Gradient regularity for widely degenerate elliptic partial differential equations

Abstract: In this paper, we investigate the regularity of weak solutions $u\colon\Omega\to\mathbb{R}$ to elliptic equations of the type \begin{equation*} \mathrm{div}\, \nabla \mathcal{F}(x,Du) = f\qquad\text{in $\Omega$}, \end{equation*} whose ellipticity degenerates in a fixed bounded and convex set $E\subset\mathbb{R}^n$ with $0\in \mathrm{Int}\, E$. Here, $\Omega\subset\mathbb{R}^n$ denotes a bounded domain, and $\mathcal{F} \colon \Omega\times\mathbb{R}^n \to\mathbb{R}_{\geq 0}$ is a function with the properties: for any $x\in\Omega$, the mapping $\xi\mapsto \mathcal{F}(x,\xi)$ is regular outside $E$ and vanishes entirely within this set. Additionally, we assume $f\in L^{n+\sigma}(\Omega)$ for some $\sigma > 0$, representing an arbitrary datum. Our main result establishes the regularity \begin{equation*} \mathcal{K}(Du)\in C^0(\Omega) \end{equation*} for any continuous function $\mathcal{K}\in C^0(\mathbb{R}^n)$ vanishing on $E$.