A complement of the Erdős-Hajnal problem on paths with equal-degree endpoints

Abstract

Answering a question of Erdős and Hajnal, Chen and Ma proved that for all \(n\geq600\) every graph with \(2n + 1\) vertices and at least \(n^2 + n+1\) edges contains two vertices of equal degree connected by a path of length three. The complete bipartite graph $K_{n,n+1}$ shows that this edge bound is sharp. In this paper, we develop a novel approach to handle graphs with large equal degrees, which enables us to establish the result for all $n\ge 2$, thereby fully resolving the problem posed by Erdős and Hajnal.