Optimisation of the lowest Robin eigenvalue in the exterior of a compact set, II: non-convex domains and higher dimensions

TL;DR

This paper extends the geometric optimization of the lowest Robin eigenvalue in exterior domains to non-convex and higher-dimensional cases, showing that the exterior of a disk or ball maximizes the eigenvalue under certain constraints.

Contribution

It proves that the exterior of a disk or ball maximizes the lowest Robin eigenvalue in various classes of domains, including non-convex and disconnected planar sets, and higher-dimensional convex exterior sets.

Findings

The exterior of a disk maximizes the eigenvalue among simply connected planar sets with fixed perimeter or area.

The exterior of a disk maximizes the eigenvalue among disconnected planar sets with fixed average perimeter.

In higher dimensions, the exterior of a ball maximizes the eigenvalue under curvature-based constraints.

Abstract

We consider the problem of geometric optimisation of the lowest eigenvalue of the Laplacian in the exterior of a compact set in any dimension, subject to attractive Robin boundary conditions. As an improvement upon our previous work (arXiv:1608.04896, to appear in J. Convex Anal.), we show that under either a constraint of fixed perimeter or area, the maximiser within the class of exteriors of simply connected planar sets is always the exterior of a disk, without the need of convexity assumption. Moreover, we generalise the result to disconnected compact planar sets. Namely, we prove that under a constraint of fixed average value of the perimeter over all the connected components, the maximiser within the class of disconnected compact planar sets, consisting of finitely many simply connected components, is again a disk. In higher dimensions, we prove a completely new result that the lowest point in the spectrum is maximised by the exterior of a ball among all sets exterior to bounded convex sets satisfying a constraint on the integral of a dimensional power of the mean curvature of their boundaries. Furthermore, it follows that the critical coupling at which the lowest point in the spectrum becomes a discrete eigenvalue emerging from the essential spectrum is minimised under the same constraint by the critical coupling for the exterior of a ball.