On the semi-infinite Deligne--Lusztig varieties for $\mathrm{GSp}$

TL;DR

This paper establishes a deep connection between Lusztig's semi-infinite Deligne--Lusztig varieties for GSp and affine Deligne--Lusztig varieties at infinite level, revealing new structural insights and reinterpretations of representation constructions.

Contribution

It proves isomorphisms between semi-infinite and affine Deligne--Lusztig varieties for GSp, generalizing previous results, and shows how certain varieties can be expressed as products, enriching the geometric understanding.

Findings

Lusztig's semi-infinite Deligne--Lusztig variety for GSp is isomorphic to an affine Deligne--Lusztig variety at infinite level.

A component of affine Deligne--Lusztig variety can be written as a product of a classical Deligne--Lusztig variety and an affine space.

Infinite level varieties can be realized as subsets of semi-infinite Deligne--Lusztig varieties, linking to Lusztig's conjectural framework.

Abstract

We prove that Lusztig's semi-infinite Deligne--Lusztig variety for $\mathrm{GSp}$ (and its inner form) is isomorphic, as a set with action, to an affine Deligne--Lusztig variety at infinite level, generalizing a result of Chan--Ivanov. Furthermore, we show that a component of some affine Deligne--Lusztig variety $X^0_{w_r}(b)_{\mathcal{L}}$ for $\mathrm{GSp}$ can be written, up to perfection, as a direct product of a classical Deligne--Lusztig variety with an affine space. We also study the varieties $X_h$ defined by Chan and Ivanov, and show that $X_h$ at infinite level can be realized as a subset of semi-infinite Deligne--Lusztig varieties defined using components of affine Deligne--Lusztig varieties such as $X^0_{w_r}(b)_{\mathcal{L}}$ above, even in the $\mathrm{GSp}$ case. This reinterprets previous constructions of representations from $X_h$ as instances of Lusztig's conjectural picture.