Kinematic formulas for sets defined by differences of convex functions

The class WDC(M) consists of all subsets of a smooth manifold M that may be expressed in local coordinates as sufficiently regular sublevel sets of DC (differences of convex) functions. If M is Riemannian and G is a group of isometries acting transitively on the sphere bundle SM, we define the invariant curvature measures of compact WDC subsets of M, and show that pairs of such subsets are subject to the array of kinematic formulas known to apply to smoother sets. Restricting to the case (M,G)=(Rd,SO(d)‾), this extends and subsumes Federer's theory of sets with positive reach in an essential way. The key technical point is equivalent to a sharpening of a classical theorem of Ewald, Larman, and Rogers characterizing the dimension of the set of directions of line segments lying in the boundary of a given convex body.