MV polytopes and reduced double Bruhat cells

TL;DR

This paper establishes a bijection between MV polytopes with a fixed highest vertex and non-negative tropical points of reduced double Bruhat cells, introducing new generalized minor functions and analyzing their combinatorial structure.

Contribution

It introduces a new class of generalized minor functions that tropicalize to MV polytope data and describes the combinatorial structure of MV polytopes with a fixed highest vertex.

Findings

MV polytopes of highest vertex w correspond to tropical points of reduced double Bruhat cells labeled by w^{-1}

Vertices of these polytopes are labeled by Weyl group elements less than w in Bruhat order

Explicit map from Weyl group to elements bounded by w in Bruhat order

Abstract

When $G$ is a complex reductive algebraic group, MV polytopes are in bijection with the non-negative tropical points of the unipotent group of $G$. By fixing $w$ from the Weyl group, we can define MV polytopes whose highest vertex is labelled by $w$. We show that these polytopes are in bijection with the non-negative tropical points of the reduced double Bruhat cell labelled by $w^{-1}$. To do this, we define a collection of generalized minor functions $\Delta_\gamma^\text{new}$ which tropicalize on the reduced Bruhat cell to the BZ data of an MV polytope of highest vertex $w$. We also describe the combinatorial structure of MV polytopes of highest vertex $w$. We explicitly describe the map from the Weyl group to the subset of elements bounded by $w$ in the Bruhat order which sends $u \mapsto v$ if the vertex labelled by $u$ coincides with the vertex labelled by $v$ for every MV polytope of highest vertex $w$. As a consequence of this map, we prove that these polytopes have vertices labelled by Weyl group elements less than $w$ in the Bruhat order.