Regular Functions on the K-Nilpotent Cone

Lucas Mason-Brown

Published 2022-04-21Version 1

Let $G$ be a complex reductive algebraic group with Lie algebra $\mathfrak{g}$ and let $G_{\mathbb{R}}$ be a real form of $G$ with maximal compact subgroup $K_{\mathbb{R}}$. Associated to $G_{\mathbb{R}}$ is a $K \times \mathbb{C}^{\times}$-invariant subvariety $\mathcal{N}_{\theta}$ of the (usual) nilpotent cone $\mathcal{N} \subset \mathfrak{g}^*$. In this article, we will derive a formula for the ring of regular functions $\mathbb{C}[\mathcal{N}_{\theta}]$ as a representation of $K \times \mathbb{C}^{\times}$. Some motivation comes from Hodge theory. In arXiv:1206.5547, Schmid and Vilonen use ideas from Saito's theory of mixed Hodge modules to define canonical good filtrations on many Harish-Chandra modules (including all standard and irreducible Harish-Chandra modules). Using these filtrations, they formulate a conjectural description of the unitary dual. If $G_{\mathbb{R}}$ is split, and $X$ is the spherical principal series representation of infinitesimal character $0$, then conjecturally $\mathrm{gr}(X) \simeq \mathbb{C}[\mathcal{N}_{\theta}]$ as representations of $K \times \mathbb{C}^{\times}$. So a formula for $\mathbb{C}[\mathcal{N}_{\theta}]$ is an essential ingredient for computing Hodge filtrations.