Reciprocal sums of Neumann eigenvalues in non-Euclidean space forms

Abstract

Let $M^n_κ$ be the simply connected space form of dimension $n\ge2$ and constant sectional curvature $κ\in\{-1,1\}$. For every bounded connected smooth domain $Ω\subset M^n_κ$, assume in the case $κ=1$ that $Ω$ is contained in an open hemisphere, and let $B_Ω$ be a geodesic ball with $|B_Ω|=|Ω|$. We prove $$ \sum_{j=1}^n \frac1{μ_j(Ω)}\ge \frac{n}{μ_1(B_Ω)}, $$ where $μ_j(Ω)$ are the positive Neumann eigenvalues of $Ω$. Equality holds if and only if $Ω$ is a geodesic ball. This proves a conjecture proposed by Xia and Wang [Math. Ann. 385, 2023, 863-879].