An indiscrete Bieberbach theorem: from amenable CAT(0) groups to Tits buildings

Abstract: Non-positively curved spaces admitting a cocompact isometric action of an amenable group are investigated. A classification is established under the assumption that there is no global fixed point at infinity under the full isometry group. The visual boundary is then a spherical building. When the ambient space is geodesically complete, it must be a product of flats, symmetric spaces, biregular trees and Bruhat--Tits buildings. We provide moreover a sufficient condition for a spherical building arising as the visual boundary of a proper CAT(0) space to be Moufang, and deduce that an irreducible locally finite Euclidean building of dimension at least 2 is a Bruhat--Tits building if and only if its automorphism group acts cocompactly and chamber-transitively at infinity.