### Thesis of Marc Heinrich

**Subject:**

**Defense date:**09/07/2019

**Advisor:**Eric Duchene

**Summary:**

This thesis explores problems related to games. The games that we consider in this study are games for which there is no hidden information: all the players have access to all the information related to the game; there is also no randomness and everything is deterministic. A few well-known board games (such as chess or go) fall in this category and are representative of the kinds of games that we consider here. Central to the study of these games is the notion of strategy, which roughly speaking, is a way of playing which ensures a certain objective. The main question of interest, when both playing and studying a game, is the problem of finding the 'best' strategy, which secures the victory for the player following it.

In this thesis, we consider both one-player games, also called combinatorial puzzles, and two-player games. Examples of combinatorial puzzles include Rubik's cube, Rush-Hour, Sokoban, the 15 puzzle, or peg solitaire. Recently, some types of one-player games in particular have received a strong regain of interest as part of the larger area of reconfiguration problems. The puzzles we described above can all be described in the following way: there is a set of configurations, which represents all the possible states of the game; and the player is allowed to transform a configuration using a prescribed set of moves. Starting from an initial configuration, the goal is to reach a target configuration by a succession of valid moves. Reconfiguration extends this definition to any search problem: the set of configuration becomes the set of solutions to an instance of a given problem, and we ask whether we can transform one given solution to another using only a prescribed set of moves. Hence, in addition to the combinatorial puzzles, reconfiguration problems also include many `games' which are not played by humans anymore but are instead mathematical problems sharing a lot of similarities with combinatorial puzzles. The study of reconfiguration problems has been driven by many different motivations. It has algorithmic applications: it can be seen as a way to adapt a current solution already in place to reach a new one by only making small local changes. It is also connected to other problems such as random sampling, approximate counting or problems coming from statistical physics. It can also be used as a tool for understanding the performance of some heuristic algorithms based on local modifications of solutions such as local search.

Two-player games, which are also called combinatorial games, have been studied since the beginning of the twentieth century, with the works of Bouton which were continued with the development of a nice theory by Berlekamp, Conway, and Guy, unifying a certain number of classical games. We focus in this study on perfect strategies (i.e., players always choosing the best possible move), and try to characterize who wins under perfect play for various games. This approach is at the heart of what is called Combinatorial Game Theory. Most of the research in this area is focused on `math games' which are games invented by mathematicians, often with simple rules and almost never played by humans. The main motivation for studying these games comes from the nice, and sometimes unexpected connections these games have with other areas of mathematics, such as for example number theory, automatons, or dynamical systems.

In this thesis, we study one- and two-player games. The questions we consider are often related to complexity. Complexity theory consists in trying to classify problems depending on their hardness. By hardness we mean to quantify how much time it would take for a computer to solve the problem. An other aspect of this research consists in investigating structural properties that these games can satisfy. Finally, one of our main tools is the notion of graph, and we use in particular methods and techniques from graph theory to answer the different questions we just mentioned.

**Jury:**

Celina De Figueiredo | Professeur(e) | Université fénérale de Rio de Janeiro | Rapporteur(e) |

Claire Mathieu | Directeur(trice) de recherche | Université Paris 7 | Rapporteur(e) |

Guerin-Lassous Isabelle | Professeur(e) | Université Lyon 1 | Président(e) |

Jan Van den Heuvel | Professeur(e) | École d’économie et de science politique de Londres | Examinateur(trice) |

Éric Duchêne | Maître de conférence | Université Lyon 1 | Directeur(trice) de thèse |

Sylvain Gravier | Directeur(trice) de recherche | Université Grenoble Alpes | Co-directeur (trice) |

Nicolas Bousquet | Chargé(e) de Recherche | Université Grenoble Alpes | Co-encadrant(e) |