. The algebra is called the
*stereotype group algebra of distributions*on the real Lie group .

- On each Stein group[5] one can consider the algebra of all holomorphic functions with the pointwise multiplication and the topology of uniform convergence on compact sets . Again, this is a stereotype algebra. Its stereotype dual space , which consists of holomorphic fuhctionals on , is a stereotype algebra with respect to the operation of convolution of functionals. The algebra is called the
*stereotype group algebra of analytic functionals*on the Stein group .

- On each affine algebraic group one can consider the algebra of all polynomials (or regular functions) with the pointwise multiplication and the strongest locally convex topology. This is again a stereotype algebra, and its stereotype dual space , which consists of currents on , is a stereotype algebra with respect to the operation of convolution of currents. The algebra is called the
*stereotype group algebra of currents*on the affine algebraic group .

The representation[6] , , , is called the *representation as delta-functionals*.

The representations , , , are defined similarly.

The following two results distinguish the stereotype group algebras among the other models of group algebras in analysis.

**Theorem (universal property)**.[7]*For any stereotype algebra the formula*

*establishes a one-to-one correspondence between*

*the continuous representations[6] of any given locally compact group in the stereotype algebra and the morphisms of stereotype algebras ,*

**Theorem**.[11]*The group algebras , , , are Hopf algebras in the monoidal category (***Ste**,,) of stereotype spaces.

## See also

## Notes

- Lang 2002, Chapter XVIII.
- Vinberg 2003, 12.4.
- Akbarov 2003, p. 272.
- If is an infinite locally compact group then the algebra of measures on is not a Fréchet algebra. In the case when is compact, is a Smith space. If is -compact, then is a Brauner space.
- A
*Stein group*is a complex Lie group which is a Stein manifold. - We use here the definition given above: a map of a group into a unital associative algebra over is called a
*representation of the group in the algebra*, if it preserves the unit and the multiplication: - Akbarov 2003, p. 275.
- A map of a smooth manifold into a stereotype space is said to be
*smooth*if for each functional the composition is a smooth function on , and the map is continuous. - A map of a Stein manifold into a stereotype space is said to be
*holomorphic*if for each functional the composition is a holomorphic function on , and the map is continuous. - A map of an affine algebraic variety over into a stereotype space is said to be
*polynomial*(or*regular*) if for each functional the composition is a polynomial on , and the map is continuous. - Akbarov 2009, p. 507.

## References

- Lang, S. (2002).
*Algebra*.*Graduate Texts in Mathematics*. Springer. ISBN 978-1-4613-0041-0. - Vinberg, E.B. (2003).
*A Course in Algebra*.*Graduate Studies in Mathematics*.**56**. ISBN 978-0-8218-3318-6. - Dixmier, J. (2003).
*C*-algebras*. North Holland. ISBN 978-0444557476. - Kirillov, A.A. (1976).
*Elements of the theory of representations*.*Grundlehren der mathematischen Wissenschaften*. Springer. ISBN 978-3-642-66243-0. - Loomis, L.H. (2011).
*Introduction to Abstract Harmonic Analysis*.*Dover Books on Mathematics*. Dover Publications. ISBN 978-0486481234. - A.I. Shtern (2001) [1994], "Group algebra of a locally compact group", in Hazewinkel, Michiel (ed.),
*Encyclopedia of Mathematics*, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4*This article incorporates material from Group $C^*$-algebra on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.* - Akbarov, S.S. (2003). "Pontryagin duality in the theory of topological vector spaces and in topological algebra".
*Journal of Mathematical Sciences*.**113**(2): 179–349. doi:10.1023/A:1020929201133. - Akbarov, S.S. (2009). "Holomorphic functions of exponential type and duality for Stein groups with algebraic connected component of identity".
*Journal of Mathematical Sciences*.**162**(4): 459–586. arXiv:0806.3205. doi:10.1007/s10958-009-9646-1.