## Properly Discontinuous Groups of Affine Transformations: A Survey

- H Abels
- Geometriae Dedicata
- 2001

We describe up to finite coverings causal flat affine complete Lorentzian manifolds such that the past and the future of any point are closed near this point. We say that these manifolds are strictly causal. In particular, we prove that their fundamental groups are virtually abelian. In dimension 4, there is only one, up to a scaling factor, strictly causal… (More)