Minkowski difference weight formulas

Abstract

Fix any complex Kac-Moody Lie algebra $\mathfrak{g}$, and Cartan subalgebra $\mathfrak{h}\subset \mathfrak{g}$. We study arbitrary highest weight $\mathfrak{g}$-modules $V$ (with any highest weight $\lambda\in \mathfrak{h}^*$, and let $L(\lambda)$ be the corresponding simple highest weight $\mathfrak{g}$-module), and write their weight-sets $\mathrm{wt} V$. This is based on and generalizes the Minkowski decompositions for all $\mathrm{wt} L(\lambda)$ and hulls $\mathrm{conv}_{\mathbb{R}}(\mathrm{wt} V)$, of Khare [J. Algebra. 2016 & Trans. Amer. Math. Soc. 2017] and Dhillon-Khare [Adv. Math. 2017 & J. Algebra. 2022]. Those works need a freeness property of the Dynkin graph nodes of integrability $J_{\lambda}$ of $L(\lambda)$: $\mathrm{wt} L(\lambda)\ -$ any sum of simple roots over $J_{\lambda}^c$ are all weights of $L(\lambda)$. We generalize it for all $V$, by introducing nodes $J_V$ that record all the lost 1-dim. weights in $V$. We show three applications (seemingly novel) for all $\big(\mathfrak{g}, \lambda, V\big)$ of our $J_V^c$-freeness: 1) Minkowski decompositions of all $\mathrm{wt} V$, subsuming those above for simples. 1$'$) Characterization of these formulas. 1$''$) For these, we solve the inverse problem of determining all $V$ with fixing $\mathrm{wt} V \ =$ weight-set of a Verma, parabolic Verma and $L(\lambda)$ $\forall$ $\lambda$. 2) At module level (by raising operators' actions), construction of weight vectors along $J_V^c$-directions. 3) Lower bounds on the multiplicities of such weights, in all $V$.