Este trabalho é sobre as geométrias finitas com 3 ou 4 pontos na cada reta e os loops e qiasigrupos relacionados. Em caso de 3 pontos na cada reta descrevemos o loop de Steiner correspondente livre e calculamos o grupo de automorfismos em caso de 3 geradores livres. Além disso descrevemos os loopos de Steiner nilpotentes de clase dois e classificamos estes loopos com 3 geradores. Em caso de 4 pontos na cada reta construimos as geometrias novas atraves de expanção central de um análogo não comutativo do quasigrupo de Steiner. Temos fortes indícios que esta construção é universal em algum sentido. / This work is about finite geometries with 3 or 4 points on every line and related loops and quasigroups. In the case of 3 points on any line we describe the structure of free loops in the variety of corresponding Steiner loops and we calculate the group of automorphisms of free Steiner loop with three generators. We describe the structure of nilpotent class two Steiner loops and classifiy all such loops with three generators. In the case of 4 points on a line we constructe new series of such geometries as central extension of corresponding non-commutative Steiner quasigroups. We conjecture that those geometries are universal in some sense.

Esse trabalho trata um pouco da teoria de A-loops comutativos finitos. No primeiro captulo estudamos propriedades básicas de loops em geral e exi- bimos exemplos de loops não associativos. No captulo 2 falamos de A-loops em geral e mesmo sem assumirmos comutatividade obtivemos resultados importantes, um exemplo é que A-loop associa potências. Também determinamos quando um isótopo e K -holomorfo de um A-loop é um A-loop. No captulo 3, nossos únicos objetos de estudo foram os A-loops comutativos finitos. Vimos que tais estruturas têm proriedades muito interessantes, por exemplo, para um A-loop comutativo finito valem os teoremas de Lagrange, Cauchy. Também, um A-loop comutativo finito, Q, tem ordem potência de um primo p se e somente se todo elemento de Q tem ordem potência de p. Mais ainda, todo A-loop comutativo finito de ordem mpar é solúvel. No último captulo, apresentamos algumas maneira de se construir um A-loop. / In the first chapter we studied basic properties of general loops and we showed some examples of nonassociative loops. In chapter 2, we talked about general A-loops (without commutativity) and even that we obtained important results, for instance, that any A-loop is power-associative. We also determined when an isotope and a K -holomorph of an A-loop is an A-loop. In chapter 3 we dealt only with finite commutative A-loops. We saw that such structures have very interesting properties, for example, for a finite commutative A- loop, Lagrange, Cauchys theorems apply. Also a finite commutative A-loop, Q, has order a power of a prime p if and only if every element of Q has order a power of p. Moreover, finite commutative A-loops of odd order are solvable. In the last chapter we introduce some ways to construct a commutative A-loop

Cette thèse consiste en trois parties que j'ai faites pendant ces trois ans.La première partie va être constituée de l'étude de la distribution de la longueur de corde sur le plan hyperbolique. Nous montrons l'identité de Pleijel pour le plan hyperbolique. En utilisant cette identité, nous remontrons l'identité de formule de Crofton et l'inégalité isopérimétrique pour le plan hyperbolique, et puis nous calculons la distribution de la longueur de corde associée à un triangle idéal et celle associée à un quadrilatère idéal. Ensuit, nous montons les résultats analogues pour les surfaces riemannienne simplement connexes avec la courbure constante. La seconde partie va contribuer aux études de la métrique de pression sur l'espace de Teichmüller d'un tore privé d'un disque. En étudiant la dégénération du tore quand la longueur du bord va à l'infini, nous trouvons la relation de cette métrique avec la métrique de pression sur l'espace modulaires des graphes métriques. Nous montrons ensuite que la fonction de l'entropie n'est pas constante sur les feuilles symplectique de l'espace Teichmüller d'une surface à bord.Finalement, la troisième partie concerne la quantification de l'espace de Teichmüller d'une surface avec les piqûres. nous montrons. Dans ce chapitre, nous étudions l'extension centrale du groupe modulaire via la quantification de Chekhov-Fock et calculons sa classe de cohomologie qui est 12 fois la classe de Meyer plus les classes d'Euler associées aux piqûres. / This thesis consists of three parts corresponding to the three subjects that I have studied during the last three years.The first part contains the study of the chord length distribution associated to a compact (or non-compact) domain in the hyperbolic plane. We prove the hyperbolic Pleijel identity. By using this identity, we find new approaches to the Crofton's formula and the isoperimetric inequality, and then compute the chord length distribution associated to an ideal triangle and that associated to an ideal quadrilateral. Then we prove the analogue results for the simply connected Riemannian surface with constant curvature.The second part of this thesis (Chapter 5) consists of the study of the pressuremetric on the Teichmüller space of one-holed torus. By studying the degeneration of the torus when the boundary length goes to infinity, we find the relation of this metric to the pressure metric on the moduli space of metric graphs. Then we study the entropy function and prove that it is not constant on the symplectic leaf of the Teichmüller space of a bordered surface.Finally, the third part concerns the quantization of the Teichmüller space of a punctured surface. In this chapter, we study the central extension of the mapping class group coming from the quantization and compute its cohomology class which is 12 times the Meyer class plus the Euler classes associated to punctures.