Nilpotent subspaces and nilpotent orbits

Abstract

Let $G$ be a semisimple algebraic group with Lie algebra $\mathfrak g$. For a nilpotent $G$-orbit $\mathcal O\subset\mathfrak g$, let $d_\mathcal O$ denote the maximal dimension of a subspace $V\subset \mathfrak g$ that is contained in the closure of $\mathcal O$. In this note, we prove that $d_\mathcal O \le \frac{1}{2}\dim\mathcal O$ and this upper bound is attained if and only if $\mathcal O$ is a Richardson orbit. Furthermore, if $V$ is $B$-stable and $\dim V= \frac{1}{2}\dim\mathcal O$, then $V$ is the nilradical of a polarisation of $\mathcal O$. Every nilpotent orbit closure has a distinguished $B$-stable subspace constructed via an $\mathfrak{sl}_2$-triple, which is called the Dynkin ideal. We then characterise the nilpotent orbits $\mathcal O$ such that the Dynkin ideal (1) has the minimal dimension among all $B$-stable subspaces $\mathfrak c$ such that $\mathfrak c\cap\mathcal O$ is dense in $\mathfrak c$, or (2) is the only $B$-stable subspace $\mathfrak c$ such that $\mathfrak c\cap\mathcal O$ is dense in $\mathfrak c$.