Positivity sets of supersolutions of degenerate elliptic equations and the strong maximum principle

Abstract

We investigate positivity sets of nonnegative supersolutions of the fully nonlinear elliptic equations $F(x,u,Du,D^2u)=0$ in $\Omega$, where $\Omega$ is an open subset of ${\mathbb R}^N$, and the validity of the strong maximum principle for $F(x,u,Du,D^2u)=f$ in $\Omega$, with $f\in\text{C}(\Omega)$ being nonpositive. We obtain geometric characterizations of positivity sets $\left\{x\in\Omega\,:\, u(x)>0\right\}$ of nonnegative supersolutions $u$ and establish the strong maximum principle under some geometric assumption on the set $\left\{x\in\Omega\,:\, f(x)=0\right\}$.