nLab
Lie algebra

Context

\infty-Lie theory

∞-Lie theory

Background

Smooth structure

Higher groupoids

Lie theory

∞-Lie groupoids

∞-Lie algebroids

Formal Lie groupoids

Cohomology

Homotopy

Examples

\infty-Lie groupoids

\infty-Lie groups

\infty-Lie algebroids

\infty-Lie algebras

Contents

Idea

A Lie algebra is the infinitesimal approximation to a Lie group.

Definition

Ordinary definition

A Lie algebra is a vector space 𝔤\mathfrak{g} equipped with a bilinear skew-symmetric map [,] 𝔤𝔤𝔤[-,-] _\mathfrak{g} \vee \mathfrak{g} \to \mathfrak{g} which satisfies the Jacobi identity:

x,y,z𝔤:[x,[y,z]]+[z,[x,y]]+[y,[z,x]]=0. \forall x,y,z \in \mathfrak{g} : [x,[y,z]] + [z,[x,y]] + [y,[z,x]] = 0 \,.

A homomorphism of Lie algebras is a linear map ϕ:𝔤𝔥\phi : \mathfrak{g} \to \mathfrak{h} such that for all x,y𝔤x,y \in \mathfrak{g} we have

ϕ([x,y] 𝔤)=[ϕ(x),ϕ(y)] 𝔥. \phi([x,y]_{\mathfrak{g}}) = [\phi(x),\phi(y)]_{\mathfrak{h}} \,.

This defines the category LieAlg of Lie algebras.

In a general linear category

The notion of Lie algebra may be formulated internal to any linear category. This general definition subsumes as special case generalizations such as super Lie algebras.

Given a commutative unital ring kk, and a (strict for simplicity) symmetric monoidal kk-linear category (C,,1)(C,\otimes,1) with the symmetry τ\tau, a Lie algebra in (C,,1,τ)(C,\otimes,1,\tau) is an object LL in CC together with a morphism [,]:AAA[,]: A\otimes A\to A such that the Jacobi identity

[,[,]]+[,[,]](id Lτ L,L)(τid L)+[,[,]](τ L,Lid L)(id Lτ L,L)=0[,[,]]+[,[,]]\circ(id_L\otimes\tau_{L,L})\circ(\tau\otimes id_L)+[,[,]]\circ (\tau_{L,L}\otimes id_L)\circ (id_L\otimes\tau_{L,L}) = 0

and antisymmetry

[,]+[,]τ L,L=0[,]+[,]\otimes\tau_{L,L} = 0

hold. If kk is the ring \mathbb{Z} of integers, then we say (internal) Lie ring, and if kk is a field and C=VecC=Vec then we say a Lie kk-algebra. Other interesting cases are super-Lie algebras, which are the Lie algebras in the symmetric monoidal category 2Vec\mathbb{Z}_2-Vec of supervector spaces and the Lie algebras in the Loday-Pirashvili tensor category of linear maps.

Alternatively, Lie algebras are the algebras over certain quadratic operad, called the Lie operad, which is the Koszul dual of the commutative algebra operad.

Cohomology

See Lie algebra cohomology.

Lie theory

See

Extra stuff, structure, properties

Notions of Lie algebras with extra stuff, structure, property includes

Properties

Examples

Examples of sequences of infinitesimal and local structures

first order infinitesimal\subsetformal = arbitrary order infinitesimal\subsetlocal = stalkwise\subsetfinite
\leftarrow differentiationintegration \to
derivativeTaylor seriesgermsmooth function
tangent vectorjetgerm of curvecurve
square-0 ring extensionnilpotent ring extensionring extension
Lie algebraformal grouplocal Lie groupLie group
Poisson manifoldformal deformation quantizationlocal strict deformation quantizationstrict deformation quantization

References

Discussion with a view towards Chern-Weil theory is in chapter IV in vol III of

Revised on August 30, 2013 17:43:27 by Urs Schreiber (89.204.154.236)