New Hardness Results in Rainbow Connectivity

TL;DR

This paper proves that determining whether a graph's rainbow connectivity or strong rainbow connectivity is at most a given number is NP-hard, confirming a longstanding conjecture and extending hardness results to bipartite graphs.

Contribution

It proves the NP-hardness of deciding rainbow connectivity and strong rainbow connectivity for all fixed k, including bipartite graphs, confirming a key conjecture in graph theory.

Findings

NP-hardness of deciding rainbow connectivity for all k

NP-hardness for strong rainbow connectivity in bipartite graphs

Confirms longstanding conjecture in graph coloring complexity

Abstract

A path in an edge colored graph is said to be a rainbow path if no two edges on the path have the same color. An edge colored graph is (strongly) rainbow connected if there exists a (geodesic) rainbow path between every pair of vertices. The (strong) rainbow connectivity of a graph $G$, denoted by ($src(G)$, respectively) $rc(G)$ is the smallest number of colors required to edge color the graph such that the graph is (strong) rainbow connected. It is known that for \emph{even} $k$ to decide whether the rainbow connectivity of a graph is at most $k$ or not is NP-hard. It was conjectured that for all $k$, to decide whether $rc(G) \leq k$ is NP-hard. In this paper we prove this conjecture. We also show that it is NP-hard to decide whether $src(G) \leq k$ or not even when $G$ is a bipartite graph.