# User:Maximilian Janisch/latexlist/Algebraic Groups/Representation of a compact group(2)

A homomorphism of a compact group into the group of continuous linear automorphisms of a (complex) Banach space that is continuous with respect to the strong operator topology.

Let $k$ be a compact group, let $V$ be a Banach space and let $\phi : G \rightarrow \operatorname { GL } ( V )$ be a representation. If $V = H$ is a Hilbert space and $\phi ( g )$ is a unitary operator for every $g \in G$, then $( 1 )$ is called a unitary representation. There always is an equivalent norm in $H$ for which $( 1 )$ is unitary.

Every irreducible unitary representation (cf. Irreducible representation) of a compact group $k$ is finite-dimensional. Let $\{ \rho ^ { \alpha } : \alpha \in I \}$ be the family of all possible pairwise inequivalent irreducible unitary representations of the group $k$. Every unitary representation $( 1 )$ of $k$ is an orthogonal sum of unique representations $\phi ^ { a }$, $\alpha \in I$, such that $\phi ^ { a }$ is an orthogonal sum, possibly zero, of a set of representations equivalent to $\rho ^ { \alpha }$.

If $k$ is finite, then the family $\{ \rho ^ { \alpha } \}$ is also finite and contains as many elements as there are distinct conjugacy classes in $k$ (moreover, $\sum _ { \alpha \in I } ( \operatorname { dim } \rho ^ { \alpha } ) ^ { 2 } = | G |$). The problem of studying these representations (computing their characters, finding explicit realizations, etc.) is the subject of an extensive theory (cf. Finite group, representation of a).

If $k$ is a connected, simply-connected, compact Lie group and $G _ { C }$ is its complexification (cf. Complexification of a Lie group), then the description of the family $\{ \rho ^ { \alpha } : \alpha \in I \}$ for $k$ amounts (by restricting the representations to $k$) to the description of the family of all irreducible pairwise inequivalent finite-dimensional rational representations of the reductive algebraic group $G _ { C }$. The latter family, in turn, allows of a complete description by considering highest weights (cf. Representation with a highest weight vector).

In modern number theory and algebraic geometry one considers $1$-adic representations of compact totally-disconnected groups (cf. [5], [6]).

#### References

 [1] L.S. Pontryagin, "Topological groups" , Princeton Univ. Press (1958) (Translated from Russian) MR0201557 Zbl 0022.17104 [2] M.A. Naimark, "Theory of group representations" , Springer (1982) (Translated from Russian) MR0793377 Zbl 0484.22018 [3] D.P. Zhelobenko, "Compact Lie groups and their representations" , Amer. Math. Soc. (1973) (Translated from Russian) MR0473097 MR0473098 Zbl 0228.22013 [4] S. Lang, "$SL _ { 2 } ( R )$" , Addison-Wesley (1975) [5] I.M. Gel'fand, M.I. Graev, I.I. Pyatetskii-Shapiro, "Generalized functions" , 6. Representation theory and automorphic functions , Saunders (1969) (Translated from Russian) Zbl 0801.33020 Zbl 0699.33012 Zbl 0159.18301 Zbl 0355.46017 Zbl 0144.17202 Zbl 0115.33101 Zbl 0108.29601 [6] J.-P. Serre, "Cohomologie Galoisienne" , Springer (1964) MR0181643 Zbl 0143.05901 Zbl 0128.26303 [7] C. Chevalley, "Theory of Lie groups" , 1 , Princeton Univ. Press (1946) MR0082628 MR0015396 Zbl 0063.00842