The Schiffer problem on the cylinder and on the $2$-sphere (2303.17036v3)

Abstract: We prove the existence of a family of compact subdomains $\Omega$ of the flat cylinder $\mathbb{R}^N\times \mathbb{R}/2\pi\mathbb{Z}$ for which the Neumann eigenvalue problem for the Laplacian on $\Omega$ admits eigenfunctions with constant Dirichlet values on $\partial \Omega$. These domains $\Omega$ have the property that their boundaries $\partial \Omega$ have nonconstant principal curvatures. In the context of ambient Riemannian manifolds, our construction provides the first examples of such domains whose boundaries are neither homogeneous nor isoparametric hypersurfaces. The functional analytic approach we develop in this paper overcomes an inherent loss of regularity of the problem in standard function spaces. With the help of this approach, we also construct a related family of subdomains of the $2$-sphere $S^2$. By this we disprove a conjecture in \cite{Souam}.