2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/131789This paper gives a new proof of a result of Geoff Mess that the linear holonomy group of a complete flat Lorentz 3-manifold cannot be cocompact in SO(2,1). The proof uses a signed marked Lorentzian length-spectrum invariant developed by G.Margulis, reinterpreted in terms of deformations of hyperbolic surfaces.to appear in "Crystallographic Groups and their Generalizations II," Contemporary MathematicsDifferential Geometrytext