Shape optimization for a nonlinear elliptic problem related to thermal insulation

In this paper we consider a minimization problem of the type $$I_{\beta,p}(D;\Omega)=\inf\biggl\{\int_\Omega \lvert{D\phi}\rvert^pdx+\beta \int_{\partial^* \Omega}\lvert{\phi}\rvert^pd\mathcal{H}^{n-1},\; \phi \in W^{1,p}(\Omega),\;\phi \geq 1 \;\textrm{in}\;D\biggl\},$$ where $\Omega$ is a bounded connected open set in $\mathbb{R}^n$, $D\subset \bar{\Omega}$ is a compact set and $\beta$ is a positive constant. We let the set $D$ vary under prescribed geometrical constraints and $\Omega \setminus D$ of fixed thickness, in order to look for the best (or worst) geometry in terms of minimization (or maximization) of $I_{\beta,p}$. In the planar case, we show that under perimeter constraint the disk maximize $I_{\beta,p}$. In the $n$-dimensional case we restrict our analysis to convex sets showing that the same is true for the ball but under different geometrical constraints.