Symmetry for a general class of overdetermined elliptic problems

Let $Ω$ be a bounded domain in $\mathbb{R} ^N$, and let $u\in C^1 (\overline{Ω})$ be a weak solution of the following overdetermined BVP: $-\nabla (g(|\nabla u|)|\nabla u|^{-1} \nabla u )=f(|x|,u)$, $u>0$ in $Ω$ and $u(x)=0, \ |\nabla u (x)| =λ(|x|)$ on $\partial Ω$, where $g\in C([0,+\infty ))\cap C^1 ((0,+\infty ) )$ with $g(0)=0$, $g'(t)>0$ for $t>0$, $f\in C([0,+\infty )) \times [0, +\infty ) )$, $f$ is nonincreasing in $|x|$, $λ\in C([0, +\infty ))$ and $λ$ is positive and nondecreasing. We show that $Ω$ is a ball and $u$ satisfies some "local" kind of symmetry. The proof is based on the method of continuous Steiner symmetrization.

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