On the eigenvalue problem for a bulk/surface elliptic system

Abstract: The paper addresses the doubly elliptic eigenvalue problem $$\begin{cases} -\Delta u=\lambda u \qquad &\text{in $\Omega$,}\ u=0 &\text{on $\Gamma_0$,}\ -\Delta_\Gamma u +\partial_\nu u =\lambda u\qquad &\text{on $\Gamma_1$,} \end{cases} $$ where $\Omega$ is a bounded open subset of $\mathbb{R}^N$ ($N\ge 2$) with $C^1$ boundary $\Gamma=\Gamma_0\cup\Gamma_1$, $\Gamma_0\cap\Gamma_1=\emptyset$, $\Gamma_1$ being nonempty and relatively open on $\Gamma$. Moreover $\mathcal{H}^{N-1}(\overline{\Gamma}_0\cap\overline{\Gamma}_1)=0$ and $\mathcal{H}^{N-1}(\Gamma_0)>0$. We recognize that $L^2(\Omega)\times L^2(\Gamma_1)$ admits a Hilbert basis of eigenfunctions of the problem and we describe the eigenvalues. Moreover, when $\Gamma$ is at least $C^2$ and $\overline{\Gamma}_0\cap\overline{\Gamma}_1=\emptyset$, we give several qualitative properties of the eigenfunctions.