Taming Nonconvexity with Epigraph Lifting

Abstract

We consider inverse problems regularized by a trained neural network with \emph{convex activations} and \emph{unrestricted weights}. The resulting optimization problem is generally nonconvex, and gradient-based methods may converge to poor local minima. We introduce a \emph{penalized epigraph lifting} of the network and show that the lifted problem admits a \emph{difference-of-convex (DC) decomposition}. For Lipschitz continuous activations, we establish \emph{finite exactness}: the original objective is recovered exactly at a finite and explicitly computable penalty threshold $\barρ$. Building on this structure, we propose \emph{H-DC} (Homotopic DC), a homotopic scheme that solves a sequence of DC subproblems with increasing $ρ$, warm-starting each level from the previous solution. Numerical experiments on synthetic problems, a manifold denoiser regularizer, and image inpainting with a learned Mumford--Shah regularizer show that H-DC consistently escapes poor local minima and reaches solutions with lower objective values than gradient-based methods from the same initialization.