Grossberg-Karshon twisted cubes and hesitant walk avoidance

TL;DR

This paper characterizes when Grossberg-Karshon twisted cubes are untwisted by translating a toric geometric condition into combinatorial terms involving hesitant $ ext{λ}$-walks and their avoidance.

Contribution

It introduces hesitant $ ext{λ}$-walks and proves that the untwisted condition corresponds to avoiding these walks in the combinatorial data.

Findings

Untwisted twisted cubes correspond to basepoint-free divisors.

Hesitant λ-walk avoidance characterizes untwisted cubes.

The combinatorial criterion simplifies understanding of the character formula.

Abstract

Let $G$ be a complex semisimple simply connected linear algebraic group. Let $\lambda$ be a dominant weight for $G$ and $\mathcal{I} = (i_1, i_2, \ldots, i_n)$ a word decomposition for an element $w = s_{i_1} s_{i_2} \cdots s_{i_n}$ of the Weyl group of $G$, where the $s_i$ are the simple reflections. In the 1990s, Grossberg and Karshon introduced a virtual lattice polytope associated to $\lambda$ and $\mathcal{I}$, which they called a twisted cube, whose lattice points encode (counted with sign according to a density function) characters of representations of $G$. In recent work, the first author and Jihyeon Yang prove that the Grossberg-Karshon twisted cube is untwisted (so the support of the density function is a closed convex polytope) precisely when a certain torus-invariant divisor on a toric variety, constructed from the data of $\lambda$ and $\mathcal{I}$, is basepoint-free. This corresponds to the situation in which the Grossberg-Karshon character formula is a true combinatorial formula in the sense that there are no terms appearing with a minus sign. In this note, we translate this toric-geometric condition to the combinatorics of $\mathcal{I}$ and $\lambda$. More precisely, we introduce the notion of hesitant $\lambda$-walks and then prove that the associated Grossberg-Karshon twisted cube is untwisted precisely when $\mathcal{I}$ is hesitant-$\lambda$-walk-avoiding.