nLab Lie algebra representation

Contents

Context

Representation theory

Lie theory

∞-Lie theory (higher geometry)

Background

Smooth structure

Higher groupoids

Lie theory

∞-Lie groupoids

∞-Lie algebroids

Formal Lie groupoids

Cohomology

Homotopy

Related topics

Examples

\infty-Lie groupoids

\infty-Lie groups

\infty-Lie algebroids

\infty-Lie algebras

Contents

Idea

The representation/action of a Lie algebra 𝔤\mathfrak{g} on a vector space VV.

Definition

A Lie algebra representation is a Lie algebra homomorphism ρ:𝔤end(V)\rho \;\colon\;\mathfrak{g} \to end(V) to the endomorphism Lie algebra of VV.

This means equivalently that

𝔤V ρ V (x,v) ρ x(v) \array{ \mathfrak{g} \otimes V & \overset{\rho}{\longrightarrow} & V \\ (x,v) &\mapsto& \rho_x(v) }

is a bilinear map such that for all x i𝔤x_i \in \mathfrak{g} and vVv \in V we have the Lie action property:

(1)ρ x 1(ρ x 2(v))ρ x 2(ρ x 1(v))=ρ [x 1,x 2](v) \rho_{x_1}(\rho_{x_2}(v)) - \rho_{x_2}(\rho_{x_1}(v)) \;=\; \rho_{[x_1,x_2]}(v)

Properties

In terms of string diagrams / Jacobi diagrams

In string diagram-notation for Lie algebra objects internal to tensor categories, the Lie action property (1) looks as follows:

ρ(f(x,y),z)=ρ(y,ρ(x,z))ρ(x,ρ(y,z)) f=[,]Liebracket ρ([x,y],z)=ρ(y,ρ(x,z))ρ(x,ρ(y,z))Lieactionproperty ρ=[,]adjointaction [[x,y],z]=[y,[x,z]]+[x,[y,z]]Jacobiidentity \begin{aligned} \Leftrightarrow & \;\;\;\;\; \rho(f(x,y),z) \;=\; \rho(y,\rho(x,z)) - \rho(x,\rho(y,z)) \\ \underset{ {f = [-,-]} \atop {Lie\;bracket} }{ \Leftrightarrow } & \;\;\;\;\; \underset{ {Lie\;action\;property} }{ \underbrace{ \rho([x,y],z) \;=\; \rho(y,\rho(x,z)) - \rho(x,\rho(y,z)) } } \\ \underset{ {\rho = -[-,-]} \atop {adjoint\;action} }{ \Leftrightarrow } & \;\;\;\;\; \underset{ {Jacobi\;identity} }{ \underbrace{ [[x,y],z] \;=\; - [y,[x,z]] + [x,[y,z]] } } \end{aligned}

Here the last line shows the equivalence to the Jacobi identity on the Lie algebra object itself in the case that the Lie action is the adjoint action.

In the language of Jacobi diagrams this is called the STU-relation, and is the reason behind the existence of Lie algebra weight systems in knot theory.

References

See also

Last revised on May 1, 2021 at 08:19:41. See the history of this page for a list of all contributions to it.