For questions about Lie algebras, an algebraic structure whose main use is in studying geometric objects such as Lie groups and differentiable manifolds.

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Decomposition of SU(n) anticommutator

In $SU(N)$, the special unitary group, the algebra generators $T_a$ are hermitian and traceless. The structure constants are fixed with $[T_a,T_b]=i f_{abc}T_c$. In the fundamental representation of ...
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21 views

question about infinitesimal transformations

Lawrence Dresner says this: (p. 10, Applications of Lie's Theory of Ordinary and Partial Differential Equations) Assume you have two infinitesimal group transformations: $$x'=x+\varepsilon(\lambda - ...
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25 views

how to compute the norm of a root from the Cartan matrix?

As far as I understand, the Cartan matrix is associated with a unique semi simple algebra. How can we compute the norm of a root $\alpha$ from it since its components are invariant under rescaling? ...
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80 views

How is the periodic structure of SO(n) reflected to its lie algebra so(n)?

An element of $SO(n)$ represents an rotation so that it must have identity with $2\pi$-like additional rotation. On the other hand, the elements of lie algebra $so(n)$ construct an noncompact vector ...
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16 views

The proportionality constant between the Casimir and the identity.

By Schur's Lemma, in any irreducible representation of a Lie algebra, the Casimir operator $J$ is proportional to the identity $Id$. How can we see that $J=j(j+1)Id$ for some natural number $j$ and ...
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30 views

The Lie algebra of special linear group of degree 2 over the set of complex numbers

I tried to compute the Lie algebra of $SL(2,\Bbb C)$. I wrote the followings: $sl(2,\Bbb C)$={$X\in M(2,\Bbb C)$: exp $tX$ $\in SL(2,\Bbb C)$}={$X\in M(2,\Bbb C)$: $det$ exp $tX$ =$1$}={$X\in M(2,\Bbb ...
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18 views

Determining the middle term of an exact sequence of Lie algebras

This is related to my previous question here. Suppose that $A_i, B_i$ are Lie algebras with $A_i$ is a sub-Lie algebra of $B_i$, $i=1,2,3$. Suppose that we have the following commutative diagram where ...
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37 views

What is a Serre presentation of a Lie algebra?

For example, as in: Give a Serre presentation of Lie algebra $\frak{g}$ of type $G_{2}$. Is it the presentation in terms of Chevalley generators, which satisfy Serre relations?
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34 views

Partial generalisation to Whitehead's second Lemma

Let $k$ be an algebraically closed field of characteristic $0$ and let $\mathfrak{g}$ be a finite dimensional semisimple $k$-Lie algebra. By Whitehead's second Lemma, we know that $H^{2}(\mathfrak{g}, ...
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14 views

How can the existence of this expression with Cartan matrix be shown using Killing form?

Let $\mathfrak{g}$ be a simple finite-dimensional Lie algebra. Let $\mathfrak{h}$ be a Cartan subalgebra, $C$ the Cartan matrix, and $R$ a system of simple roots ...
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49 views

Root Systems and Dynkin diagrams.

On page 142, the textbook An Introduction to Lie Groups and Lie Algebras (by Kirillov) determines the fundamental group of the root system $A_2$. Basically, the author says we have two simple roots ...
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27 views

SO(2,1) invariance algebra.

Please excuse me for my ignorance. I would like to know how $SO(2,1)$ Lie algebra is derived from operators and commutators. I have some notes, that the Lie algebra of $SO(2,1)$ is derived from: ...
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15 views

Gauge theory H(P) and V(P)

In general you get a connection out of a one-form $\omega$ where $\omega = g^{-1}dg+g^{-1}Ag$ and $A= A^{\alpha}_{\mu}\frac{\lambda_{\alpha}}{2i}dx^{\mu}$ is given and the base ...
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172 views

Lie groups, Lie algebra and left invariant vector fields

Hi I'm learning about Lie Groups to understand gauge theory (in the principal bundle context) and I'm having trouble with some concepts. Now let $a$ and $g$ be elements of a Lie group $G$, the left ...
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15 views

gradation of lie algebras

Let $ H(n,m) $ for $n=2r$ and $K(n,m)$ for $n=2r+1$ be hamilton and contact lie algebras over finite fields. $ H(n,m) $ is a graded subalgebra of $W(n,m)$ with length $s=\sum _{i=1} ^{2r} (p^{m_i} ...
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16 views

A refernce about Cartan matrix

There exist an approach to "Cartan Matrix" in Carter's book "Finite groups of Lie type, conjugacy classes an complex characters" p.23, which seems be different to other definitions of Cartan matrix I ...
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22 views

Cayley-Hamilton type decomposition of SL(3,R) matrices

Given an element $\lambda = \theta_a T_a$ of SL(3,R) Lie algebra, where $T_a$s are the generators and $\theta_a$s are parameters, is there a general formula to determine the coefficients A,B and C ...
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34 views

Convolution and Characters

I am confused about the purpose of the Formal Character, character functions, and the convolution in representation theory of Lie algebras. Is the Character function different than just the Character? ...
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37 views

Weyl's construction for symplectic groups--an exercise in Fulton and Harris's book

This is an exercise in section 17.3 in Fulton and Harris's book:Representation theory-a first course. Let $V=\mathbb{C}^{2n}$ and $Sp(2n)$ be the symplectic group w.r.t the nondegenerate bilinear ...
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28 views

Isometries of S^3 and some Lie algebras

By considering $S^3$ as the group of unit quaternions, and letting it act on itself from both the left and right, one can get an isomorphism $SO(4)\cong (S^3\times S^3)/C_2$, where the $C_2$ subgroup ...
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31 views

Lie algebra: If ad(g) is solvable then g solvable?

I'm trying to prove that if the image of the adjoint representation of a Lie algebra g is solvable then g is solvable, ie, if for some n (ad(g))^(n) = 0 then there exists a m such that g^(m) = 0 My ...
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29 views

Closed Connected Subgroup of $SO(5)$

I was reading a paper in which a part of it they want to classify the closed connect subgroups of $SO(5)$. What they write is this: Let $G^0$ be a closed connected subgroup of $SO(5)$. Let $T$ be a ...
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71 views

Lattices in Lie Algebras

I am having a little confusion with the different types of lattices involved with Lie algebras. Root system: represented as euclidian vector arrows. However I have seen the same arrangement with ...
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29 views

Codimension of $\ker $ $\alpha $

Can someone explain why the codimension of $\ker $ $\alpha $ is $1$ in $ H $, with complement $ Fh_\alpha $? Is this because $ h_\alpha $ when $ \alpha $ is simple is part of the dual basis to ...
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44 views

Toral sub algebra

It seems to me, I could be wrong, that the toral sub algebra goes against the following rules: For a semisimple Lie algebra: If the killing form is nondegenerate the Lie algebra is semi simple-> the ...
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50 views

Dimension of a weight space which is of weight $0$.

Let $V$ be a module of a Lie algebra $\mathfrak{g}$ and $V_{0}$ be the weight space of $V$ of weight $0$. $$ V_0 = \{ v\in V: h.v = 0, h \in \mathfrak{h} \}, $$ $\mathfrak{h}$ is a Cartan subalgebra ...
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24 views

Proof for a corollary from PBW theorem

I need to know how we can prove the following corollary : If $x_1, \ldots, x_n$ is a vector space basis for Lie algebra $L$ then a vector space basis for $U(L)$, $U(L)$ is universal enveloping ...
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29 views

$\mathfrak so(V,B)$ as subalgebra and trace if subsets of it.

I'm studying lie algebras, and got stuck on this one: Let $B$ be a bilinear form on a finite-dimensional vector space $V$ over $\mathbb F$. I've seen many books that say that $\mathfrak so(V,B)$ ...
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51 views

Formal proof of Clebsch Gordon sum

physicist here. When looking at the irreducible representations of $so(3)$, i.e. the set of all real valued anti-symmetric matrices, one can parametrize those irreps with an index $j$ which can be ...
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23 views

What is the image of an automorphism of Lie algebra?

Let $L$ be a simple Lie algebra over ${\rm GF}(2)$. If $α$ is an automorphism of $L$ then for any element of $L$ we must have $α[a,b]=[α(a),α(b)]$. Now I want to have a clear understanding of image of ...
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31 views

For a nilpotent Lie subalgebra, $\mathfrak{h}$, is $ad(\mathfrak{h})$ simultaneously diagonalizable if each $ad(H)$ is diagonalizable?

Let $\mathfrak{g}$ be a Lie algebra and $\mathfrak{h}\subseteq \mathfrak{g}$ be a nilpotent subalgebra such that for every $H \in \mathfrak{h}$, the adjoint map $ad(H): \mathfrak{g} \rightarrow ...
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18 views

How to prove that $U(\mathfrak{h})$ is isomorphic to $\mathcal{O}(\mathfrak{h}^*)$.

Let $\mathfrak{h}$ be a Cartan subalgebra of a Lie group $G$. It is said that $U(\mathfrak{h})$ is isomorphic to $\mathcal{O}(\mathfrak{h}^*)$. Here $\mathcal{O}(\mathfrak{h}^*)$ is the ring of ...
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38 views

Root space of a Semi simple group an LVS?

A semi-simple Lie group has a Cartan Subalgebra ($H$) (CSA) -an LVS, Dual to this CSA LVS is root space($H^*$), which is set funtionals that map elements of CSA to real numbers and hence useful in ...
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18 views

For a matrix group $G$ of dimension $d$, I am trying to prove that the function $Ad : G \rightarrow GL_d(\mathbb{R})$ is smooth.

For a matrix group $G$ of dimension $d$, I am trying to prove that the function $Ad : G \rightarrow GL_d(\mathbb{R})$ is smooth. So where I am starting is by extending $Ad : G \rightarrow ...
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78 views

Proof of Horn theorem with moment map

Please look at this problem: Let $\mathcal{H}$ be the space of $(n,n)$ hermitian matrix. $\phi:\begin{align*} &\mathcal{H} \to \mathfrak{u}(n):=Lie(U(n)) \\&A \mapsto iA \end{align*}$ ...
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36 views

Weyl Character Formula to find $M_\lambda(\mu)$

In Lie algebra book by Humphreys, he has used Weyl Character Formula to find the dimension of $V(\lambda)$ in the examples followed by the proof of this formula. But how to find the dimensions of the ...
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39 views

irreducible representations of lie algebras

We have the following criterion for the irreducibility of a Lie algebra representation (we work with $L$-modules here). Let $L$ be a Lie algebra, $V$ a finite dimensional vector space, and let $L ...
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12 views

Is the matrix of every set of base vectors of $\Bbb{C}^n$ symmetric?

The book "Theory of Lie Groups" by Chevalley says A linear endomorphism $\alpha$ of $C^n$ is determined when the elements $\alpha e_i=\sum\limits_{j=1}^n a_{ji}e_j$ are given. There corresponds ...
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55 views

definition of derived algebra $[L,L]$ of a Lie algebra $L$

Definition of derived algebra of a Lie algebra $L$ is given by linear span of commutators $[x,y]$ for $x,y \in L$. but here why do we take linear span and why cant we just consider collection of all ...
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186 views

Schur's lemma and Invariant subspaces of direct sums of irreducible representations

There is a corollary to Schur's lemma which says that : If $V$ is a finite dimensional irreducible complex representation of a group G or Lie algebra and $\phi :V \rightarrow V$ is an intertwining ...
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103 views

Kernel of the Lie bracket $[,]\colon\wedge^2\mathfrak g\to\mathfrak g$

I believe the following is probably well-known, but so far I couldn't find the answer by myself: Let $\mathfrak g$ be a real (finite-dimensional) Lie algebra, and $\wedge^2\mathfrak g$ its second ...
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37 views

Can I find a finite increasing filtration for every $V\in\mathfrak{O}$?

Let $V\in{O}$, I want to proof there exists a finite increasing filtration by submodules $0=V_0\subset V_1\subset\cdots\subset V_n=V$ such that $V_{i+1}/V_i$ is a highest weight module. Since $V\in ...
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70 views

Calculating an expression for the trace of generators of two Lie algebra.

Suppose we have $$[Q^a,Q^b]=if^c_{ab}Q^c$$ where Q's are generators of a Lie algebra associated a SU(N) group. So Q's are traceless. Also we have $$[P^a,P^b]=0$$ where P's are generators of a Lie ...
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100 views

Which Lie group / algebra is generated by these three matrices?

This is a beginner question (and not any homework). I want to get a feeling for Lie group/algebra generators. Do the three matrices $$A=\begin{pmatrix} 0 & 0 & 0 \\ 0 & 0 & 1 \\ 0& ...
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26 views

Closed formula for the coefficients of a series obtained from an expansion.

The Heisenberg algebra is generated by $h_i, i\in \mathbb{Z}\backslash\{0\}$ and the central element $c$. We expand the function $$\exp (\sum_{n=1}^{\infty}h_{-n}\frac{z^n}{n}) = 1 + ...
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93 views

Definitions of Semisimple Lie Algebra

We usually define semisimplicity of a Lie algebra $\mathfrak{g}\subset M_n ({\bf R})$ from two ways. I want to know the relation between them. One of the definitions of semisimple Lie algebra is ...
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41 views

The simplicity of $\bigwedge^i \mathbb{C}^{n+1}$ as a representation of $\mathfrak{sl_{n+1}}$ and its weight vectors

I want to show that $\bigwedge^i \mathbb{C}^{n+1}$ is a simple representation for $\mathfrak{sl}(n+1,\mathbb{C})$ for each $1\le i \le n+1$ but I'm already stuck at determining the weight vectors. So ...
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50 views

Map algebras between scheme and Lie algebra

Kindly asking for any hints about the following questions: Suppose, $X$ be an arbitrary scheme over an algebraically closed field $k$. 1- In general, what is the structure of $A= ...
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59 views

Lie algebra homomorphism as an scheme morphism

Kindly asking for any hints about the following questions: Assume $g$ is a finite-dimensional Lie algebra. We denote the group of Lie algebra automorphisms of $g$ by $\rm Aut_k g$. Any Lie algebra ...
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79 views

a question on weyl group and its action on $\mathfrak{t}$

$\mathfrak{g}$ is a complex semisimple lie algebra which is a subalgebra of some $\mathfrak{g}l(n,\mathbb{C})$, we have chosen a compact real form $\mathfrak{l}$ of $\mathfrak{g}$ and let $K$ be the ...