On the Maximum Density of Graphs with Unique-Path Labelings

A unique-path labelling of a simple, finite graph is a labelling of its edges with real numbers such that, for every ordered pair of vertices (u, v), there is at most one nondecreasing path from u to v. In this paper we prove that any graph on n vertices that admits a unique-path labelling has at most n log2(n)/2 edges, and that this bound is tight for infinitely many values of n. Thus we significantly improve on the previously best known bounds. The main tool of the proof is a combinatorial lemma which might be of independent interest. For every n we also construct an n-vertex graph that admits a unique-path labelling and has n log2(n)/2−O(n) edges.