Clarke subgradients of stratifiable functions

Abstract

We establish the following result: if the graph of a (nonsmooth) real-extended-valued function $f:{\mathbb{R}}^n\to \mathbb{R}\cup \{+\infty \}$ is closed and admits a Whitney stratification, then the norm of the gradient of $f$ at $x\in domf$ relative to the stratum containing $x$ bounds from below all norms of Clarke subgradients of $f$ at $x$. As a consequence, we obtain some Morse-Sard type theorems as well as a nonsmooth Kurdyka-Łojasiewicz inequality for functions definable in an arbitrary o-minimal structure.