Serrin-type overdetermined problem for the $p$-Laplacian with Robin boundary conditions

Let $Ω$ be an open bounded connected subset of $\mathbb{R}^N$, $N \geq 2$, of class $C^{2,α}$, for $α\in (0,1)$. Let $p \geq 2$ and $β>0$. We prove the symmetry of the solution of the $p$-torsion problem with Robin boundary condition subject to the natural overdetermined condition coming from a shap