A concavity condition for existence of a negative value in Neumann-Poincaré spectrum in three dimensions
Yong-Gwan Ji, Hyeonbae Kang · Proceedings of the American Mathematical Society · 2018
It is proved that if a bounded domain in three dimensions satisfies a certain concavity condition, then the Neumann-Poincaré operator on either the boundary of the domain or its inversion in a sphere has a negative value in its spectrum. The concavity condition is quite simple, and is satisfied if there is a point on the boundary at which the Gaussian curvature is negative.