The Gomory-Hu inequality and trees

TL;DR

This paper investigates the relationship between graph structures and ultrametric spaces generated by vertex labelings, establishing key inequalities and conditions for ultrametricity.

Contribution

It proves a new inequality relating the size of the distance set to the number of edges and characterizes when equality holds, also linking graph labelings to ultrametric spaces.

Findings

Proves that |D(V)| ≤ |E| + 1 for ultrametric spaces generated by graph labelings.

Identifies necessary and sufficient conditions for equality in the inequality.

Shows that connected graphs with non-negative labels generate pseudoultrametric spaces and provides conditions for ultrametricity.

Abstract

Let $G=(V,E)$ be a finite connected graph with vertex set $V$ and edge set $E$, and let $U(G)$ be the set of all ultrametric spaces $(V,d_l)$ generated by vertex labelings $l\colon V \to \mathbb R^+$. We prove that the inequality $$ |D(V)| \le |E| + 1 $$ holds for all $(V,d_l) \in U(G)$, where $D(V)$ is the distance set of $(V,d_l)$. The necessary and sufficient conditions under which the above inequality turns to an equality are found. Moreover, we prove that each connected graph with non-negative vertex labeling generates a pseudoultrametric space and find some sufficient conditions under which this space is ultrametric.