A contractible Schiffer counterexample on the half-sphere

Abstract

We show the existence of a family of nontrivial smooth contractible domains on the sphere that admit Neumann eigenfunctions of the Laplacian which are constant on the boundary. These domains are contained on the half-sphere, in stark contrast with the rigidity literature for Serrin-type problems. The proof relies on a local bifurcation argument around the family of geodesic disks centered at the north pole. We combine the use of anisotropic H\"older spaces for the functional setting with computer-assisted techniques to check the bifurcation conditions.