The weights of simple modules in Category $\mathscr{O}$ for Kac-Moody algebras

TL;DR

This paper provides the first explicit formulas for the weights of all simple highest weight modules over Kac-Moody algebras, linking various module invariants and extending results to quantum groups.

Contribution

It introduces positive formulas for weights, relates integrability, convex hulls, and Weyl symmetry, and extends findings to quantum groups, many of which are new even in finite type.

Findings

Explicit weight formulas for all simple modules

Equivalence of integrability, convex hull, and Weyl symmetry

Extension of results to quantum groups

Abstract

We give the first positive formulas for the weights of every simple highest weight module $L(\lambda)$ over an arbitrary Kac-Moody algebra. Under a mild condition on the highest weight, we also express the weights of $L(\lambda)$ as an alternating sum similar to the Weyl-Kac character formula. To obtain these results, we show the following data attached to a highest weight module are equivalent: (i) its integrability, (ii) the convex hull of its weights, (iii) the Weyl group symmetry of its character, and (iv) when a localization theorem is available, its behavior on certain codimension one Schubert cells. We further determine precisely when the above datum determines the weights themselves. Moreover, we use condition (iv) to relate localizations of the convex hull of the weights with the introduction of poles of the corresponding $D$-module on certain divisors, which answers a question of Brion. Many of these results are new even in finite type. We prove similar assertions for highest weight modules over a symmetrizable quantum group.