Robin eigenvalue ratios: monotonicity on boxes and counterexamples

Abstract

We disprove Laugesen's conjecture that the ratio of the first two Robin eigenvalues decreases with the positive boundary parameter. On any prescribed compact interval of positive parameters, the ratio is strictly increasing on a suitable connected polyhedral domain in every dimension at least two; in the plane, the domain can be a simply connected polygon. In contrast, we prove strict decrease on every rectangular box. Both results follow from a comparison of the relative growth of interval eigenvalues.