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Linear representations of algebras and groups, Lie theory, associative algebras, multilinear algebra.

5 votes
Accepted

what information of a representation was killed by Jacquet functor?

This is a comment, mostly on terminology that I think caused some confusion, not an answer, but I don't have enough "influence" to post this as a comment. For real reductive groups, the Jacquet funct …
Victor Protsak's user avatar
0 votes

What is a representation?

You can use Atlas of Lie groups software to compute representations.
Victor Protsak's user avatar
5 votes
Accepted

Prehomogeneous vector spaces

Your two representations of $GL_n$ are not isomorphic, because one of them contains $\Lambda_1$ with multiplicity 2 and the other with multiplicity 1, $\Lambda_1^*$ and $\Lambda_1$ being non-isomorphi …
Victor Protsak's user avatar
2 votes

What is Gelfand-Tsetlin basis for an irreducible representation of sl(n)?

It's more common to talk about the GT basis of $\mathfrak{gl}_n$-modules. In the case of the adjoint representation, the first row of the GT scheme is $(1,0,\ldots,0,-1)$ and each subsequent row satis …
Victor Protsak's user avatar
2 votes

About composition factors of Verma modules

The answer is affirmative. It is enough to provide a way to uniquely determine the highest weight $\lambda$ a Verma module $M(\lambda)$ from the composition factors of $M(\lambda)$. Every composition …
Victor Protsak's user avatar
2 votes

Intertwiners and Clebsch-Gordan coefficients

I have some trouble understanding notation (what is $K$?), so here is a very general answer. The case of ${\rm SL}(2,\Bbb{R})$ was studied in the old work of Repka and, if I am not mistaken, describ …
Victor Protsak's user avatar
9 votes
Accepted

Invariant polynomials for a product of algebraic groups

There is a fruitful way of thinking about the algebra $\mathbb{C}[M_{n,m}]^{{O_n}\times O_m}$ that originates from the $(GL_n, GL_m)$-duality. Namely, $$ \mathbb{C}[M_{n,m}]=\bigoplus_{\lambda}V_ …
Victor Protsak's user avatar
2 votes

Weight multiplicity formulae for $(\mathfrak g,B)$-irreps

The "trivial" case is when the simple highest weight module is a generalized Verma module (i.e. the module induced from a character of a parabolic). The multiplicity is given by the variant of Kostant …
Victor Protsak's user avatar
5 votes
Accepted

Lie algebra with cyclic structure constants

Endow your algebra with the symmetric bilinear form making $\{e_i\}$ an orthornormal basis (the Gram matrix is the identity matrix). Since $c_{ij}^k=([e_i,e_j],e_k)$, the cyclicity condition is equiva …
Victor Protsak's user avatar
12 votes
Accepted

Is every G-invariant function on a Lie algebra a trace?

The answer to the general question is "no": If $\mathfrak{g}$ is solvable, by Lie's theorem its commutant $\mathfrak{g}^{\prime}=[\mathfrak{g},\mathfrak{g}]$ is represented by strictly upper triang …
Victor Protsak's user avatar
9 votes
Accepted

How to calculate symmetric tensor products of SO(10) representations?

Based on comments from Eric Rowell and José Figueroa-O'Farrill, I assume that your $V$ is a half-spinor representation of $Spin_{10}$. The key issue is that your hypothetical decomposition of $S(V)$ i …
Victor Protsak's user avatar
3 votes
Accepted

Decomposition of GL(2,p) into irreducible representations

Note: This was written up concurrently with David's answer, but wasn't proofread and didn't get past the captcha stage due to technical problems. It is more common to consider the representation of …
Victor Protsak's user avatar
4 votes

irreducible subgroup of SL(n,R)

A representation $\rho$ over a field $K$ is called absolutely irreducible if for any algebraic field extension $L/K$, the representation $\rho\otimes_K L$ obtained by extension of scalars is irreducib …
Victor Protsak's user avatar
13 votes
Accepted

to test equivalence of representations under automorphism

I assume that $\phi$ is an automorphism of $G.$ Note that if $\phi$ is inner then trivially $\rho$ and $\rho\circ\phi$ are equivalent, thus the answer depends only on the image of $\phi$ in the outer …
Victor Protsak's user avatar
2 votes

Is there an analog of Clifford Theorem in the setting of Lie algebras?

If the module $M$ is finite-dimensional then the answer seems to be affirmative. Let $Soc_I(M)$ be the maximal semisimple $I$-submodule of $M,$ which is non-zero by the finite-dimensio-nality of $M. …
Victor Protsak's user avatar

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