Affine Coxeter Extensions of the Two-Holed Projective Plane

Abstract

A Margulis spacetime is a complete flat affine Lorentzian 3-manifold with free fundamental group. Associated to $M$ is a noncompact complete hyperbolic surface $\Sigma$. We study double extensions of $\pi_1 (M) \cong \pi_1 (\Sigma)$ when $\Sigma$ is homeomorphic to a projective plane minus two discs. We classify proper actions of this double extension on Minkowski space and show that there exist proper actions that do not admit crooked fundamental domains.