Clique Number of Tournaments II

Abstract

The directed clique number $\vec{\omega }(T)$ of a tournament $T$ is the minimum, over all orderings of the vertices of $T$, of the clique number of the graph whose edges are the arcs that point backward with respect to the ordering. In this paper, we prove that, for every integer $k\ge 3$, deciding whether $\vec{\omega }(T)\le k$ is NP-complete. This answers a question of Nguyen, Scott, and Seymour, and contrasts with the classical undirected setting, where deciding whether $\omega (G)\le k$ is polynomial-time solvable for every fixed integer $k$. On the other hand, we give a polynomial-time algorithm distinguishing tournaments with $\vec{\omega }(T)\le 2$ from those with $\vec{\omega }(T)>100$. We also study the tournament analogue of the Gyárfás--Sumner conjecture. We construct new $\vec{\chi }$-bounding tournaments and thereby prove a conjecture of Aboulker, Aubian, Charbit, and Lopes stating that every class of tournaments with bounded twin-width is $\vec{\chi }$-bounded. We then exhibit new tournaments that are not $\vec{\chi }$-bounding, disproving another conjecture of Aboulker, Aubian, Charbit, and Lopes, as well as two conjectures of Kim. Finally, we present infinite families of 3-$\vec{\omega }$-critical and 4-$\vec{\omega }$-critical tournaments.