Weinstock Inequality on Regular Trees

Source abstract

Let $T_n$ be the infinite $n$-regular tree, $n\ge3$. We prove that every finite connected vertex set $Ω\subset T_n$ satisfies the sharp inequality $$σ_1(Ω)\le \frac{n}{(n-1)|Ω|+1}.$$ Equality holds if and only if $Ω$ 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 $T_n$.