Functions with Bounded Hessian-Schatten Variation: Density, Variational, and Extremality Properties

In this paper we analyze in detail a few questions related to the theory of functions with bounded p-Hessian-Schatten total variation, which are relevant in connection with the theory of inverse problems and machine learning. We prove an optimal density result, relative to the p-Hessian-Schatten total variation, of continuous piecewise linear (CPWL) functions in any space dimension d, using a construction based on a mesh whose local orientation is adapted to the function to be approximated. We show that not all extremal functions with respect to the p-Hessian-Schatten total variation are CPWL. Finally, we prove the existence of minimizers of certain relevant functionals involving the p-Hessian-Schatten total variation in the critical dimension d=2.