Matrix recursion for positive characteristic diagrammatic Soergel bimodules for affine Weyl groups

TL;DR

This paper introduces a matrix recursion functor for diagrammatic Soergel bimodules over affine Weyl groups in positive characteristic, revealing self-similarity and providing new bounds on the p-canonical basis and tilting module characters.

Contribution

It constructs a functor from the diagrammatic Hecke category to a matrix category that categorifies a recursive representation, linking bimodules of different sizes and explaining p-canonical basis self-similarity.

Findings

Provides a new functor categorifying a recursive representation.

Explains self-similarity in the p-canonical basis.

Establishes lower bounds on p-canonical basis and tilting characters.

Abstract

Let $W$ be an affine Weyl group, and let $\Bbbk$ be a field of characteristic $p>0$. The diagrammatic Hecke category $\mathcal{D}$ for $W$ over $\Bbbk$ is a categorification of the Hecke algebra for $W$ with rich connections to modular representation theory. We explicitly construct a functor from $\mathcal{D}$ to a matrix category which categorifies a recursive representation $\xi: \mathbb{Z}W \rightarrow M_{p^r}(\mathbb{Z}W)$, where $r$ is the rank of the underlying finite root system. This functor gives a method for understanding diagrammatic Soergel bimodules in terms of other diagrammatic Soergel bimodules which are ``smaller'' by a factor of $p$. It also explains the presence of self-similarity in the $p$-canonical basis, which has been observed in small examples. By decategorifying we obtain a new lower bound on the $p$-canonical basis, which corresponds to new lower bounds on the characters of the indecomposable tilting modules by the recent $p$-canonical tilting character formula due to Achar-Makisumi-Riche-Williamson.