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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3
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11 views

Does a choice of measure on $\mathfrak{g}$ induce a measure on $G$?

Let $G$ be a Lie group with Lie algebra $\mathfrak{g}$. One can put a (left) Haar measure $\mu$ on $G$ and a Lebesgue measure $\lambda$ on $\mathfrak{g}$ which are both unique up to constants. My ...
0
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0answers
12 views

Quadratic Casimir of SO(5)

In the article A Four Dimensional Generalization of the Quantum Hall Effect, arXiv:cond-mat/0110572, by Zhang and Hu Quadratic Casimir operator for $SO(5)$ is given as $$p^2/2+q^2/2+2p+q .$$ When ...
2
votes
0answers
56 views

Why doesn't the “naive” scalar product for $SO(n)$ yield something invariant?

By definition, for $SO(n)$ we have $g^T g=1$ for $g \in SO(n)$. Given some vector $v \in V$ and some representation $R: SO(N) \rightarrow \mathrm{Lin}(V)$, the defining condition above tells us ...
1
vote
1answer
64 views

Where does the ambiguity in choosing a basis for a Lie algebra come from?

This is a follow-up to this question. For matrix Lie algebras, we can define the Lie algebra $g$ of a group $G$ as the set $T_a \in g$ that yield an element of $G$ when put into the exponential map: ...
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0answers
21 views

Proof check - $L(\mathbb{F})\to L_{V}(\mathbb{F})$ for $L=\mathfrak{sl}_{2}$ is an isomorphism.

Let $L=\mathfrak{sl}_{2}$ with basis $(x,y,h)$, $char\mathbb{F}>2$ and $V=V(m)$ an irreducible $L$-module with highest weight $m\in\mathbb{Z}^{+}$. Let also ...
3
votes
2answers
88 views

The generators of $SO(n)$ are antisymmetric, which means there are no diagonal generators and therefore rank zero for the Lie algebra?

Okay, this may be a silly question but I can't figure it out myself right now. By definition $O \in SO(n)$ fulfils $O^T O=1$ and $\det(O)=1$. For the generators of the group $ T_a \in so(n)$, this ...
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0answers
31 views

Question related to the Campbell-Baker-Haussdorf formula

I have two operators $A$ and $B$ such that $[A,B] = C$ $[A,C] = -2A$ $[B,C] = +2B$ and I would like to obtain an expression for $\log(\exp(A+B)\exp(-B)\exp(-A))$. Is it a linear combination of ...
3
votes
1answer
48 views

Tangent space of quotient space

Let $\pi : M \rightarrow M/G$ be the canonical projection, where $M$ is a manifold and $M/G$ is a quotient manifold. Now, what can we say about $d \pi (p) : T_pM \rightarrow T_p(M/G)$? From my ...
4
votes
0answers
36 views

Basic application of Weyl-Character-Formula

(I did not find a solution of my problem in any forum so far. Sorry if it exists...) I am new to Lie-Algebras and representations and actually do not need the mathematical background... I need only ...
2
votes
0answers
67 views

Fields of Research in Algebra [closed]

I'm a last-year student in mathematics and I'm looking for a master degree in algebra. So I'm trying to understand what are the most interesting fields of research in algebra all around the world. ...
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1answer
27 views

Lie group and stabilizer quotient

Let $G$ be a Lie group and $$G_x:=\{g \in G; Ad^*_g(x)=x\}$$ the stabilizer, where $Ad_g^*$ is the adjoint of the adjoint representation. Now, I was wondering why $G/G_x$ has a manifold structure. ...
1
vote
1answer
48 views

Orbits form a manifold?

A prominent example are the coadjoint orbits $O_x = \{Ad_u^*(x);u \in G\}$ where $x \in \mathfrak{g}$ and $G$ a Lie group with Adjoint map $Ad.$ Could anybody give me an easy argument why $O_x$ is a ...
0
votes
0answers
38 views

Exponential map only for matrix Lie algebras?

Recently, I stumbled over some proofs in Lie algebra theory and noticed that they often use the notion of an exponential map $e^{t \zeta}$ for $\zeta \in \mathfrak{g}$ such that $e^{t \zeta} \in G$ ...
0
votes
1answer
17 views

Tangent vectors to coadjoint orbits

Let $O_x:=\{Ad^*_g (x); g \in G\}$ be the orbit of $x \in \mathfrak{g}^*$ and $Ad$ the adjoint map. Now take $\xi \in \mathfrak{g}$ then $g(t):=Ad^*_{e^{t \xi}}(x)$ defines a map $g: I \rightarrow ...
1
vote
1answer
26 views

Lie algebra of affine linear maps

Let $G$ be the Lie group of affine transformations, $$\{x \mapsto Ax+b,A \in GL(n), b \in \mathbb{R}^n\}.$$ We can represent these maps as matrices $$\begin{pmatrix} A & b \\ 0 & 1 ...
2
votes
0answers
33 views

Tangent space of coadjoint orbit

Let $\xi \in T_xOx$ be a tangent vector at $x \in O_x :=\{\mathrm{ Ad} _{u}^*(x); u \in G\}$ for $x \in g^*.$ ($g$ is the Lie-Algebra) Then I read that this $\xi$ can be represented as the velocity ...
-1
votes
0answers
17 views

Lie algebra affine transformations [duplicate]

Let $G$ be the Lie group of affine transformations $$\{x \mapsto Ax+b,A \in GL(n), b \in \mathbb{R}^n\}.$$ Then we can represent these maps as matrices $\begin{pmatrix} A & b \\ 0 & 1 ...
1
vote
1answer
26 views

“Semi-simplicity” of Lie algebra elements.

Why are diagonalizable elements of Lie algebra called "semi-simple"? Is there a notion of "simple" elements? Is it related to "semi-simplicity" of the Lie algebra?
0
votes
0answers
15 views

Adjoint and coadjoint orbits

I just read that for the Lie algebras $\mathfrak{gl}(N),\mathfrak{sl}(N),\mathfrak{so}(N),\mathfrak{sp}(2N)$ the adjoint and coadjoint orbits coincide. Now, the adjoint orbits are $O_{\xi} = ...
0
votes
1answer
47 views

Killing form - strange definition

I was just reading about Killing forms. In my opinion, the definition of these forms is quite strange. I mean why would one define $B(X,Y) = \mathrm{tr} (\mathrm{ad} (X) \circ \mathrm{ad} (Y))$? I ...
3
votes
1answer
28 views

Adjoint representation is Lie algebra homomorphism

Let $T_g:=L_g R_{g^{-1}}: G \rightarrow G$ be the standard automorphism of a Lie algebra, then $Ad_g:=DT_g(e): \mathfrak{g} \rightarrow \mathfrak{g} $is called the adjoint representation. Now, I want ...
0
votes
0answers
12 views

Stabilizer subgroup in adjoint action

Given $b \in \mathfrak{su}(n)$, how can I find the stabilizer $\text{stab}(b)$ for the adjoint action of $SU(n)$ on $\mathfrak{su}(n)$ given by $Ad_U(b) = UbU^{\dagger}$ without using coordinates? The ...
3
votes
1answer
33 views

Level set as the orbit of the action of a Lie Group?

I'm wondering the following. Given a smooth function $f:\mathbb R^n\rightarrow \mathbb R^m$ with $m<n$ and level sets $\mathcal O(y)=\{x\in\mathbb R^n| f(x)=y \}$. What are the conditions on $f$ ...
0
votes
0answers
19 views

Why $exp(0_{T_eG})=e$, where $exp$ is the exponential map of a Lie group?

I wonder if this fact is true: I consider the exponential map of a Lie group $G$. $$exp: \mathfrak{g} \rightarrow G.$$ Is it true that $exp(0_{T_eG})=e$, where $e$ is the identity element of $G$? ...
2
votes
3answers
46 views

Group actions on manifolds - exponential map

Let $M$ be a smooth manifold. Suppose $K$ is a Lie group (with Lie algebra $\mathfrak{k}$) acting EDIT: TRANSITIVELY on $M$ from the left and $G$ is a Lie group (with Lie algebra $\mathfrak{g}$) ...
2
votes
2answers
46 views

Realizing the oscillator algebra as a matrix Lie algebra

In Hilgert's & Neeb's Structure and Geometry of Lie Groups, they introduce a Lie algebra, which they call the "oscillator algebra," as an extension of the Heisenberg algebra. They give a basis ...
4
votes
0answers
32 views

Understanding $G_2$ inside Spin(7)?

This is a rather embarrassing question, so please let me know of any duplication and I will happily remove it. I am seeking to understand the $\mathbb Q$-split form of the algebraic group $G_2$, and ...
1
vote
1answer
38 views

Proof check - infinite-dimensional $\mathfrak{sl}(2, \mathbb{F})$-module

Let $L=\mathfrak{sl}(2,\mathbb{F})$ with the usual basis $(x, \ y, \ h)$ and $\text{char}\,\mathbb{F}=0$. Let $Z(\lambda)$, $\lambda\in\mathbb{F}$ the infinite-dimensional $L$-module spanned by ...
0
votes
1answer
28 views

Determine the exponential map of the direct product of two Lie groups.

I know that the direct product of two Lie groups $G$ and $H$ is a Lie group. Knowing the exponential map of $G$ and $H$, I would like to find an expression for the exponential map of $G \times H$. ...
1
vote
0answers
15 views

Branching rules without previous knowledge of the projection matrix?

Given a representation $R$ of some group $G$ one can find in many books and papers (e.g. page 96ff here) the decomposition under certain subgroups: $$ R= R_1 + R_2 + \ldots$$ This is often called a ...
1
vote
0answers
14 views

Inner Automorphism of Lie algebras in Terms of Roots and Weights?

An automorphism is a homomorphism of a group $G$ onto itself. For Lie groups this induces a Lie algebra $g$ automorphism, i.e. a map of the Lie alegbra onto itself that preserves the Lie bracket. An ...
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0answers
13 views

Maximal subalgebras of simple Lie algebras.

Does anyone know how I can access Dynkin's papers on the classification of maximal subalgebras of simple finite dimensional complex Lie algebras? V.V. Morozov also worked on this topic, how can I ...
1
vote
0answers
67 views

Orbit of a Weight Vector?

Given some element $\phi$ of a representation $R$ of a group $G$, the orbit $G(\phi)$ of $\phi$ is defined as the set $g \phi \ \forall \ g \in G$. We can write every element of a given ...
1
vote
1answer
27 views

Automorphism group of a lie algebra as a lie subgroup of $GL(\frak g)$

Let $G$ be a lie group with lie algebra $\frak{g}$. Let $Aut(\frak g)$ be the automorphism group of $\frak{g}$. Its clear to me that $Aut(\frak{g})$ $\subset GL(\frak{g})$ since any automorphism of ...
0
votes
1answer
33 views

Duality and tensor product of the Lie algebra

I would like to know how to compute the tensor product of the matrices below and how to deal with duality of vector spaces. The vector space I concern is the Lie algebra $\mathscr{sl_2}$ with basis ...
1
vote
1answer
34 views

How to proof that the $\mathbb{Z}$-span of weights of a faithful $L$-modul contains the root lattic?

Let $L$ be a semisimple Lie Algebra with root system $\Phi$ and base $\Delta$ of $\Phi$. Let $V$ be a finit dimensional, faithful $L$-modul with weights $\Pi(V)$. I am trying to show that the ...
0
votes
2answers
12 views

Adjoint action notations $\operatorname{ad}(X)(Y)$, $\operatorname{ad}(X)$ and $\operatorname{ad}_x(Y)$ are equivalent??

As the title says I'm a bit confused with these notations of adjoint action of Lie algebra on itself. Are these notations ($\operatorname{ad}(X)(Y)$, $\operatorname{ad}(X)$ and ...
0
votes
1answer
45 views

How to understand the definition of Killing form?

Define the matrix commutator $\text{ad}_X$ as $$\text{ad}_XY=[X,Y]=XY-YX$$ where $X,Y\in\mathfrak{g}$ and $\mathfrak{g}$ is the Lie algebra associated to Lie group $G$. Then on Lie group $G$, the ...
0
votes
0answers
33 views

Killing form question.

Studying Lie algebras (Cartan-Weyl basis) I've stumbled upon the expression of the Killing form: $g_{\rho\alpha}=c^\sigma_{\rho\tau} c^\tau_{\alpha\sigma}$. The question is are these ...
1
vote
2answers
67 views

Solving $\lambda U^{\dagger}V -\bar{\lambda} V^{\dagger}U = A$

Given an arbitrary $A \in \mathfrak{su}(n)$, is it always possible to solve for $U,V \in SU(n)$ and $\lambda \in \mathbb{C}$ such that $\lambda U^{\dagger}V -\bar{\lambda} V^{\dagger}U = A$? Update: ...
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vote
1answer
46 views

Stabilizer subgroup of adjoint action

Given $b \in \mathfrak{su}(n)$, how can I find the stabilizer $\text{stab}(b)$ for the adjoint action of $SU(n)$ on $\mathfrak{su}(n)$ given by $Ad_U(b) = UbU^{\dagger}$ without using coordinates? The ...
1
vote
2answers
55 views

Representation of $sl(2,R)$.

I am interested in the unique (up to isomorphism) $5$-dimensional representation of the Lie algebra $sl(2,R)$. I understand that one can choose the module $V_4 = ...
1
vote
1answer
30 views

research in special function with Lie algebra

First of all, I don't know if this is the right place to ask about this. If not, please direct me somewhere I can get more help. I have to research in the field of special functions with a lie ...
2
votes
0answers
25 views

List of simple roots in the H-basis for various Lie algebras?

There are four usual bases one can use to express the roots and weights of a given algebra. The $\alpha$-basis, where we write the roots and weights in terms of the simple roots $\alpha_i$. The ...
0
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0answers
26 views

What exactly is lower central series of Lie algebra?

I've read several definitions of $LCS$ and derived series of Lie algebra, but I'm not sure if i get it right: In case of $LCS$, the relationship is given as $g_{k+1}=[g,g_k]$, does $``g_k"$ stand for ...
1
vote
1answer
27 views

Explicit Representation of the SU(N) Simple Roots in with redundant coefficents?

Commonly the simple roots for $SU(n)$ groups are given as $n$ dimensional vectors, although root-space is $n-1$ dimensional. The $SU(n)$ Wikipedia article explains: Here, we use n redundant ...
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0answers
19 views

Under what conditions is the homology of a dg coalgebra a graded coalgebra?

I'm trying to get a feel for some differential graded (dg) structures. Suppose $C$ is a differential graded coalgebra over a commutative ring $k$, i.e. a graded $k$-module equipped with a coproduct ...
2
votes
1answer
22 views

Universal enveloping algebra as bialgebra

If $\mathfrak{g}$ is a Lie algebra (over $k$ ), then we can construct its universal enveloping algebra $U(\mathfrak{g})$. We can define $\Delta:U(\mathfrak{g})\rightarrow U(\mathfrak{g})\otimes ...
0
votes
0answers
17 views

$SO(n)$ algebra relations in the vector rep

The $\mathfrak{so}(n)$ algebra has some relations between generators always indicated as $$\left[T_{ij}, T_{kl}\right] = \delta_{ik}T_{jl} - \delta_{jk}T_{li} - \delta_{jl} T_{ik} + ...
2
votes
1answer
63 views

If $\mathfrak{g}=\bigoplus\mathfrak{g}_i$ is a semisimple Lie algebra, why does $\mathfrak{h}=\bigoplus\left(\mathfrak{h}\cap\mathfrak{g}_i\right)$?

There is this property about Cartan subalgebras that is not clear to me. Suppose $\mathfrak{g}$ is a semisimple Lie algebra. Then I know we can decompose it uniquely as ...