Standard parabolic subsets of highest weight modules

Abstract

In this paper we study certain fundamental and distinguished subsets of weights of an arbitrary highest weight module over a complex semisimple Lie algebra. We call these sets "standard parabolic subsets of weights". It is shown that for any highest weight module, the sets of simple roots whose corresponding standard parabolic subsets of weights are equal form intervals. Moreover, we provide the first closed-form expressions for the maximum and minimum elements of the aforementioned intervals for all highest weight modules $\mathbb{V}^\lambda$ over semisimple Lie algebras $\mathfrak{g}$. Surprisingly, these formulas only require the Dynkin diagram of $\mathfrak{g}$ and the integrability data of $\mathbb{V}^\lambda$. As a consequence, we extend previous work by Vinberg and Cellini-Marietti to all highest weight modules. We further compute the dimension, stabilizer, and vertex set of standard parabolic faces of highest weight modules, and show that they are completely determined by the aforementioned closed-form expressions. We also compute the $f$-polynomial and a minimal half-space representation of the convex hull of the set of weights. Some of these results were recently shown for the adjoint representation of a simple Lie algebra, but analogues remain unknown for any other finite- or infinite-dimensional highest weight module. Our analysis is uniform and type-free, across all semisimple Lie algebras and for arbitrary highest weight modules.