On a conjectured reverse Faber-Krahn inequality for a Steklov-type Laplacian eigenvalue

TL;DR

This paper proves a local version of a reverse Faber-Krahn inequality for a Steklov-type Laplacian eigenvalue, showing that near a ball, the ball minimizes the eigenvalue among sets of fixed measure.

Contribution

It establishes the validity of a conjectured reverse Faber-Krahn inequality for Lipschitz sets close to a ball, highlighting the local minimality of the ball for the eigenvalue.

Findings

Reverse Faber-Krahn inequality holds near a ball

Balls are local minimizers of the embedding constant

Result applies to Lipschitz sets close to a ball

Abstract

For a given bounded Lipschitz set $\Omega$, we consider a Steklov--type eigenvalue problem for the Laplacian operator whose solutions provide extremal functions for the compact embedding $H^1(\Omega)\hookrightarrow L^2(\partial \Omega)$. We prove that a conjectured reverse Faber--Krahn inequality holds true at least in the class of Lipschitz sets which are "close" to a ball in a Hausdorff metric sense. The result implies that among sets of prescribed measure, balls are local minimizers of the embedding constant.