A Note on the Complexity of Directed Clique

TL;DR

This paper investigates the computational complexity of determining the directed clique number in a directed graph, proving it is -complete when the parameter is part of the input, extending known complexity results.

Contribution

It establishes -completeness of the directed clique number decision problem when the parameter is input-dependent, generalizing previous fixed-parameter complexity results.

Findings

The problem is polynomial-time solvable for t=1.

The problem is NP-complete for fixed t.

The problem is -complete when t is part of the input.

Abstract

For a directed graph $G$, and a linear order $\ll$ on the vertices of $G$, we define backedge graph $G^\ll$ to be the undirected graph on the same vertex set with edge $\{u,w\}$ in $G^\ll$ if and only if $(u,w)$ is an arc in $G$ and $w \ll u$. The directed clique number of a directed graph $G$ is defined as the minimum size of the maximum clique in the backedge graph $G^\ll$ taken over all linear orders $\ll$ on the vertices of $G$. A natural computational problem is to decide for a given directed graph $G$ and a positive integer $t$, if the directed clique number of $G$ is at most $t$. This problem has polynomial algorithm for $t=1$ and is known to be \NP-complete for every fixed $t\ge3$, even for tournaments. In this note we prove that this problem is $\Sigma^\mathsf{P}_{2}$-complete when $t$ is given on the input.