Automorphisms of Lie incidence geometries with spectral gaps

TL;DR

This paper characterizes uniclass automorphisms of simply laced spherical buildings using spectral gap properties, linking automorphism fixed structures to spectral conditions on root subgroup geometries.

Contribution

It provides a spectral gap criterion for identifying uniclass automorphisms in simply laced spherical buildings, extending previous fixed-structure characterizations.

Findings

Uniclass automorphisms correspond to spectral gap conditions.

Automorphisms do not map points to distance or codistance 1.

Characterization applies to thick irreducible spherical buildings of simply laced type.

Abstract

An automorphism of a building is called uniclass if the Weyl distance between any chamber and its image lies in a unique (twisted) conjugacy class of the Coxeter group. In a previous paper we characterised uniclass automorphisms of spherical buildings in terms of their fixed structure. In the present paper we restrict to the simply laced case and characterise uniclass automorphisms in terms of a spectral gap property. More precisely, we show that an automorphism of a thick irreducible spherical building of simply laced type is uniclass if and only if no point of the long root subgroup geometry is mapped to distance $1$ or codistance $1$.