Avoidance Games Are PSPACE-Complete

Avoidance games are games in which two players claim vertices of a hypergraph and try to avoid some structures. These games are studied since the introduction of the game of SIM in 1968, but only few complexity results are known on them. In 2001, Slany proved some partial results on Avoider-Avoider games complexity, and in 2017 Bonnet et al. proved that short Avoider-Enforcer games are Co-W[1]-hard. More recently, in 2022, Miltzow and Stojakovi\'c proved that these games are NP-hard. As these games corresponds to the mis\`ere version of the well-known Maker-Breaker games, introduced in 1963 and proven PSPACE-complete in 1978, one could expect these games to be PSPACE-complete too, but the question remained open since then. We prove here that both Avoider-Avoider and Avoider-Enforcer conventions are PSPACE-complete, and as a consequence of it that some particular Avoider-Enforcer games also are

A note on the flip distance between non-crossing spanning trees

We consider spanning trees of $n$ points in convex position whose edges are pairwise non-crossing. Applying a flip to such a tree consists in adding an edge and removing another so that the result is still a non-crossing spanning tree. Given two trees, we investigate the minimum number of flips required to transform one into the other. The naive $2n-\Omega(1)$ upper bound stood for 25 years until a recent breakthrough from Aichholzer et al. yielding a $2n-\Omega(\log n)$ bound. We improve their result with a $2n-\Omega(\sqrt{n})$ upper bound, and we strengthen and shorten the proofs of several of their results

Incidence, a Scoring Positional Game on Graphs

Positional games have been introduced by Hales and Jewett in 1963 and have been extensively investigated in the literature since then. These games are played on a hypergraph where two players alternately select an unclaimed vertex of it. In the Maker-Breaker convention, if Maker manages to fully take a hyperedge, she wins, otherwise, Breaker is the winner. In the Maker-Maker convention, the first player to take a hyperedge wins. In both cases, the game stops as soon as Maker has taken a hyperedge. By definition, this family of games does not handle scores and cannot represent games in which players want to maximize a quantity. In this work, we introduce scoring positional games, that consist in playing on a hypergraph until all the vertices are claimed, and by defining the score as the number of hyperedges a player has fully taken. We focus here on Incidence, a scoring positional game played on a 2-uniform hypergraph, i.e. an undirected graph. In this game, two players alternately claim the vertices of a graph and score the number of edges for which they own both end vertices. In the Maker-Breaker version, Maker aims at maximizing the number of edges she owns, while Breaker aims at minimizing it. In the Maker-Maker version, both players try to take more edges than their opponent. We first give some general results on scoring positional games such that their membership in Milnor's universe and some general bounds on the score. We prove that, surprisingly, computing the score in the Maker-Breaker version of Incidence is PSPACE-complete whereas in the Maker-Maker convention, the relative score can be obtained in polynomial time. In addition, for the Maker-Breaker convention, we give a formula for the score on paths by using some equivalences due to Milnor's universe. This result implies that the score on cycles can also be computed in polynomial time

Vertex covering under constraints

Cette thèse porte sur le problème de la couverture d'ensembles finis dans une structure discrète. Cette problématique très générale permet de nombreuses approches et nous faisons l'étude de certaines d'entre elles. Le premier chapitre introduit les notions qui seront indispensables à la bonne compréhension de cette thèse et fait un bref état de l'art sur certains problèmes de couvertures, en particulier le problème de domination dans les graphes. Le second chapitre aborde la domination de puissance, une variante du problème de domination qui a la particularité qu'on lui adjoint un phénomène de propagation. Nous étudions ce problème pour les grilles triangulaires et les grilles carrées de dimension 3. Dans le troisième chapitre, nous revenons à la domination classique mais dans un contexte ludique, avec le jeu de domination Maker-Breaker. Nous étudions la complexité du problème consistant à décider quel joueur gagne, la durée minimale d'une partie si les deux joueurs jouent parfaitement, et dérivons ce jeu pour la domination totale et dans une version Avoider-Enforcer. Le quatrième chapitre traite du nombre géodésique fort, un problème qui a la particularité de se couvrir à l'aide de plus courts chemins dans le graphe. Nous étudions le nombre géodésique fort de plusieurs classes de graphes ainsi que son comportement en relation avec le produit cartésien. Enfin, dans le cinquième chapitre, nous quittons le domaine des graphes pour étudier l'identification de points dans le plan par des disques. En plus de couvrir chaque point d'un certain ensemble par des disques, nous souhaitons que l'ensemble des disques couvrants chaque point soit unique. Nous donnons des résultats dans certains cas particuliers, des bornes dans le cas général et étudions la complexité du problème quand le rayon des disques est fixéThis PhD thesis concerns the problem of covering finite sets in a discrete structure. This very general issue allows numerous approaches and we study some of them. The first chapter introduces the notions that are essentials to the understanding of this thesis and makes a brief state of the art on some covering problems, including the domination problem. The second chapter addresses the power dominating problem, a variation of the dominating problem with a propagation process. We study this problem on triangular grids and square grids of dimension 3. In the third chapter, we come back to the classical domination but in the context of a game, with the Maker-Breaker domination game. We study the complexity of the problem of deciding which player has a winning strategy and the minimum duration of a game if both players play perfectly. We also derive this problem for total domination and for an Avoider-Enforcer version. The fourth chapter is about the strong geodetic number: a problem with the distinctive characteristic that the covering is made by shortest paths in the graph. We study the strong geodetic number of several graph classes and its behaviour for the Cartesian product. Lastly, in the fifth chapter, we leave the realm of graphs to study the identification of points using disks. More than just covering every point of a certain set, the subset of disks covering each point must be unique to that point. We give results on particular configurations, bounds on the general case and we study the complexity of the problem when the radius of the disks is fixe