Global Structures on CR Manifolds via Nash Blow-ups

A generic compact real codimension two submanifold X of Cn+2 will have a CR structure at all but a finite number of points (failing at the complex jump points J). The main theorem of this paper gives a method of extending the CR structure on the non-jump points X − J to the jump points. We examine a Gauss map from X − J to an appropriate flag manifold F and take the closure of the graph of this map in X × F . This is a version of a Nash blow-up. We give a clean criterion for when this closure is a smooth manifold and see that the local differential properties at the points X − J can now be naturally extended to this new smooth manifold, allowing global techniques from differential geometry to be applied to compact CR manifolds. As an example, we find topological obstructions for the manifold to be Levi nondegenerate.