Geometric and homological properties of affine Deligne-Lusztig varieties

TL;DR

This paper investigates the geometric and homological aspects of affine Deligne-Lusztig varieties, providing a reduction method to relate their structure for arbitrary elements to minimal length cases, and proves a conjecture linking their dimension to affine Hecke algebra polynomials.

Contribution

It generalizes previous results to the affine case, introduces a reduction technique, and proves a conjecture connecting variety dimensions with affine Hecke algebra properties.

Findings

Describes the structure of affine Deligne-Lusztig varieties for minimal length elements.

Provides a reduction method relating arbitrary elements to minimal length cases.

Establishes a connection between the dimension of varieties and the degree of class polynomials.

Abstract

This paper studies affine Deligne-Lusztig varieties $X_{\tw}(b)$ in the affine flag variety of a quasi-split tamely ramified group. We describe the geometric structure of $X_{\tw}(b)$ for a minimal length element $\tw$ in the conjugacy class of an extended affine Weyl group, generalizing one of the main results in \cite{HL} to the affine case. We then provide a reduction method that relates the structure of $X_{\tw}(b)$ for arbitrary elements $\tw$ in the extended affine Weyl group to those associated with minimal length elements. Based on this reduction, we establish a connection between the dimension of affine Deligne-Lusztig varieties and the degree of the class polynomial of affine Hecke algebras. As a consequence, we prove a conjecture of G\"ortz, Haines, Kottwitz and Reuman in \cite{GHKR}.