Convexity for free boundaries with singular term (nonlinear elliptic case)

Abstract: We consider a free boundary problem in an exterior domain \begin{cases}\begin{array}{cc} Lu=g(u) & \text{in }\Omega\setminus K, \ u=1 & \text{on }\partial K,\ |\nabla u|=0 &\text{on }\partial \Omega, \end{array}\end{cases} where $K$ is a (given) convex and compact set in $\mathbb{R}^n$ ($n\ge2$), $\Omega={u>0}\supset K$ is an unknown set, and $L$ is either a fully nonlinear or the $p$-Laplace operator. Under suitable assumptions on $K$ and $g$, we prove the existence of a nonnegative quasi-concave solution to the above problem. We also consider the cases when the set $K$ is contained in ${x_n=0}$, and obtain similar results.