Representations of Lie Groups and Lie Algebras |

**In General**
> s.a. examples of lie groups; lorentz
and poincaré group; SU(2).

* __Approaches__: Infinitesimal
(Cartan-Weyl), and global; The latter gives simple canonical realizations
of the carrier space in terms of polynomials in complex variables.

* __Decomposition of tensor
products__: Formulas for multiplicites are known for some cases, such as
*n*-fold products of fundamental (*j* = 1/2) representations
of GL(*n*), U(*n*), SU(*n*);
> s.a. Clebsh-Gordan Coefficients.

* __Connected topological solvable__:
Every finite-D irr is 1D (Lie); __Examples__: 2D Poincaré
group; Heisenberg group.

* __Semisimple connected__:
Any representation is fully reducible.

* __Semisimple connected compact__:
There are no non-trivial finite-dimensional faithful unitary irr's.

* __Simple connected__:
The only 1D irreducible representation is the trivial one.

* __Simple connected compact__:
There are no non-trivial finite-dimensional unitary irr's.

@ __Texts__: Varadarajan 84;
Bröcker & tom Dieck 85;
Fulton & Harris 91;
Vilenkin & Klimyk 94 [and special functions].

@ __General references__:
Chaturvedi et al RVMP(06) [Schwinger rep];
Kurnyavko & Shirokov a1710
[constructing infinitesimal invariants].

**Adjoint Representation**

$ __Def__: The mapping Ad: *G* → L(\(\cal G\), \(\cal G\)),
where \(\cal G\) is the Lie algebra of *G*, defined by Ad(*g*):
\(\cal G\) → \(\cal G\), corresponding to (L_{g}
R_{g}^{−1})'(*e*)
in the isomorphism between T_{e}*G* and \(\cal G\)
given by *α*_{e} ∈
T_{e}*G* (*α* = left-invariant
vector field generated by *α*_{e}).

* __Notation__: Often represented by Ad(*g*)
*γ* = *g* *γ* *g*^{−1},
literally correct if *G* is (a subgroup of) GL(*n*).

@ __References__: in Choquet-Bruhat et al 82.

**Specific Groups** > s.a. lie groups [generalizations].

* __SL(2, C)__:

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**Representations of Lie Algebras**
> s.a. group representations / lie algebras;
simplex [polytope]; Special Functions.

$ __Adjoint representation__:
The differential of the adjoint representation of the Lie group *G*
on \(\cal G\) at the identity *e* (modulo the isomorphism of
T_{e}*G* onto \(\cal G\)),
ad: \(\cal G\) → L(\(\cal G\), \(\cal G\)),
defined by ad:= Ad'_{e}.

* __Matrix form__: If *f*_{ijk}
are the structure constants corresponding to the generators *T*_{i},
then (*T*_{i}^{adj})\(_{jk}\)
= −i *f*_{ijk}.

@ __References__: Turbiner qa/97 [in Fock space];
Humphreys BAMS(98) [simple, modular representations];
de Azcárraga & Macfarlane NPB(00)ht [fermionic];
Popovych et al JPA(03)mp [as vector fields on a manifold];
Lau m.RT/04 [bosonic and fermionic];
Henderson 12.

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