Transitive Actions on Lorentz Manifolds with Noncompact Stabilizer

If a topological group G acts on a topological space X, then we say that the action is orbit nonproper provided that, for some x ∈ X, the orbit map g → gx: G → X is nonproper. We consider the problem of classifying the connected, simply connected real Lie groups G admitting a locally faithful, orbit nonproper, isometric action on a connected Lorentz manifold. In an earlier paper, we found three collections of groups such that G admits such an action iff G is in one of the three collections. In another paper, we effectively described the first collection. In this paper, we show that the second collection contains a small, effectively described collection of groups, and, aside from those, it is contained in the union of the first and third collections. Finally, in a third paper, we effectively describe the third collection, thus solving the stated problem.