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28 votes
4 answers
2k views

Matrices: characterizing pairs $(AB, BA)$

Let $A$ be an $m\times n$-matrix and $B$ an $n \times m$-matrix over the same field. Consider the matrices $C=AB$ and $D=BA$. It is probably well known (and not difficult to show) that the only ...
Frieder Ladisch's user avatar
10 votes
2 answers
1k views

How to decompose a composition of representations?

Background I would like to know if there is some slick machinery to solve the following representation-theoretic problem. Let $\left(V,\langle-,-\rangle\right)$ be a finite-dimensional real inner ...
José Figueroa-O'Farrill's user avatar
5 votes
3 answers
5k views

Spherical Harmonics - a bunch of questions about them

Hi there, Please tell me if I should divide these into individual questions next time. Short intro: Spherical Harmonics are a nice collection of functions. They are orthogonal and allow you to take ...
Ofer's user avatar
  • 151
6 votes
2 answers
1k views

Which tensor fields on a symplectic manifold are invariant under all Hamiltonian vector fields?

Consider a connected symplectic manifold $(M, \omega)$ of dimension $m=2n$. A few preliminary reminders (mostly to fix the notation): A vector field $X$ is symplectic if its flow preserves the ...
villemoes's user avatar
  • 957
5 votes
2 answers
1k views

Howe duality for exceptional algebras

There is a nice tool in representation theory, the Howe duality, which as I know works for certain pairs of classical Lie algebras (the reference to the complete list of Howe dual pairs is appreciated ...
John's user avatar
  • 605