Maximizing the Steklov eigenvalues on trees with a diameter constraint
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2026
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| _version_ | 1866917406430986240 |
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| author | Ai, Jiangdong Lin, Huiqiu Shi, Yongtang |
| author_facet | Ai, Jiangdong Lin, Huiqiu Shi, Yongtang |
| contents | We study the first nonzero Steklov eigenvalue $λ_2(T,δΩ)$ of the Dirichlet-to-Neumann operator on a finite tree $T$ with leaf boundary $δΩ$, under a constraint on the diameter $D$. He and Hua [Calc. Var. PDE, 2022] showed that $λ_2(T) \leq 2/D$ for any tree of diameter $D$, with the even-diameter equality case fully characterized. For odd $D$, the geometric picture underlying the sharp configurations has remained unclear beyond diameter three. We determine this picture completely for all odd diameters $D = 2r+1 \geq 5$. The sharp value of $λ_2$ is achieved on spider trees with nearly-equidistributed branch lengths, forming the family of \emph{generalized almost seesaw trees} $\mathrm{AS}(r,q+2,c,t)$, prescribed by the arithmetic of $n$ relative to $\lceil r/2 \rceil$. Together with the results of He-Hua and Lin-Zhao [Bull. Lond. Math. Soc., 2025] for even diameters and diameter three, this completes the geometric classification for every diameter. The argument is based on a scalar root equation for one-center profiles, an inverse boundary quadratic form on boundary fluxes, and a reduction scheme from arbitrary trees to two-center profiles, and then to the one-center class. The inverse variational viewpoint may be regarded as a boundary analogue of the classical distance-matrix formalism for trees initiated by Graham and Lovász [Adv. Math., 1978]. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2604_12404 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Maximizing the Steklov eigenvalues on trees with a diameter constraint Ai, Jiangdong Lin, Huiqiu Shi, Yongtang Combinatorics We study the first nonzero Steklov eigenvalue $λ_2(T,δΩ)$ of the Dirichlet-to-Neumann operator on a finite tree $T$ with leaf boundary $δΩ$, under a constraint on the diameter $D$. He and Hua [Calc. Var. PDE, 2022] showed that $λ_2(T) \leq 2/D$ for any tree of diameter $D$, with the even-diameter equality case fully characterized. For odd $D$, the geometric picture underlying the sharp configurations has remained unclear beyond diameter three. We determine this picture completely for all odd diameters $D = 2r+1 \geq 5$. The sharp value of $λ_2$ is achieved on spider trees with nearly-equidistributed branch lengths, forming the family of \emph{generalized almost seesaw trees} $\mathrm{AS}(r,q+2,c,t)$, prescribed by the arithmetic of $n$ relative to $\lceil r/2 \rceil$. Together with the results of He-Hua and Lin-Zhao [Bull. Lond. Math. Soc., 2025] for even diameters and diameter three, this completes the geometric classification for every diameter. The argument is based on a scalar root equation for one-center profiles, an inverse boundary quadratic form on boundary fluxes, and a reduction scheme from arbitrary trees to two-center profiles, and then to the one-center class. The inverse variational viewpoint may be regarded as a boundary analogue of the classical distance-matrix formalism for trees initiated by Graham and Lovász [Adv. Math., 1978]. |
| title | Maximizing the Steklov eigenvalues on trees with a diameter constraint |
| topic | Combinatorics |
| url | https://arxiv.org/abs/2604.12404 |