A Lie group is a group (in the sense of abstract algebra) that is also a differentiable manifold, such that the group operations (addition and inversion) are smooth, and so we can study them with differential calculus. They are a special type of topological group. Consider using with the ...

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33 views

Lie Group Structure on the 2-Sphere: does the following argument hold?

Being inspired by the existence of a Lie group structure on the circle $\Bbb{S}^{1}$, I was looking for a group law that would make the two-sphere $\Bbb{S}^{2}$ into a Lie group. I found out that no ...
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7 views

Lie assignment-explanation of a proof

http://www.math.ucla.edu/~vsv/liegroups2007/9.pdf I am trying to understand the proof of theorem 1 in this file.The proof is not perfectly clear to me. I would like to get some explanations. what ...
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15 views

Is the Lie algebra morphism induced by surjective Lie group morphism also surjective?

Let $G$ be a matrix lie group and $\Pi : G\to \Pi(G)$ a surjective Lie group morphism. Let $\mathfrak g$ and $\mathfrak h$ be the respective Lie algebras of $G$ and $\Pi(G)$. Then there is a unique ...
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26 views

Lie Algebra and Tangent Space

For a matrix Lie group $G\subset GL_n(\mathbb C)$ we define the Lie algebra to be the set of matrices $X\in M_n(\mathbb C)$ such that for all $t\in \mathbb R$ we have $\quad \exp(tX)\in G$. For ...
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1answer
36 views

Vector field left-invariant then also its respective flow?

I was wondering whether left invariance of a vector field $X$ to a respective Lie group $G$ (so $dL(a)(x)(X(x))= X(ax))$ is transfered to the respective flow defined by $\frac{d}{dt} \phi^{t}(x) = ...
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16 views

Compactness and noncompactness of Lie groups

I have read that if one complexifies a Lie group, (i.e. a group which has elements written as $G=e^{i\lambda_a T^a}$, where $T^a$ are the Lie algebra elements which generate the group, and $\lambda_a$ ...
2
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1answer
22 views

Evolution operator always in $SU(n)$?

Think about the evolution operator $U$ in Quantum Mechanics for finite dimensional systems. Then this operator satisfies an equation $$U'(t) = -iHU(t)..$$ Here, I assume that $H$ is ...
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1answer
31 views

Diffeomorphism between groups

I would like to prove that the map $$\phi :O_{n}\times H\rightarrow GL_{n}(\mathbb{R})$$ $$\phi(B,A)=BA$$ is a diffeomorphism, while $O_{n}$ is the orthogonal group and $H$ is the group of all upper ...
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29 views

Relation between compact Lie group and Lie algebra representation

Currently I'm studying representation theory for compact Lie groups and I don't know how to link representations of Lie algebra to representations of corresponding Lie group, ie. suppose I have a ...
3
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2answers
68 views

Lie algebra of R^n

Until now the only example of lie groups I have seen are subgroups of $GL_n$. Today I had the idea, that also $G=(\mathbb R^n,+)$ must be a lie group ($(\mathbb R^n,+)$ is a group with the ...
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60 views

Why is this not an inconsistency in elementary Lie theory?

I made an observation last week, and it has bothered me ever since. Recall the formulae ...
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1answer
25 views

Continuous Group of Transformations

I read in chapter 2 "Weisner Method" in the book "Obtaining Generating Functions" by Elna Browning McBride In the Introduction,I did not understand the meaning of this statement " The method is ...
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1answer
51 views

$O_{2n}(\mathbb{R}) \cap GL_{n}(\mathbb{C})=U(n)$

During a lecture of a Lie Algebras yesterday, the professor of the class stated the following fact without proof $O_{2n}(\mathbb{R}) \cap GL_{n}(\mathbb{C})=U(n)$ Note that we are viewing ...
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1answer
31 views

The complex structure of a complex torus

A complex torus $X$ of complex dimension $1$ is just a quotient of $\mathbb{C}$ by a lattice $\Lambda=\mathbb{Z}\oplus \tau\mathbb{Z}$ where $\tau=a+ib$ is a complex number with $b>0$. The complex ...
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25 views

Does the “differential” of a unitary representation give continuous operators on the space of smooth vectors?

Let $\pi : G \rightarrow U(H)$ be a strongly continuous unitary representation of a Lie group, $G$, on a Hilbert space, $H$. Let $H_\infty$ be the space of smooth vectors in $H$, those $v$ for which ...
4
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1answer
96 views

Symplectic group and Quaternion Inner product

I have a problem understanding a passage from "Naive Lie theory"(by John Stillwell), here is the passage from section $3.9$ ,page $71$: The idea of treating orthogonal, unitary, and symplectic ...
3
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2answers
29 views

Why do “the Dynkin components of a weight play the role of eigenvalues with respect to the generators $H^i$ of the Cartan subalgebra”?

In the book "Symmetries, Lie Algebras and Representations: A Graduate Course for Physicists" by Jürgen Fuchs,Christoph Schweigert the authors write "In the description of representations, the ...
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30 views

Representation ring of circle group over complex field

Can someone please describe how to find a representation algebra of circle group over complex field ? I am reading " representation theory of compact Lie group" chapter 3 section 7. It will be great ...
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1answer
26 views

General Element of U(4)

Relating back to previous question about how to write a general element of $U(2)$, I am now wondering about how to write a general element of $U(4)$. Define $\Gamma_{(i,j)}:=\sigma_i\otimes\sigma_j$ ...
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30 views

Bochner-style theorem for SO(3)

Bochner's Theorem essentially provides necessary/sufficient conditions for when something is the Fourier transform of a nonnegative measure on a compact abelian group. I'm looking for a similar ...
4
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1answer
41 views

Understanding $G_2$ inside Spin(7)? (EDIT: problem solved)

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 ...
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20 views

In what sense are complex representations of a real Lie algebra and complex representations of the complexified Lie algebra equivalent?

In this book I read Proposition A.1. The irreducible complex representations of a real Lie algebra $\mathfrak{g}$ are in one-to-one correspondence with the irreducible complex-linear ...
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1answer
52 views

Are the physics and math definitions of a complex representation equivalent?

I was astonished to read at Wikipedia that The term complex representation has slightly different meanings in mathematics and physics. In mathematics, a complex representation is a group ...
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28 views

What does it really mean to complexify the $10$-dimensional representation of $ \mathfrak{so}(10)$?

A commonly used "trick" in $SO(10)$ Grand Unified Theories is to use a "complex" instead of a "real" $10$-dimensional representation for the Higgs fields. My problem is understanding what this ...
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1answer
398 views

Map from $SL(2,\mathbb C)/SU(2)$ to $SL(2,\mathbb C)$

I am trying to write a map $s$, such that $s: SL(2,\mathbb{C})/SU(2) \to SL(2,\mathbb C)$. Using Iwasawa decomposition, I see that any element $g$ in $SL(2,\mathbb{C})$ can be written as $g = su$, ...
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11 views

Complexification of a Lie algebra representation in terms of weights?

EDIT: I found in this book the sentence: The weight system of a real representation of $G$ is defined to be the weight system of its complexification I think if someone can explain what this ...
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37 views

Why is a weight automatically a complex weight?

EDIT: I think the partial answer to my question is that in order to talk about weights we always need a complex Lie algebra. If the Lie algebra is real, we use the complexification, This is necessary, ...
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38 views

Where to the degrees of freeedom go when a complex representation becomes a real representation of a subalgebra?

As an example consider the complex $16$-dimensional representation of $\mathfrak{so}(10)$. When $\mathfrak{so}(10)$ is reduced to the subalgebra $\mathfrak{so}(9)$, the complex $16$-dimensional ...
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53 views

About the diameter of a Riemannian manifold

My exact problem is a Riemannian manifold defined on $SU(2^n)$, where the metric is defined as follows: If $U(t)$ is a curve so that $U′(t)=−iH(t)U(t)$ (so just a unitary evolution of a quantum system ...
1
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1answer
47 views

Finite normal subgroups of $SO(4)$

What are the finite normal subgroups of $SO(4)$? If these do not exist (or if they are trivial, e.g. from some projection to $SO(2)$), are there different finite normal subgroups of $O(4),$ $U(4)$, ...
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7 views

Regular elements on Lie algebra and on Lie group level

I want to understand (better) the definition and meaning of regular elements of semisimple Lie groups resp. Lie algebras. A regular element $X$ of $\mathfrak g$ is one whose centralizer has smallest ...
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2answers
40 views

Decomposition of an unitary operator by simple operators

For quantum computation, it's well known that any unitary operator can be approximated with an arbitrary accuracy by simple operators, for example to approximate an unitary operator on n qubits by no ...
2
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1answer
33 views

Orbit closures of real symmetric bilinear form

Let $\alpha$ and $\beta$ be two real symmetric bilinear forms in $\operatorname{sym}(\mathbb{R}^n)$, with signatures $(p_{\alpha},n_{\alpha},z_{\alpha})$ and $(p_{\beta},n_{\beta},z_{\beta})$. I ...
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25 views

An incorrect proof that the Lie algebra matrix exponential is always injective. What's wrong?

Suppose we have two square complex matrices $X,Y $ in the lie algebra $\mathcal{G}$ of matrix Lie group $\mathbb G$ such that $e^X = e^Y$. Then $e^{tX}$ and $e^{tY}$ define the same one-parameter ...
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27 views

Lie group is parallelizable

While going through the proof given by Alex Youcis at http://math.stackexchange.com/a/308798/86801 , I found the part where the local representation of the map $\ \Phi\ $ is shown to be the identity ...
3
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1answer
186 views

Lie bracket; confusing proof from lecture

I am having some difficulties understanding this proof. Let $G$ be a closed matrixsubgroup of the general linear group. We have a right translation $Y(g):=dR_g(e) Y(e)$ on the Lie algebra $Y \in ...
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14 views

Reductions of structure groups and sections of coset bundles

I'm looking for a reference for the following proposition: Let $G$ be a Lie group and $H$ a (closed) Lie subgroup of $G$. Let $E \to B$ be a principal $G$-bundle. Then reductions of the ...
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1answer
131 views

Parallelizability of Lie groups

I have tried a lot to prove this well-known result. The basic idea behind the proof is clear to me. But I'm stuck at showing either of the following conditions: The map $\ G \rightarrow TG \ $ given ...
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19 views

Orbit closures of symmetric bilinear form

Let $A$ and $B$ be two real symmetric matrices in $M_n(\mathbb{R})$. I would like to learn about necessary and sufficient conditions for knowing when $B \in \overline{GL_n(\mathbb{R})\cdot A}$; where: ...
0
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2answers
319 views

Is there a general expression for the adjoint representation of $U(N)$ or $u(N)$?

At least for low values of $N$ like $2$ or $3$ and such I would like to know if there are explicit matrices known giving the representation of $u(N)$ or $U(N)$ in the adjoint? (..a related query: ...
2
votes
1answer
27 views

Curvature of $K$-invariant connection (principal bundles)

Here is a proposition from Kobayashi & Nomizu's Foundations of Differential Geometry. I don't understand how they obtain the final line of the proof. They write: \begin{align} ...
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27 views

Lie group action from the Lie algebra

want to find the corresponding lifting f the standard U(n) lifting on $C^n$ to $L=C^n \times C$ with hermitian metric $e^(-|z|^2)$. I try to follow the method in Donaldson, and I find if B in u(n) ...
5
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3answers
213 views

Lie Group Decomposition as Semidirect Product of Connected and Discrete Groups

I've believed for a long time that every Lie group can be decomposed as the semidirect product of a connected Lie group and a discrete Lie group. However, in this Math Overflow thread, it is mentioned ...
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24 views

Formal adjoint of curvature (Yang Mills)

Currently reading a paper on finding solutions to the Yang Mills equation $D^*\Omega=0$, where $\Omega$ is the curvature and $D^*$ is the formal adjoint of the exterior covariant derivative $D$. ...
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13 views

Do there exist equidistributed countable subgroups in (compact) Lie groups?

By an equidistributed countable subgroup I mean a countable subgroup (with a finite or possibly countable set of generators) that is dense in $G$ such that for any sufficiently nice function (Haar ...
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20 views

Relation between Iwasawa and Cartan decompositions

Given a real semi-simple Lie group, one have the two decompositions - $$G=KAN \text{ - Iwasawa Decomposition} $$ and $$G=KA^{+}K \text{ - Cartan Decomposition} $$ I'm interested in an explicit ...
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40 views

Level sets on $SU(n)$

Given $G \in SU(n)$, what are the level sets of the function $F(V) = |tr(G^{\dagger}V)|^2$? Can they be written only in terms of abstract linear maps, not in terms of the components of $V$ and $G$ in ...
2
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1answer
54 views

Scalar product on Lie algebra of compact Lie group [duplicate]

I am studying Differential Geometry and I am facing with a lemma in which there is a step that I do not understand. In particular, let $G$ be a connected compact Lie group, is used "$\langle\ \cdot , ...
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37 views

Lie group representation and inner product

Let $G$ be a connected semisimple Lie group.Now let $\theta$ be the Cartan involution of $G$ and let $(\pi,V)$ be a finite dimensional representation of $G$. On page 22 of Analysis and geometry on ...
6
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1answer
72 views

Showing $U(n)/Z(U(n))=SU(n)/Z(SU(n))$

I was working on the following problem from Stillwell's Naive Lie Theory. Prove that $U(n)/Z(U(n))=SU(n)/Z(SU(n))$. It was shown earlier in the text that $Z(U(n))=\{ e^{i\theta} \textbf{1}: ...