On the Steiner $k$-diameter and Steiner ($k,k^{\prime}$)-radius of trees
Qingnan Zhang, Yingzhi Tian
TL;DR
This work generalizes a classical bound relating Steiner diameter and Steiner radius on trees by proving Sd_k(T) ≤ (k/(k−k')) Sr_{k,k'}(T) for k≥3 and k>k'≥1, extending Chartrand et al.'s result. It then delivers tight bounds for the special cases k'∈{2,3}, establishing exact inequalities: Sd_k(T) ≤ (k/(k−2)) Sr_{k,2}(T) − 2/(k−2) with equality for T ≅ P_2(a,b;x), and Sd_k(T) ≤ (k/(k−3)) Sr_{k,3}(T) − 6/(k−3) with equality for T ≅ P_3(a,b;x). The paper also conjectures a further refined universal bound Sd_k(T) ≤ (k/(k−k')) Sr_{k,k'}(T) − k'(k'−1)/(k−k') holding for all k'≥1, supported by the tightness results in the k' = 2,3 cases. These results deepen the understanding of Steiner-distance-based parameters on trees and provide precise tools for analyzing their extremal behavior.
Abstract
Given a connected graph $G=(V,E)$ and a $k$-set $S\subseteq V(G)$, the $Steiner$ $distance$ $d_{G}(S)$ of $S$ is defined as the size of a minimum tree including $S$ in $G$. The $Steiner$ $k$-$eccentricity$ of a vertex $v$ in $G$ is the maximum value of $d_G(S)$ over all $S\subseteq V(G)$ with $|S|=k$ and $v\in S$. The minimum Steiner $k$-eccentricity over all vertices, denoted by $Sr_k(G)$, is called the $Steiner$ $k$-$radius$ of $G$ and the maximum Steiner $k$-eccentricity over all vertices, denoted by $Sd_k(G)$, is its $Steiner$ $k$-$diameter$. The $Steiner$ $(k,k^{\prime})$-$eccentricity$ of a $k^{\prime}$-subset $S^{\prime}$ of $V(G)$, which is an extension of the Steiner $k$-eccentricity of a vertex $v$, is defined as the maximum Steiner distance over all $k$-subsets of $V(G)$ containing $S^{\prime}$. The minimum Steiner $(k,k^{\prime})$-eccentricity among all $k^{\prime}$-subsets of $V(G)$, denoted by $Sr_{k,k^{\prime}}(G)$, is called the $Steiner$ $(k,k^{\prime})$-$radius$ of $G$. In 1989, Chartrand, Oellermann, Tian and Zou showed that for any $k\geq3$, $Sd_k(T)\leq \frac{k}{k-1}Sr_k(T)$ for any tree $T$. In this paper, we generalize this result and show that $Sd_k(T)\leq \frac{k}{k-k^{\prime}}Sr_{k,k^{\prime}}(T)$ for any $k\geq3$, $k>k^{\prime}\geq1$. Furthermore, for $k^{\prime}=2$ and $k^{\prime}=3$, we obtain a tight upper bound of the Steiner $k$-diameter by the Steiner $(k,k^{\prime})$-radius for all trees.