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quadratic Lie algebra (Definition)

A Lie algebra $ \mathfrak{g}$ is said to be quadratic if $ \mathfrak{g}$ as a representation (under the adjoint action) admits a non-degenerate, invariant scalar product $ ( \cdot \mid \cdot )$ .

$ \mathfrak{g}$ being quadratic implies that the adjoint and co-adjoint representations of $ \mathfrak{g}$ are isomorphic.

Indeed, the non-degeneracy of $ ( \cdot \mid \cdot )$ implies that the induced map $ \phi \colon \mathfrak{g} \to \mathfrak{g}^*$ given by $ \phi(X)(Z) = (X \mid Z)$ is an isomorphism of vector spaces. Invariance of the scalar product means that $ ( [X,Y] \mid Z) = -(Y \mid [X,Z] ) = (Y \mid [Z,X] )$. This implies that $ \phi$ is a map of representations:

$\displaystyle \phi(ad_X(Y))(Z) = \phi([X,Y])(Z) = ( [X,Y] \mid Z ) = ( Y \mid [Z,X] ) = ad^*_X( \phi(Y)(Z) ) $



"quadratic Lie algebra" is owned by benjaminfjones.
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See Also: quadratic algebra

Also defines:  quadratic Lie algebra
Keywords:  quadratic, invariant scalar product, Lie algebra
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Cross-references: scalar product, vector spaces, isomorphism, map, induced, isomorphic, adjoint, implies, invariant scalar product, non-degenerate, adjoint action, representation, Lie algebra

This is version 3 of quadratic Lie algebra, born on 2005-09-20, modified 2005-09-21.
Object id is 7379, canonical name is QuadraticLieAlgebra.
Accessed 1540 times total.

Classification:
AMS MSC17B01 (Nonassociative rings and algebras :: Lie algebras and Lie superalgebras :: Identities, free Lie algebras)
 17B10 (Nonassociative rings and algebras :: Lie algebras and Lie superalgebras :: Representations, algebraic theory )

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