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

7 votes

A linear representation of the group of jets at 0 under composition

I think you want to look at Faà di Bruno's formula, and the description of composition of formal power series, particularly the historical remarks. The linear representation of this group that allows …
Robert Bryant's user avatar
7 votes

Orbit space of $\mathrm{SO}(3)$ irreducible representations

I don't know where the orbit types in this case were first explicitly classified, but it is done in my paper Second order families of special Lagrangian 3-folds, Perspectives in Riemannian geometry, 6 …
Robert Bryant's user avatar
6 votes
Accepted

Invariants for the isotropy representation of a Riemannian symmetric space

One reference is in Helgason's 1984 book Groups and Geometric Analysis. The result you want appears there as Corollary 5.12. The notation he uses is $X=G/K$ is a symmetric space where $G$ is connecte …
Robert Bryant's user avatar
29 votes
Accepted

Is it possible to realize the Moebius strip as a linear group orbit?

Yes. Here is one way: Consider standard $\mathbb{R}^3$ endowed with the Lorentzian quadratic form $Q = x^2+y^2-z^2$, and let $G\simeq\mathrm{O}(2,1)\subset\mathrm{GL}(3,\mathbb{R})$ be the symmetry …
Robert Bryant's user avatar
2 votes

Injective group homomorphism on $\frac{Spin(4k+2)\times U(1)}{\mathbf{Z}/2}$ or $\frac{Spin(...

You really should have a look at F. Reese Harvey's book Spinors and Calibrations, where all of your questions are answered. For example, your 'inclusion' (1) is not correct for sufficiently large $n$. …
Robert Bryant's user avatar
10 votes
Accepted

Is there a unique "natural" action of $\mathsf{SL}_{n+1}$ on $\mathbb{R}^n$?

There are two aspects to this question, the global question and the local question. Also, the case $n=1$ is different from $n>1$. Basically, the answer is 'essentially yes, but with some caveats'. H …
Robert Bryant's user avatar
11 votes
Accepted

The normalizer of $\operatorname{Spin}(2N)$ in $\operatorname{U}(2^{N-1})$?

You can work out the answers to these questions using the material in Chapter 11 of the book Spinors and Calibrations by F. Reese Harvey. You will also need to recall that, for $N\not=4$, the group o …
Robert Bryant's user avatar
7 votes
Accepted

Invariant ring of $\textrm{Sym}^2(\wedge^2\mathbb{R}^4)$ under $\textrm{SO}(4)$

The answer is 'no', though I don't know an easy way to see this without doing an explicit calculation. Here is where to look though, if you want to do the calculation yourself: Things work out a bit …
Robert Bryant's user avatar
12 votes
Accepted

To describe an invariant trivector in dimension 8 geometrically

Here's another very nice (but still algebraic) interpretation that explains some of the geometry: Recall that $\operatorname{SL}(2,\mathbb{C})$ has a $2$-to-$1$ representation into $\operatorname{SL} …
LSpice's user avatar
  • 12.9k
8 votes
Accepted

Nilpotent orbits in representations of exceptional groups

As per the OP's comment, we are to assume that $\mathrm{G}_2$ and $\mathrm{F}_4$ mean the complex simple Lie groups. Let's start with $\mathrm{G}_2\subset\mathrm{SO}(7,\mathbb{C})$, in its standard re …
Robert Bryant's user avatar
6 votes
Accepted

Subgroup $\mathrm{E}_6$ generated by $\mathrm{Spin_7}$ and $\mathrm{SL}_3$

N.B.: I am revising my response for clarity. (The actual answer to the question asked by the OP is still the same, but I think that this re-organization, particularly at the end, makes the structure …
Robert Bryant's user avatar
6 votes

Subgroup of $\mathrm{GL}_n$ stabilizing linear subspace skew-symmetric matrices

Here is an outline of the argument that shows that the $\mathrm{SL}_6(\mathbb{C})$-stabilizer of the generic $3$-plane $W\subset\Lambda^2(\mathbb{C}^6)$ has dimension $1$, not $0$, as (apparently) cla …
Robert Bryant's user avatar
12 votes
Accepted

Invariants of symmetric matrices

This is a trick question, right? It's not true when $m=2$ because, then $\mathrm{SO}(m{-}1)=\mathrm{SO}(1)$ is trivial, so that all polynomials on $2$-by-$2$ symmetric matrices are invariants, and th …
Robert Bryant's user avatar
13 votes
Accepted

Stabilizer of Sp(n) and U(n) in GL(n)

First, let me fix a misunderstanding: $\mathrm{Sp}(n)$ does not sit in $\mathrm{GL}(n,\mathbb{C})$, but in $\mathrm{GL}(2n,\mathbb{C})$, so I'll assume that you mean, for the second part that $A$ lie …
Robert Bryant's user avatar
23 votes

First Explicit Irreducible Representations

If all you really want to know are the irreducible representations of ${\frak{so}}(7)$ and ${\frak{so}}(8)$ up to dimension $30$, I can save you the trouble of looking up these tables: For ${\frak{so …
Robert Bryant's user avatar

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