Path Cover, Hamiltonicity, and Independence Number: An FPT Perspective

TL;DR

This paper extends the Gallai-Milgram theorem with an FPT algorithm that determines minimal path covers and finds large independent sets, also providing a fixed-parameter algorithm for Hamiltonian path detection in graphs.

Contribution

It introduces an FPT algorithm for path cover optimization and Hamiltonian path detection, advancing algorithmic graph theory and addressing longstanding open problems.

Findings

An FPT algorithm for minimum path cover with certification.

An FPT algorithm for deciding Hamiltonian path in graphs with small independence number.

Application of techniques to Hamiltonian Cycle, Path Cover, and Topological Minor problems.

Abstract

The classic theorem of Gallai and Milgram (1960) generalizes several fundamental results in Graph Theory, such as Dilworth's theorem on posets and K\H{o}nig's theorem on matchings in bipartite graphs. The theorem asserts that for every graph G, the vertex set of G can be partitioned into at most \alpha(G) vertex-disjoint paths, where \alpha(G) is the maximum size of an independent set in G. The proof of the Gallai-Milgram theorem is constructive and yields a polynomial-time algorithm that computes a covering of G by at most \alpha(G) vertex-disjoint paths. While the Gallai-Milgram theorem is tight, it was not known prior to our work whether deciding if a graph G could be covered by fewer than \alpha(G) vertex-disjoint paths can be done in polynomial time. We resolve this question by proving the following algorithmic extension of the Gallai-Milgram theorem for undirected graphs: There is an algorithm that, for an n-vertex graph G and an integer parameter k \ge 1, runs in time 2^{2^{O(k^4\log{k})}} \cdot n^{O(1)} and outputs a path cover P of G together with - a correct conclusion that P is a minimum-size path cover, or - an independent set of size |P|+k, certifying that P contains at most \alpha(G) - k paths. The proof of our algorithmic extension of the Gallai-Milgram theorem is non-trivial and builds on several novel algorithmic ideas. One of the key subroutines in our algorithm is an FPT algorithm, parameterized by \alpha(G), for deciding whether G contains a Hamiltonian path. This result is of independent interest - prior to our work no polynomial-time algorithm for deciding Hamiltonicity was known even for graphs with independence number at most three. Moreover, the algorithmic techniques we develop apply to a wide array of problems in undirected graphs, including Hamiltonian Cycle, Path Cover, Largest Linkage, and Topological Minor Containment.