A graph theoretic proof of the complexity of colouring by a local tournament with at least two directed cycles

In this paper we give a graph theoretic proof of the fact that deciding whether a homomorphism exists to a fixed local tournament with at least two directed cycles is NP-complete. One of the main reasons for the graph theoretic proof is that it showcases all of the techniques that have been built up over the years in the study of the digraph homomorphism problem

A graph theoretic proof of the complexity of colouring by a local tournament with at least two directed cycles

In this paper we give a graph theoretic proof of the fact that deciding whether a homomorphism exists to a fixed local tournament with at least two directed cycles is NP-complete. One of the main reasons for the graph theoretic proof is that it showcases all of the techniques that have been built up over the years in the study of the digraph homomorphism problem

A classification of locally semicomplete digraphs

Recently, Huang (1995) gave a characterization of local tournaments. His characterization involves arc-reversals and therefore may not be easily used to solve other structural problems on locally semicomplete digraphs (where one deals with a fixed locally semicomplete digraph). In this paper we derive a classification of locally semicomplete digraphs which is very useful for studying structural properties of locally semicomplete digraphs and which does not depend on Huang's characterization. An advantage of this new classification of locally semicomplete digraphs is that it allows one to prove results for locally semicomplete digraphs without reproving the same statement for tournaments. We use our result to characterize pancyclic and vertex pancyclic locally semicomplete digraphs and to show the existence of a polynomial algorithm to decide whether a given locally semicomplete digraph has a kernel

Trouver de bonnes 2-partitions des digraphes I. Propriétés héréditaires.

We study the complexity of deciding whether a given digraph D has a vertex-partition into two disjoint subdigraphs with given structural properties. Let H and E denote following two sets of natural properties of digraphs: H ={acyclic, complete, arcless, oriented (no 2-cycle), semicomplete, symmetric, tournament} and E ={strongly connected, connected, minimum out- degree at least 1, minimum in-degree at least 1, minimum semi-degree at least 1, minimum degree at least 1, having an out-branching, having an in-branching}. In this paper, we determine the complexity of deciding, for any fixed pair of positive integers k1,k2, whether a given digraph has a vertex partition into two digraphs D1,D2 such that |V(Di)| ≥ ki and Di has property Pi for i = 1, 2 when P1 ∈ H and P2 ∈ H ∪ E. We also classify the complexity of the same problems when restricted to strongly connected digraphs. The complexity of the problems when both P1 and P2 are in E is determined in the companion paper [2].when both $\mathbb{P}_1$ and $\mathbb{P}_2$ are in ${\cal E}$ is determined in the companion paper (INRIA Research report RR-8868).Nous étudions la complexité de décider si un digraphe donné D admet une partition en deux sous-digraphes ayant des propriétés structurelles fixées. Dénotons par H et E les deux ensembles de propriétés de digraphes naturelles : H ={acyclique, complet, sans arcs, orienté, semicomplet, symétrique, tournoi} et E ={fortement connexe, connexe, degré sortant minimum au moins 1, degré entrant minimum au moins 1, semi-degré entrant minimum au moins 1, degré minimum au moins 1, avoir une arborescence sortante couvrante, avoir une arborescence entrante couvrante}. Dans ce rapport, nous déterminons la complexité de décider, pour toute paire d’entiers k1,k2, si un digraphie donné admet une partition en deux digraphes D1,D2 tels que |V(Di)|≥ki et Di a la propriété Pi pour i=1,2lorsque P1 ∈H et P2 ∈H∪E. Nous classifions également la complexité des mêmes problèmes restreints aux digraphies fortement connexes. La complexité des problèmes lorsque P1 et P2 sont toutes deux dans E est déterminée dans le rapport suivant (Rapport de Recherche INRIA RR-8868)

Finding good 2-partitions of digraphs I. Hereditary properties

International audienceWe study the complexity of deciding whether a given digraph D has a vertex-partition into two disjoint subdigraphs with given structural properties. Let H and E denote following two sets of natural properties of digraphs: H ={acyclic, complete, arcless, oriented (no 2-cycle), semicomplete, symmetric, tournament} and E ={strongly connected, connected, minimum out-degree at least 1, minimum in-degree at least 1, minimum semi-degree at least 1, minimum degree at least 1, having an out-branching, having an in-branching}. In this paper, we determine the complexity of of deciding, for any fixed pair of positive integers k1, k2, whether a given digraph has a vertex partition into two digraphs D1, D2 such that |V (Di)| ≥ ki and Di has property Pi for i = 1, 2 when P1 ∈ H and P2 ∈ H ∪ E. We also classify the complexity of the same problems when restricted to strongly connected digraphs. The complexity of the problems when both P1 and P2 are in E is determined in the companion paper [2]