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,[1]
the comultiplication , generated by the operation of multiplication in [2]
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. Since the delta-functionals
,
form a basis in the vector space , one can describe the Hopf operations in by their actions on the elements :[3]
The algebra (denoted also as ) is called the group algebra (or the group ring) of the group over the field . The following construction connects the representations of the group with the representations of the algebra :
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)[6][7]. 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.[8][7]
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).
Examples
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:[9]
The algebra is called the stereotype group algebra of measures on the locally compact group .[10]
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[11] 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[12], , , 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.
the continuous representations[12] of any given locally compact group in the stereotype algebra and the morphisms of stereotype algebras ,
the smooth[14] representations[12] of any given real Lie group in the stereotype algebra and the morphisms of stereotype algebras ,
the holomorphic[15] representations[12] of any given Stein group in the stereotype algebra and the morphisms of stereotype algebras ,
the polynomial (regular)[16] representations[12] of any given affine algebraic group in the stereotype algebra and the morphisms of stereotype algebras .
^The unit for this multiplication is defined by the formula
where 1 means the identity function on :
^The counit for this comultiplication is defined by the formula
where means the unit in .
^In particular, the multiplication turns out to be the usual convolution of functionals :
or
where
are the expansions of and along the basis in the vector space .
^The unit for this multiplication is defined by the formula
where means the unit in
^The counit for this comultiplication is defined by the formula
^Lang 2002, Chapter XVIII § 1. sfn error: no target: CITEREFLang2002 (help)
^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.
^ abcdeWe 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:
^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, 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. {{cite journal}}: Invalid |ref=harv (help)
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. {{cite journal}}: Invalid |ref=harv (help)