Center of U(n), Cascade of Orthogonal Roots, and a Construction of Lipsman-Wolf

Abstract

Let $G$ be a complex simply-connected semisimple Lie group and let $\g=\hbox{\rm Lie}\,G$. Let $\g = \n_- +\hh + \n$ be a triangular decomposition of $\g$. One readily has that $\hbox{\rm Cent}\,U(\n)$ is isomorphic to the ring $S(\n)^{\n}$ of symmetric invariants. Using the cascade $\mathcal{B}$ of strongly orthogonal roots, some time ago we proved (see [K]) that $S(\n)^{\n}$ is a polynomial ring $\mathbb{C}[{\xi }_1,...,{\xi }_m]$ where $m$ is the cardinality of $\mathcal{B}$. The authors in [LW] introduce a very nice representation-theoretic method for the construction of certain elements in $S(\n)^{\n}$. A key lemma in [LW] is incorrect but the idea is in fact valid. In our paper here we modify the construction so as to yield these elements in $S(\n)^{\n}$ and use the [LW] result to prove a theorem of Tony Joseph.

arXiv comment10 pages, plain.tex; key words: cascade of orthogonal roots, Borel subgroups, nilpotent coadjoint action, dedicated to Joe Wolf; msc keywords: Representation theory, invariant theory