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Dual representation : ウィキペディア英語版
Dual representation

In mathematics, if is a group and is a linear representation of it on the vector space , then the dual representation is defined over the dual vector space as follows:〔Lecture 1 of 〕
: is the transpose of , that is, = for all .
The dual representation is also known as the contragredient representation.
If is a Lie algebra and is a representation of it on the vector space , then the dual representation is defined over the dual vector space as follows:〔Lecture 8 of 〕
: = for all .
In both cases, the dual representation is a representation in the usual sense.
==Motivation==
In representation theory, both vectors in and linear functionals in are considered as ''column vectors'' so that the representation can act (by matrix multiplication) from the ''left''. Given a basis for and the dual basis for , the action of a linear functional on , can be expressed by matrix multiplication,
:\langle\varphi, v\rangle \equiv \varphi(v) = \varphi^Tv,
where the superscript is matrix transpose. Consistency requires
:\langle^
*(g)\varphi, \rho(g)v\rangle = \langle\varphi, v\rangle.〔Lecture 1, page 4 of 〕
With the definition given,
:\langle^
*(g)\varphi, \rho(g)v\rangle = \langle\rho(g^)^T\varphi, \rho(g)v\rangle = (\rho(g^)^T\varphi)^T \rho(g)v = \varphi^T\rho(g^)\rho(g)v = \varphi^Tv = \langle\varphi, v\rangle.
For the Lie algebra representation one chooses consistency with a possible group representation. Generally, if is a representation of a Lie group, then given by
:\pi(X) = \frac\Pi(e^)|_.
is a representation of its Lie algebra. If is dual to , then its corresponding Lie algebra representation is given by
:\pi^
*(X) = \frac\Pi^
*(e^)|_ = \frac\Pi(e^)^T|_ = -\pi(X)^T..〔Lecture 8, page 111 of 〕

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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