Glossary of Lie algebras

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This is a glossary for the terminology applied in the mathematical theories of Lie algebras. The statements in this glossary mainly focus on the algebraic sides of the concepts, without referring to Lie groups or other related subjects.

Lie algebra

A vector space ${\displaystyle \mathfrak{𝔤}}$ over a field ${\displaystyle F}$ with a binary operation [·, ·] (called the Lie bracket or abbr. bracket) , which satisfies the following conditions: ${\displaystyle \mathrm{\forall }a,b\in F,x,y,z\in \mathfrak{𝔤}}$,

1. ${\displaystyle [ax+by,z]=a[x,z]+b[y,z]}$ (bilinearity)
2. ${\displaystyle [x,x]=0}$ (alternating)
3. ${\displaystyle x,y],z]+[[y,z],x]+[[z,x],y]=0}$ ([[Jacobi identity)

associative algebra

An associative algebra ${\displaystyle A}$ can be made to a Lie algebra by defining the bracket ${\displaystyle [x,y]=xy-yx}$ (the commutator of ${\displaystyle x,y}$) ${\displaystyle \mathrm{\forall }x,y\in A}$ .

homomorphism

A vector space homomorphism ${\displaystyle \varphi :{\mathfrak{𝔤}}_1\to {\mathfrak{𝔤}}_2}$ is said to be a Lie algebra homomorphism if ${\displaystyle \varphi ([x,y])=[\varphi (x),\varphi (y)]\mathrm{\forall }x,y\in {\mathfrak{𝔤}}_1.}$

adjoint representation

Given ${\displaystyle x\in \mathfrak{𝔤}}$, define map ${\displaystyle {\text{<mrow data-mjx-texclass="ORD">ad</mrow>}}_x}$ by

${\displaystyle \begin{array}{rl}{\text{<mrow data-mjx-texclass="ORD">ad</mrow>}}_x:& \mathfrak{𝔤}\to \mathfrak{𝔤}\\ & y\mapsto [x,y]\end{array}}$

${\displaystyle {\text{<mrow data-mjx-texclass="ORD">ad</mrow>}}_x}$ is a Lie algebra derivation. The map

${\displaystyle \begin{array}{rl}\text{<mrow data-mjx-texclass="ORD">ad</mrow>}:& \mathfrak{𝔤}\to \mathfrak{𝔤}\mathfrak{𝔩}(\mathfrak{𝔤})\\ & x\mapsto {\mathrm{a}\mathrm{d}}_x\end{array}}$

thus defined is a Lie algebra homomorphism.

${\displaystyle \text{<mrow data-mjx-texclass="ORD">ad</mrow>}:\mathfrak{𝔤}\to \text{<mrow data-mjx-texclass="ORD">End</mrow>}(\mathfrak{𝔤})}$ is called adjoint representation.

Jacobi identity

The identity [[x, y], z] + [[y, z], x] + [[z, x], y] = 0.

To say Jacobi identity holds in a vector space is equivalent to say adjoint of all elements are derivations : ${\displaystyle {\text{<mrow data-mjx-texclass="ORD">ad</mrow>}}_x([y,z])=[{\text{<mrow data-mjx-texclass="ORD">ad</mrow>}}_x(y),z]+[y,{\text{<mrow data-mjx-texclass="ORD">ad</mrow>}}_x(z)]}$ .

subalgebra

A subspace ${\displaystyle {\mathfrak{𝔤}}^{\prime }}$ of a Lie algebra ${\displaystyle \mathfrak{𝔤}}$ is called the subalgebra of ${\displaystyle \mathfrak{𝔤}}$ if it is closed under bracket, i.e. ${\displaystyle [{\mathfrak{𝔤}}^{\prime },{\mathfrak{𝔤}}^{\prime }]\subseteq {\mathfrak{𝔤}}^{\prime }.}$

ideal

A subspace ${\displaystyle {\mathfrak{𝔤}}^{\prime }}$ of a Lie algebra ${\displaystyle \mathfrak{𝔤}}$ is the ideal of ${\displaystyle \mathfrak{𝔤}}$ if ${\displaystyle [{\mathfrak{𝔤}}^{\prime },\mathfrak{𝔤}]\subseteq {\mathfrak{𝔤}}^{\prime }.}$

In particular, every ideal is also a subalgebra. Every kernel of a Lie algebra homomorphism is an ideal. Unlike in ring theory, there is no distinguishability of left ideal and right ideal.

derived algebra

The derived algebra of a Lie algebra ${\displaystyle \mathfrak{𝔤}}$ is ${\displaystyle [\mathfrak{𝔤},\mathfrak{𝔤}]}$. It is a subalgebra.

normalizer

The normalizer of a subspace ${\displaystyle K}$ of a Lie algebra ${\displaystyle \mathfrak{𝔤}}$ is ${\displaystyle N_{\mathfrak{𝔤}}(K):=\{x\in \mathfrak{𝔤}|[x,K]\subseteq K\}}$ .

centralizer

The centralizer of a subset ${\displaystyle X}$ of a Lie algebra ${\displaystyle \mathfrak{𝔤}}$ is ${\displaystyle C_{\mathfrak{𝔤}}(X):=\{x\in \mathfrak{𝔤}|[x,X]=\{0\}\}}$ .

center

The center of a Lie algebra is the centralizer of itself : ${\displaystyle Z(L):=\{x\in \mathfrak{𝔤}|[x,\mathfrak{𝔤}]=0\}}$

radical

The radical ${\displaystyle \text{<mrow data-mjx-texclass="ORD">Rad</mrow>}(\mathfrak{𝔤})}$ is the maximum solvable ideal of ${\displaystyle \mathfrak{𝔤}}$.

abelian

A Lie algebra is said to be abelian if its derived algebra is zero.

nilpotent Lie algebra

A Lie algebra ${\displaystyle L}$ is said to be nilpotent if ${\displaystyle C^N(L)=\{0\}}$ for some positive integer ${\displaystyle N}$ .

The following conditions are equivalent:

• ${\displaystyle C^N(L)=\{0\}}$ for some positive integer ${\displaystyle N}$, i.e. the descending central series eventually terminates to ${\displaystyle \{0\}}$.
• ${\displaystyle C_N(L)=L}$ for some positive integer N, i.e. the ascending central series eventually terminates to L.
• There exists a chain of ideals of ${\displaystyle L}$, ${\displaystyle L=I_1\supseteq I_2\supseteq I_3\supseteq \cdots \supseteq I_n=\{0\}}$, such that ${\displaystyle [L,I_k]\subseteq I_{k+1}}$ .
• There exists chain of ideals of ${\displaystyle L}$,${\displaystyle L=I_1\supseteq I_2\supseteq I_3\cdots \supseteq I_n=\{0\}}$, such that ${\displaystyle I_k/I_{k+1}\subseteq Z(L/I_{k+1})}$.
• ${\displaystyle \text{<mrow data-mjx-texclass="ORD">ad</mrow>}x}$ is nilpotent ${\displaystyle \mathrm{\forall }x\in L}$ . (Engel's theorem)
• ${\displaystyle \text{<mrow data-mjx-texclass="ORD">ad</mrow>}L}$ is a nilpotent Lie algebra.

In particular, every nilpotent Lie algebra is solvable.

If ${\displaystyle L}$ is nilpotent, any subalgebra and quotient of ${\displaystyle L}$ are nilpotent.

nilpotent element in a Lie algebra

An element ${\displaystyle x\in L}$ is said to be nilpotent in ${\displaystyle L}$ if ${\displaystyle a{d}_x}$ is a nilpotent endomorphism, i.e. viewing ${\displaystyle {\text{<mrow data-mjx-texclass="ORD">ad</mrow>}}_x}$ as a matrix in ${\displaystyle {\mathfrak{𝔤}\mathfrak{𝔩}}_{\mathfrak{𝔤}}}$, ${\displaystyle \mathrm{\exists }N\in {\mathbb{\mathbb{Z}}}^{+},({\text{<mrow data-mjx-texclass="ORD">ad</mrow>}}_x{)}^N=0}$. It is equivalent to ${\displaystyle ({\text{<mrow data-mjx-texclass="ORD">ad</mrow>}}_x{)}^{N}y=[x[x\dots [x[x,y]\dots ]=0\mathrm{\forall }y\in L}$

descending central series

a sequence of ideals of a Lie algebra ${\displaystyle L}$ defined by ${\displaystyle C^0(L)=L,C^1(L)=[L,L],C^{n+1}(L)=[L,C^n(L)]}$

ascending central series

a sequence of ideals of a Lie algebra ${\displaystyle L}$ defined by ${\displaystyle C_0(L)=\{0\},C_1(L)=Z(L)}$ (center of L) , ${\displaystyle C_{n+1}(L)={\pi }_n^{-1}(Z(L/C_n(L)))}$, where ${\displaystyle {\pi }_i}$ is the natural homomorphism ${\displaystyle L\to L/C_n(L)}$

solvable Lie algebra

A Lie algebra ${\displaystyle L}$ is said to be solvable if ${\displaystyle L^{(N)}=0}$ for some positive integer ${\displaystyle N}$ , i.e. the derived series eventually terminates to ${\displaystyle \{0\}}$.

The following condition is equivalent to solvability:

* There exists chain of ideals of ${\displaystyle L}$, ${\displaystyle L=I_1\supseteq I_2\supseteq I_3\cdots \supseteq I_n=\{0\}}$, such that ${\displaystyle [I_k,I_k]\subseteq I_{k+1}}$ .

If ${\displaystyle L}$ is solvable, any subalgebra and quotient of ${\displaystyle L}$ are solvable.

Let ${\displaystyle I}$ is an ideal of a Lie algebra ${\displaystyle L}$. If ${\displaystyle L/I,I}$ are solvable, ${\displaystyle L}$ is solvable.

derived series

a sequence of ideals of a Lie algebra L defined by ${\displaystyle L^{(0)}=L,L^{(1)}=[L,L],L^{(n+1)}=[L^{(n)},L^{(n)}]}$

simple

A Lie algebra is said to be simple if it is non-abelian and has only two ideals, itself and ${\displaystyle \{0\}}$.

semisimple Lie algebra

A Lie algebra is said to be semisimple if its radical is ${\displaystyle \{0\}}$.

semisimple element in a Lie algebra

split Lie algebra

free Lie algebra

toral Lie algebra

Lie's theorem

Let ${\displaystyle \mathfrak{𝔤}}$ be a finite-dimensional complex solvable Lie algebra over algebraically closed field of characteristic ${\displaystyle 0}$, and let ${\displaystyle V}$ be a nonzero finite dimensional representation of ${\displaystyle \mathfrak{𝔤}}$. Then there exists an element of ${\displaystyle V}$ which is a simultaneous eigenvector for all elements of ${\displaystyle \mathfrak{𝔤}}$.

Corollary: There exists a basis of ${\displaystyle V}$ with respect to which all elements of ${\displaystyle \mathfrak{𝔤}}$ are upper triangular.

Killing form

The Killing form on a Lie algebra ${\displaystyle \mathfrak{𝔤}}$ is a symmetric, associative, bilinear form defined by ${\displaystyle \kappa (x,y):=\text{<mrow data-mjx-texclass="ORD">Tr</mrow>}(\text{<mrow data-mjx-texclass="ORD">ad</mrow>}x\text{<mrow data-mjx-texclass="ORD">ad</mrow>}y)\mathrm{\forall }x,y\in \mathfrak{𝔤}}$.

Cartan criterion for solvability

A Lie algebra ${\displaystyle \mathfrak{𝔤}}$ is solvable iff ${\displaystyle \kappa (\mathfrak{𝔤},[\mathfrak{𝔤},\mathfrak{𝔤}])=0}$.

Cartan criterion for semisimplity

If ${\displaystyle \kappa (\cdot ,\cdot )}$ is nondegenerate, then ${\displaystyle \mathfrak{𝔤}}$ is semisimple.

If ${\displaystyle \mathfrak{𝔤}}$ is semisimple and the underlying field ${\displaystyle F}$ has characteristic 0 , then ${\displaystyle \kappa (\cdot ,\cdot )}$ is nondegenerate.

lower central series

synonymous to "descending central series".

upper central series

synonymous to "ascending central series".

In the below section, denote ${\displaystyle (\cdot ,\cdot )}$ as the inner product of a Euclidean space E.

In the below section, ${\displaystyle <\cdot ,\cdot >}$ denoted the function defined as ${\displaystyle <\beta ,\alpha >=\frac{(\beta ,\alpha )}{(\alpha ,\alpha )}\mathrm{\forall }\alpha ,\beta \in E}$ .

Cartan subalgebra

A Cartan subalgebra ${\displaystyle \mathfrak{𝔥}}$ of a Lie algebra ${\displaystyle \mathfrak{𝔤}}$ is a nilpotent subalgebra satisfying ${\displaystyle N_{\mathfrak{𝔤}}(\mathfrak{𝔥})=\mathfrak{𝔥}}$ .

regular element of a Lie algebra

maximal toral subalgebra

Borel subalgebra

root of a semisimple Lie algebra

Let ${\displaystyle \mathfrak{𝔤}}$ be a semisimple Lie algebra, ${\displaystyle \mathfrak{𝔥}}$ be a Cartan subalgebra of ${\displaystyle \mathfrak{𝔤}}$. For ${\displaystyle \alpha \in {\mathfrak{𝔥}}^{*}}$, let ${\displaystyle {\mathfrak{g}}_{\mathfrak{\alpha }}:=\{x\in \mathfrak{𝔤}|[h,x]=\alpha (h)x\mathrm{\forall }h\in \mathfrak{𝔥}\}}$. \alpha is called a root of ${\displaystyle \mathfrak{𝔤}}$ if it is nonzero and ${\displaystyle {\mathfrak{g}}_{\mathfrak{\alpha }}\ne \{0\}}$

The set of all roots is denoted by ${\displaystyle \mathrm{\Phi }}$ ; it forms a root system.

Root system

A subset ${\displaystyle \mathrm{\Phi }}$ of the Euclidean space ${\displaystyle E}$ is called a root system if it satisfies the following conditions:

• ${\displaystyle \mathrm{\Phi }}$ is finite, ${\displaystyle \text{<mrow data-mjx-texclass="ORD">span</mrow>}(\mathrm{\Phi })=E}$ and ${\displaystyle 0\notin \mathrm{\Phi }}$.
• For all ${\displaystyle \alpha \in \mathrm{\Phi }}$ and ${\displaystyle c\in \mathbb{ℝ}}$, ${\displaystyle c\alpha \in \mathrm{\Phi }}$ iff ${\displaystyle c=\pm 1}$.
• For all ${\displaystyle \alpha ,\beta \in \mathrm{\Phi }}$, ${\displaystyle <\alpha ,\beta >}$ is an integer.
• For all ${\displaystyle \alpha ,\beta \in \mathrm{\Phi }}$, ${\displaystyle S_{\alpha }(\beta )\in \mathrm{\Phi }}$, where ${\displaystyle S_{\alpha }}$ is reflection through hyperplane normal to ${\displaystyle \alpha }$ i.e. ${\displaystyle S_{\alpha }(x)=x-<x,\alpha >\alpha }$.

Cartan matrix

Cartan matrix of root system ${\displaystyle \mathrm{\Phi }}$ is matrix ${\displaystyle (<{\alpha }_i,{\alpha }_j>{)}_{i,j=1}^n}$ where ${\displaystyle \mathrm{\Delta }=\{{\alpha }_1\dots {\alpha }_n\}}$ is a set of simple roots of ${\displaystyle \mathrm{\Phi }}$.

Dynkin diagrams

Simple Roots

A subset ${\displaystyle \mathrm{\Delta }}$ of a root system ${\displaystyle \mathrm{\Phi }}$ is called a set of simple roots if it satisfies the following conditions:

• ${\displaystyle \mathrm{\Delta }}$ is linear basis of ${\displaystyle E}$.
• Each element of ${\displaystyle \mathrm{\Phi }}$ is a linear combination of elements of ${\displaystyle \mathrm{\Delta }}$ with coefficients which are either all nonnegative or all nonpositive.

a partial order on the Eucliean space E defined by the set of simple root

${\displaystyle \mathrm{\forall }\lambda ,\mu \in E,\lambda >\mu ⟺\lambda -\mu >0⟺\mathrm{\exists }{k}_1,k_2,...,k_n\in {\mathbb{\mathbb{Z}}}^{+},{\alpha }_1,{\alpha }_2,...,{\alpha }_n\in \mathrm{\Delta },\lambda -\mu =\sum _i{k}_i{\alpha }_i}$

regular element with respect to a root system

Let ${\displaystyle \mathrm{\Phi }}$ be a root system. ${\displaystyle \gamma \in E}$ is called regular if ${\displaystyle (\gamma ,\alpha )\ne 0\mathrm{\forall }\gamma \in \mathrm{\Phi }}$.

For each set of simple roots ${\displaystyle \mathrm{\Delta }}$ of ${\displaystyle \mathrm{\Phi }}$, there exists a regular element ${\displaystyle \gamma \in E}$ such that ${\displaystyle (\gamma ,\alpha )>0\mathrm{\forall }\gamma \in \mathrm{\Delta }}$, conversely for each regular ${\displaystyle \gamma }$ there exist a unique set of base roots ${\displaystyle \mathrm{\Delta }(\gamma )}$ such that the previous condition holds for ${\displaystyle \mathrm{\Delta }=\mathrm{\Delta }(\gamma )}$. It can be determined in following way: let ${\displaystyle {\mathrm{\Phi }}^{+}(\gamma )=\{\alpha \in \mathrm{\Phi }|(\alpha ,\gamma )>0\}}$. Call an element ${\displaystyle \alpha }$ of ${\displaystyle {\mathrm{\Phi }}^{+}(\gamma )}$ decomposable if ${\displaystyle \alpha ={\alpha }^{\prime }+{\alpha }^{″}}$ where ${\displaystyle {\alpha }^{\prime },{\alpha }^{″}\in {\mathrm{\Phi }}^{+}(\gamma )}$, then ${\displaystyle \mathrm{\Delta }(\gamma )}$ is the set of all indecomposable elements of ${\displaystyle {\mathrm{\Phi }}^{+}(\gamma )}$

positive roots

Positive root of root system ${\displaystyle \mathrm{\Phi }}$ with respect to a set of simple roots ${\displaystyle \mathrm{\Delta }}$ is a root of ${\displaystyle \mathrm{\Phi }}$ which is a linear combination of elements of ${\displaystyle \mathrm{\Delta }}$ with nonnegative coefficients.

negative roots

Negative root of root system ${\displaystyle \mathrm{\Phi }}$ with respect to a set of simple roots ${\displaystyle \mathrm{\Delta }}$ is a root of ${\displaystyle \mathrm{\Phi }}$ which is a linear combination of elements of ${\displaystyle \mathrm{\Delta }}$ with nonpositive coefficients.

long root

short root

Weyl group

Weyl group of a root system ${\displaystyle \mathrm{\Phi }}$ is a (necessarily finite) group of orthogonal linear transformations of ${\displaystyle E}$ which is generated by reflections through hyperplanes normal to roots of ${\displaystyle \mathrm{\Phi }}$

inverse of a root system

Given a root system ${\displaystyle \mathrm{\Phi }}$. Define ${\displaystyle {\alpha }^v=\frac{2\alpha }{(\alpha ,\alpha )}}$, ${\displaystyle {\mathrm{\Phi }}^v=\{{\alpha }^v|\alpha \in \mathrm{\Phi }\}}$ is called the inverse of a root system.

${\displaystyle {\mathrm{\Phi }}^v}$ is again a root system and have the identical Weyl group as ${\displaystyle \mathrm{\Phi }}$.

base of a root system

synonymous to "set of simple roots"

dual of a root system

synonymous to "inverse of a root system"

<--

weight in a root system

${\displaystyle \lambda \in E}$ is called a weight if ${\displaystyle <\lambda ,\alpha >\in \mathbb{\mathbb{Z}}\mathrm{\forall }\alpha \in \mathrm{\Phi }}$ . -->

weight lattice

weight space

dominant weight

A weight \lambda is dominant if ${\displaystyle <\lambda ,\alpha >\in {\mathbb{\mathbb{Z}}}^{+}}$ for some ${\displaystyle \alpha \in \mathrm{\Phi }}$

fundamental dominant weight

Given a set of simple roots ${\displaystyle \mathrm{\Delta }=\{{\alpha }_1,{\alpha }_2,...,{\alpha }_n\}}$, it is a basis of ${\displaystyle E}$. ${\displaystyle {\alpha }_1^v,{\alpha }_2^v,...,{\alpha }_n^v\in {\mathrm{\Phi }}^v}$ is a basis of ${\displaystyle E}$ too; the dual basis ${\displaystyle {\lambda }_1,{\lambda }_2,...,{\lambda }_n}$ defined by ${\displaystyle ({\lambda }_i,{\alpha }_j^v)={\delta }_{ij}}$ , is called the fundamental dominant weights.

highest weight

minimal weight

multiplicity (of weight)

radical weight

strongly dominant weight

module

Define an action of ${\displaystyle \mathfrak{𝔤}}$ on a vector space ${\displaystyle V}$ ( i.e. an operation ${\displaystyle \mathfrak{𝔤}\times V\to V,(x,v)\mapsto xv}$) such that: ${\displaystyle \mathrm{\forall }a,b\in F,x,y\in \mathfrak{𝔤},v,w\in V}$ satisfy

# ${\displaystyle (ax+by)v=a(xv)+b(yv)}$

# ${\displaystyle x(av+bw)=a(xv)+b(xw)}$

# ${\displaystyle [x,y]v=x(yv)-y(xv)}$

Then ${\displaystyle V}$ is called a ${\displaystyle \mathfrak{𝔤}}$-module. (Remark: ${\displaystyle V,\mathfrak{𝔤}}$ have the same underlying field ${\displaystyle F}$.)

Each ${\displaystyle \mathfrak{𝔤}}$-module corresponds to a representation ${\displaystyle \mathfrak{𝔤}\to {\mathfrak{𝔤}\mathfrak{𝔩}}_V}$.

A subspace W is a submodule (more precisely, sub ${\displaystyle \mathfrak{𝔤}}$-module) of ${\displaystyle V}$ if ${\displaystyle \mathfrak{𝔤}}$-module ${\displaystyle W\subset V}$ .

representation

For a vector space ${\displaystyle V}$, if there is a Lie algebra homomorphism ${\displaystyle \pi :\mathfrak{𝔤}\to {\mathfrak{𝔤}\mathfrak{𝔩}}_V}$, then ${\displaystyle \pi }$ is called a representation of ${\displaystyle \mathfrak{𝔤}}$.

Each representation ${\displaystyle \mathfrak{𝔤}\to {\mathfrak{𝔤}\mathfrak{𝔩}}_V}$ corresponds to a ${\displaystyle \mathfrak{𝔤}}$-module ${\displaystyle V}$.

A subrepresentation is the representation corresponding to a submodule.

homomorphism

Given two ${\displaystyle \mathfrak{𝔤}}$-module V, W, a ${\displaystyle \mathfrak{𝔤}}$-module homomorphism ${\displaystyle \varphi }$ is a vector space homomorphism satisfying ${\displaystyle \varphi (xv)=x\varphi (v)\mathrm{\forall }x\in \mathfrak{𝔤},v\in V}$.

trivial representation

A representation is said to be trivial if the image of ${\displaystyle \mathfrak{𝔤}}$ is the zero vector space. It corresponds to the action of ${\displaystyle \mathfrak{𝔤}}$ on module ${\displaystyle V}$ by ${\displaystyle xv=0\mathrm{\forall }x\in \mathfrak{𝔤},v\in V}$ .

faithful representation

If the representation ${\displaystyle \mathfrak{𝔤}\to {\mathfrak{𝔤}\mathfrak{𝔩}}_V}$ is injective, it is said to be faithful.

tautology representation

If a Lie algebra ${\displaystyle \mathfrak{𝔤}}$ is defined as a subalgebra of ${\displaystyle \mathfrak{𝔤}\mathfrak{𝔩}(n,F)}$ , like ${\displaystyle \mathfrak{𝔰}\mathfrak{𝔩}(n,F),\mathfrak{𝔬}(2l,F),\mathfrak{𝔱}(n,F)}$ (the upper triangular matrices), the tautology representation is the imbedding ${\displaystyle \mathfrak{𝔤}\to \mathfrak{𝔤}\mathfrak{𝔩}(n,F)}$ . It corresponds to the action on module ${\displaystyle F^n}$ by the matrix multiplication.

adjoint representation

The representation ${\displaystyle \begin{array}{rl}\text{<mrow data-mjx-texclass="ORD">ad</mrow>}:& \mathfrak{𝔤}\to {\text{<mrow data-mjx-texclass="ORD">gl</mrow>}}_{\mathfrak{𝔤}}\\ & x\mapsto {\text{<mrow data-mjx-texclass="ORD">ad</mrow>}}_x\end{array}}$. It corresponds viewing ${\displaystyle \mathfrak{𝔤}}$ as a ${\displaystyle \mathfrak{𝔤}}$-module - the action on the module is given by the adjoint endomorphism.

irreducible modules

A module is said to be irreducible if it has only two submodules, itself and zero.

indecomposable module

A module is said to be indecomposable if it cannot be written as direct sum of two non-zero submodules.

An irreducible module need not be indecomposable but the converse is not true.

completely reducible module

A module is said to be completely reducible if it can be written as direct sum of irreducible modules.

simple module

Synonymous as irreducible module.

quotient module / quotient representation

Given a ${\displaystyle \mathfrak{𝔤}}$-module V and its submodule W, an action ${\displaystyle \mathfrak{𝔤}}$ on V/W can be defined by ${\displaystyle x(v+W)=xv+W\mathrm{\forall }x\in \mathfrak{𝔤},v\in V}$ . V/W is said to be a quotient module in this case.

Schur's lemma

Statement in the language of module theory: Given V an irreducible ${\displaystyle \mathfrak{𝔤}}$-module, ${\displaystyle \varphi V\to V}$ is a ${\displaystyle \mathfrak{𝔤}}$-module homomorphism iff ${\displaystyle \varphi =\lambda 1_V}$ for some ${\displaystyle \lambda \in F}$ .

Statement in the language of representation theory: Given an irreducible representation ${\displaystyle \varphi :L\to \mathfrak{𝔤}\mathfrak{𝔩}(V)}$, for ${\displaystyle \theta \in \text{<mrow data-mjx-texclass="ORD">End</mrow>}(V)}$, ${\displaystyle \theta \varphi (x)=\varphi (x)\theta }$ iff ${\displaystyle \theta =\lambda 1_V}$ for some ${\displaystyle \lambda \in F}$.

simple module

synonymous to "irreducible module".

factor module

synonymous to "quotient module".

PBW theorem (Poincaré–Birkhoff–Witt theorem)

BGG category \mathcal{O}

a basis constructed by Claude Chevalley with the property that all structure constants are integers. Chevalley used these bases to construct analogues of Lie groups over finite fields, called Chevalley groups.

The generators of a Lie group are split into the generators H and E such that:

${\displaystyle [H_{{\alpha }_i},H_{{\alpha }_j}]=0}$

${\displaystyle [H_{{\alpha }_i},E_{{\alpha }_j}]=A_{ij}{E}_{{\alpha }_j}}$

${\displaystyle [E_{{\alpha }_i},E_{{\alpha }_j}]=H_{{\alpha }_j}}$

${\displaystyle [E_{\beta },E_{\gamma }]=\pm (p+1)E_{\beta +\gamma }}$

where p = m if β + γ is a root and m is the greatest positive integer such that γ − mβ is a root.

general linear algebra ${\displaystyle gl(n,F)}$

Ado's theorem

Any finite-dimensional Lie algebra is isomorphic to a subalgebra of ${\displaystyle {\mathfrak{𝔤}\mathfrak{𝔩}}_V}$ for some finite-dimensional vector space V.

Classical Lie algebras:

Exceptional Lie algebras:

• Poisson algebra
• Kac–Moody algebra

• Lie group
• Glossary of semisimple groups
• Linear algebraic group
• Particle physics and representation theory

• Erdmann, Karin & Wildon, Mark. Introduction to Lie Algebras, 1st edition, Springer, 2006. ISBN 1-84628-040-0
• Humphreys, James E. Introduction to Lie Algebras and Representation Theory, Second printing, revised. Graduate Texts in Mathematics, 9. Springer-Verlag, New York, 1978. ISBN 0-387-90053-5

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