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Adjoint representation of a Lie algebra
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Adjoint representation of a Lie algebra : ウィキペディア英語版
Adjoint representation of a Lie algebra

In mathematics, the adjoint endomorphism or adjoint action is a homomorphism of Lie algebras that plays a fundamental role in the development of the theory of Lie algebras.
Given an element of a Lie algebra \mathfrak, one defines the adjoint action of on \mathfrak as the map
:\operatorname_x :\mathfrak\to \mathfrak \qquad \text \qquad \operatorname_x (y) = ()
for all in \mathfrak.
The concept generates the adjoint representation of a Lie group Ad. In fact, ad is the differential of Ad at the identity element of the group.
== Adjoint representation ==

Let \mathfrak be a Lie algebra over a field . Then the linear mapping
:\operatorname:\mathfrak \to \operatorname(\mathfrak)
given by is a representation of a Lie algebra and is called the adjoint representation of the algebra. (Its image actually lies in Der(\mathfrak). See below.)
Within End(\mathfrak), the Lie bracket is, by definition, given by the commutator of the two operators:
:()=\operatorname_x \circ \operatorname_y - \operatorname_y \circ \operatorname_x
where ○ denotes composition of linear maps.
If \mathfrak is finite-dimensional, then End(\mathfrak) is isomorphic to \mathfrak(\mathfrak), the Lie algebra of the general linear group over the vector space \mathfrak and if a basis for it is chosen, the composition corresponds to matrix multiplication.
Using the above definition of the Lie bracket, the Jacobi identity
:]+\right)(z) = \left(\operatorname_\right)(z)
where , , and are arbitrary elements of \mathfrak.
This last identity says that ad really is a Lie algebra homomorphism; i.e., a linear mapping that takes brackets to brackets.
In a more module-theoretic language, the construction simply says that \mathfrak is a module over itself.
The kernel of ad is, by definition, the center of \mathfrak. Next, we consider the image of ad. Recall that a derivation on a Lie algebra is a linear map \delta:\mathfrak\rightarrow \mathfrak that obeys the Leibniz' law, that is,
:\delta (()) = () + (\delta(y) )
for all and in the algebra.
That ad''x'' is a derivation is a consequence of the Jacobi identity. This implies that the image of \mathfrak under ad is
a subalgebra of Der(\mathfrak), the space of all derivations of \mathfrak.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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