On Laplacian eigenvalue equation with constant Neumann boundary data

• DOI: 10.1007/s00208-024-02873-1
• Publication Date: 2024-04-26T13:02:05Z
• Authors: Yong Huang; Qinfeng Li; Qiuqi Li; Ruofei Yao
• Keywords: Constant (computer programming)

Let $Ω$ be a bounded Lipshcitz domain in $\mathbb{R}^n$ and we study boundary behaviors of solutions to the Laplacian eigenvalue equation with constant Neumann data. \begin{align} \label{cequation0} \begin{cases} -Δu=cu\quad &\mbox{in $Ω$}\\ \frac{\partial u}{\partial ν}=-1\quad &\mbox{on $\partial Ω$}. \end{cases} \end{align}First, by using properties of Bessel functions and proving new inequalities on elementary symmetric polynomials, we obtain the following inequality for rectangular boxes, balls and equilateral triangles: \begin{align} \label{bbb} \lim_{c\rightarrow μ_2^-}c\int_{\partial Ω}u_c\, dσ\ge \frac{n-1}{n}\frac{P^2(Ω)}{|Ω|}, \end{align}with equality achieved only at cubes and balls. In the above, $u_c$ is the solution to the eigenvalue equation and $μ_2$ is the second Neumann Laplacian eigenvalue. Second, let $κ_1$ be the best constant for the Poincaré inequality with mean zero on $\partial Ω$, and we prove that $κ_1\le μ_2$, with equality holds if and only if $\int_{\partial Ω}u_c\, dσ>0$ for any $c\in (0,μ_2)$. As a consequence, $κ_1=μ_2$ on balls, rectangular boxes and equilateral triangles, and balls maximize $κ_1$ over all Lipschitz domains with fixed volume. As an application, we extend the symmetry breaking results from ball domains obtained in Bucur-Buttazzo-Nitsch[J. Math. Pures Appl., 2017], to wider class of domains, and give quantitative estimates for the precise breaking threshold at balls and rectangular boxes. It is a direct consequence that for domains with $κ_1<μ_2$, the above boundary limit inequality is never true, while whether it is valid for domains on which $κ_1=μ_2$ remains open.<br/>A revised version compared to the previous one<br/>