Level sets of nonsmooth functions, part 2: Lipschitz and piecewise-differentiable manifolds

Title

Level sets of nonsmooth functions, part 2: Lipschitz and piecewise-differentiable manifolds

Journal Article
2025
Journal
Journal of Mathematical Analysis and Applications
543 (2)
Pagination
128920
This paper introduces piecewise-differentiable manifolds according to a unified general framework that also applies to nonsmooth Lipschitz manifolds and smooth manifolds. We present definitions of nonsmooth manifolds and embedded submanifolds for abstract sets as well as for subsets of R^n, and explore the relationships between them. The piecewise-differentiable and Lipschitz Rank Theorems from Part 1 of this series, in terms of the Clarke Jacobian and B-subdifferential generalized derivative sets, are used to characterize level sets of functions between nonsmooth manifolds as embedded submanifolds. We illustrate how the Level Set Theorems developed in this paper can be applied to functions on Euclidean space, including a piecewise-differentiable process model for distillation columns.