On the geometry of star domains and the spectra of Hodge-Laplace operators under domain approximation

Abstract: We study the eigenvalues of Hodge--Laplace operators over bounded convex domains. Generalizing contributions by Guerini and Savo, we show that the Poincar\'e--Friedrichs constants of the Sobolev de~Rham complexes increase with the degree of the forms. We establish that the spectra of Hodge--Laplace operators converge when the domain is approximated by a sequence of convex domains up to a bi-Lipschitz deformation of the boundary. As preparatory work with independent relevance, we study the gauge function and the reciprocal radial function of convex sets, providing new proofs for Lipschitz estimates by Vre\'{c}ica and Toranzos for the reciprocal radial function and improving a Lipschitz estimate for the gauge function due to Beer.