Weinstock Inequality on Regular Trees

Abstract

Let $T_n$ be the infinite $n$-regular tree, $n\ge 3$. We prove that every finite connected vertex set $\Omega \subset T_n$ satisfies the sharp inequality \[ σ_1(Ω)\le \frac{n}{(n-1)|Ω|+1}. \] Equality holds if and only if $\Omega$ is a ball. Since \[ |δΩ|=(n-2)|Ω|+2, \] the result is equivalently a sharp upper bound at fixed external boundary cardinality, and hence a discrete Weinstock inequality on $T_n$.

17 pages