Elsevier

Discrete Mathematics

Volume 87, Issue 1, 19 January 1991, Pages 29-40
Discrete Mathematics

Label-connected graphs and the gossip problem

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Abstract

A graph with m edges is called label-connected if the edges can be labeled with real numbers in such a way that, for every pair (u, v) of vertices, there is a (u, v)-path with ascending labels. The minimum number of edges of a label-connected graph on n vertices equals the minimum number of calls in the gossip problem for n persons, which is known to be 2n − 4 for n ⩾ 4. A polynomial characterization of label-connected graphs with n vertices and 2n − 4 edges is obtained. For a graph G, let θ(G) denote the minimum number of edges that have to be added to E(G) in order to create a graph with two edge-disjoint spanning trees. It is shown that for a graph G to be label-connected, θ(G) ⩽ 2 is necessary and θ(G) ⩽ 1 is sufficient. For i = 1, 2, the condition θ(G) ⩽ i can be checked in polynomial time. Yet recognizing label-connected graphs is an NP-complete problem. This is established by first showing that the following problem is NP-complete: Given a graph G and two vertices u and v of G, does there exist a (u, v)-path P in G such that GE(P) is connected?

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On leave from Instituto Superior de Agronomia, Tapada da Ajuda, 1399 Lisboa Codex, Portugal.