Relative Bruhat decomposition of wonderful compactification

TL;DR

This paper extends the classical Bruhat decomposition results to the setting of wonderful compactifications of reductive groups, focusing on the geometric and order-theoretic properties of Bruhat cells at infinity.

Contribution

It establishes analogous relative Bruhat decomposition results for wonderful compactifications over arbitrary base fields, generalizing prior work that was limited to algebraically closed fields.

Findings

Proves relative Bruhat decomposition for wonderful compactifications.

Describes the Zariski and topological closure relations of Bruhat cells.

Extends classical results to a more general geometric setting.

Abstract

In the seminal paper of Borel and Tits about reductive groups, they show some fundamental results about Bruhat cells with respect to a minimal parabolic subgroup, e.g., relative Bruhat decomposition and its geometrization, relative Bruhat order and the relation of Zariski closure and topological closure. In this paper, we show analogous results for Bruhat cells of wonderful group compactification in the sense of De Concini and Procesi. Our results can be viewed as the version at infinity of those of Borel and Tits. Our main focus is general base field. When the base field is algebraically closed, most of our results are proved by Brion and Springer.