Localization of Singularities and Universal Geometric Rank Bounds in the Satake Correspondence

TL;DR

This paper develops a framework for analyzing singularities in the affine Grassmannian, providing a factorization of transition matrices and establishing universal bounds on their rank based on local geometric data.

Contribution

It introduces a novel factorization of the transition matrix between bases in the affine Grassmannian and proves a universal rank bound using a new geometric efficiency metric.

Findings

Factorization of the transition matrix into four geometric components

Introduction of the Geometric Efficiency metric ($ta$)

Universal rank bound based on local BMP stalks

Abstract

This article introduces a framework for the localization and isolation of singularities in the affine Grassmannian. Our primary result is a structural factorization of the transition matrix $C$ between the Mirkovi\'c--Vilonen (MV) basis and the convolution basis into $C = P \cdot M \cdot A \cdot Q^{-1}$, where the four factors represent: equivariant localization ($Q$), fusion via nearby cycles ($A$), local intersection cohomology stalks ($M$), and diagonal normalization ($P$). Utilizing this factorization, and by introducing the Geometric Efficiency metric ($\eta$) we establish a Universal Geometric Rank Bound, proving that the rank of the transition matrix $C$ is bounded by the dimension of the local Braden--MacPherson (BMP) stalks.