Components of $V(\rho) \otimes V(\rho)$ and dominant weight polyhedra for affine Kac-Moody Lie algebras

TL;DR

This paper extends the understanding of tensor product components and dominant weight polyhedra from simple Lie algebras to affine Kac-Moody Lie algebras, providing new results especially for affine sl_n.

Contribution

It generalizes a known criterion for tensor components from simple Lie algebras to affine Kac-Moody algebras, removing the saturation factor in the affine sl_n case.

Findings

Confirmed the tensor component criterion for affine sl_n without saturation.

Extended the polyhedral description of dominant weights to affine Kac-Moody algebras.

Developed new techniques involving the Goddard-Kent-Olive construction.

Abstract

Kostant asked the following question: Let $\mathfrak{g}$ be a simple Lie algebra over the complex numbers. Let $\lambda$ be a dominant integral weight. Then, $V(\lambda)$ is a component of $V(\rho)\otimes V(\rho)$ if and only if $\lambda \leq 2 \rho$ under the usual Bruhat-Chevalley order on the set of weights. In an earlier work with R. Chirivi and A. Maffei the second author gave an affirmative answer to this question up to a saturation factor. The aim of the current work is to extend this result to untwisted affine Kac-Moody Lie algebra $\mathfrak{g}$ associated to any simple Lie algebra $\mathring{\mathfrak{g}}$ (up to a saturation factor). In fact, we prove the result for affine $sl_n$ without any saturation factor. Our proof requires some additional techniques including the Goddard-Kent-Olive construction and study of the characteristic cone of non-compact polyhedra.