“…Results concerning the first positive eigenvalue and Neumann boundary conditions are available in [16,22] for Euclidean domains. Concerning higher eigenvalues, results in the case of Dirichlet conditions for Euclidean domains can be found in [1,4,36] (continuity and limits with respect to p, multiplicity, etc.). As for Riemannian manifolds, we mention [12,30,32,35,41] for sharp estimates for the first positive eigenvalue in case of compact manifolds or domains and Neumann boundary conditions, under a given lower bound on the Ricci curvature.…”