On the local representation of piecewise smooth equations as a Lipschitz manifold

Abstract

We study systems of equations, F(x)=0, given by piecewise differentiable functions F:Rn→Rk, k⩽n. The focus is on the representability of the solution set locally as an (n−k)-dimensional Lipschitz manifold. For that, nonsmooth versions of inverse function theorems are applied. It turns out that their applicability depends on the choice of a particular basis. To overcome this obstacle we introduce a strong full-rank assumption (SFRA) in terms of Clarkeʼs generalized Jacobians. The SFRA claims the existence of a basis in which Clarkeʼs inverse function theorem can be applied. Aiming at a characterization of SFRA, we consider also a full-rank assumption (FRA). The FRA insures the full rank of all matrices from the Clarkeʼs generalized Jacobian. The article is devoted to the conjectured equivalence of SFRA and FRA. For min-type functions, we give reformulations of SFRA and FRA using orthogonal projections, basis enlargements, cross products, dual variables, as well as via exponentially many convex cones. The equivalence of SFRA and FRA is shown to be true for min-type functions in the new case k=3.