Symmetry and stability of non-negative solutions to degenerate elliptic equations in a ball

We consider non-negative distributional solutions $u \in C^1(B_R)$ to the equation $−\mbox{ div } [g(|\nabla u|)|\nabla u|^{−1} \nabla u] = f(|x|, u)$ in a ball $B_R$, with $u = 0$ on $\partial B_R$, where $f$ is continuous and non-increasing in the first variable and $g \in C^1(0, +\infty) \cap C[0, +\infty)$, with $g(0) = 0$ and $g(t) >0$ for $t > 0$. The solutions satisfy a certain ‘local’ type of symmetry. Moreover, this also implies that the solutions are radially symmetric if $f$ satisfies appropriate growth conditions near its zeros. In a second part we study the autonomous case, $f = f(u)$. The solutions of the equation are critical points for an associated variation problem. We show under rather mild conditions that global and local minimizers of the variational problem are radial. This is joint work with Peter Takac.