Extremal domains and Pólya-type inequalities for the Robin Laplacian on rectangles and unions of rectangles

Abstract

We show that eigenvalues of the Robin Laplacian with a positive boundary parameter $α$ on rectangles and unions of rectangtes satisfy Pólya-type inequalities, albeit with an exponent smaller than that of the corresponding Weyl asympotics for a fixed domain. We determine the optimal exponents in either case, showing that they are different in the two situations. Our approach to proving these results includes a characterisation of the corresponding extremal domains for the $k$th eigenvalue in regions of the $(k,α)$-plane.