Recently, it has become increasingly clear that boundaries play a significant role in the understanding of the non-perturbative phase of the dynamics of strings. In this thesis we propose to study the effects of boundaries in non-critical string theory. We thus analyse boundary conformal field theories on random surfaces using the conformal gauge approach of David, Distler and Kawai. The crucial point is the choice of boundary conditions on the Liouville field. We discuss the Weyl anomaly cancellation for Polyakov's non-critical open bosonic string with Neumann, Dirichlet and free boundary conditions. Dirichlet boundary conditions on the Liouville field imply that the metric is discontinuous as the boundary is approached. We consider the semi-classical limit and argue how it singles out the free boundary conditions for the Liouville held. We define the open string susceptibility, the anomalous gravitational scaling dimensions and a new Yang-Mills Feynman mass critical exponent. Finally, we consider an application to the theory of non-critical dual membranes. We show that the strength of the leading stringy non-perturbative effects is of the order e(^-o(1/βst)), a result that mimics those found in critical string theory and in matrix models. We show how this restricts the space of consistent theories. We also identify non-critical one dimensional D-instantons as dynamical objects which exchange closed string states and calculate the order of their size. The extension to the minimal c ≤ 1 boundary conformal models is also briefly discussed.

Σκοπός της παρούσας εργασίας είναι η ανάλυση προβλημάτων συνοριακών τιμών. Αρχικά αναφέρονται στοιχεία γραμμικής ανάλυσης και συγκεκριμένα εισάγεται η έννοια ενός τελεστή και τα είδη τελεστών που υπάρχουν, καθώς και η σημασία τους στη Φυσική. Επίσης, δίνεται ο ορισμός της διαφορικής εξίσωσης (Σ.Δ.Ε), ο ορισμός ενός προβλήματος αρχικών τιμών και ο ορισμός ενός προβλήματος συνοριακών τιμών. Έπειτα, αναλύεται η θεωρία Sturm-Liouville και περιγράφονται παραδείγματα συνοριακών τιμών τα οποία επιλύονται με αυτή. Ακόμη, μελετώνται οι συναρτήσεις Green και δίνονται παραδείγματα εφαρμογών τους. Στη συνέχεια εξάγεται η κυματική εξίσωση με τη βοήθεια του μοντέλου της ταλαντούμενης χορδής και επιλύεται με τη μέθοδο του χωρισμού των μεταβλητών για διάφορους τύπους αρχικών και συνοριακών τιμών. Κατόπιν, περιγράφονται μέθοδοι για την επίλυση προβλημάτων συνοριακών τιμών που συνδέονται με την εξίσωση της θερμότητας και μετά αναφέρονται εφαρμογές που προκύπτουν από την επίλυση προβλημάτων διάδοσης θερμότητας. Τέλος αναφέρεται η θεωρία Fredholm και η έννοια της κατανομής και δίνονται παραδείγματα λύσεων των διαφορικών εξισώσεων με την έννοια των κατανομών. Η θεωρία Fredholm είναι ιδιαίτερα σημαντική σε προβλήματα διαφορικών εξισώσεων που είναι μη ομογενή. / In the present project, the initial boundary value problems are analyzed. Firstly, elements of linear analysis are introduced. Particularly the concept of an operator and its types are introduced as well as the importance in the physics sector. Also, the definition of a differential equation and the initial boundary value problems are presented. Additionally, the theory of Sturm-Liouville and its example are described. Moreover, Green function and their applications are introduced. Furthermore, the wave equation was elicited with the basis of vibrating spring model and solved with the method of separating variables. Also with this method and by using Fourier series the heat equation was solved. Finally the theory of Fredholm and the concept of distribution are described. The theory of Fredholm is important in problems of not homogeneous differential equation problems.

In this paper we investigate (d + 1)-point D-optimal designs for d-th degree polynomial regression with weight function w(x) > 0 on the interval [a, b]. Suppose that w'(x)/w(x) is a rational function and the information of whether the optimal support contains the boundary points a and b is available. Then the problem of constructing (d + 1)-point D-optimal designs can be transformed into a differential equation problem leading us to a certain matrix with k auxiliary unknown constants. We characterize the weight functions corresponding to the cases when k= 0 and k= 1. Then, we can solve (d + 1)-point D-optimal designs directly from differential equation (k = 0) or via eigenvalue problems (k = 1). The numerical results show us an interesting relationship between optimal designs and ordered eigenvalues.

Le but de cette thèse est d'étudier lagravité quantique bidimensionnelle. Nousnous intéressons plus particulièrement auxapproches dans le continu. Ces dernièresreposent principalement sur l'action deLiouville qui décrit le couplage entre théorieconforme et gravité. Si cette action, bienconnue, est très bien comprise, la mesure del'intégrale fonctionnelle sur l'espace desmétriques pose plus de problèmes. Toutefois,sous l'hypothèse simplificatrice d'une mesurede champ libre, la dépendance en l'aire de lafonction de partition de la gravité quantiqueen présence de matière conforme a pu êtreétablie. Malgré l'hypothèse assez forte sur lamesure d'intégration, cette formule (diteKPZ), a été confirmée par des calculs issusde méthodes discrètes, et ce dans plusieurscas particuliers. Grâce à une nouvelle méthode derégularisation spectrale en espace courbe,cette mesure d'intégration a récemment puêtre proprement définie. Dans cette thèse,un calcul perturbatif de la fonction departition à aire fixée est mené, jusqu'à troisboucles, en considérant l'action de Liouvilleet des surfaces de Kähler de genrequelconque (qui coïncident avec l'ensembledes surfaces à deux dimensions). Desdivergences apparaissant dans les calculs, ilest nécessaire de renormaliser les actions.Cette renormalisation peut être interprétéecomme une renormalisation de la mesured'intégration. Nos résultats à deux bouclessont finis, indépendants de la régularisationet compatibles avec le résultat KPZ, maisdépendent d'un paramètre libre. L'étude àtrois boucles suggère que la théorie resterenormalisable aux ordres supérieurs maisdépend de nouveaux paramètres à chaqueordre. Ces résultats ont été généralisé dansle cas du tore au couplage à de la matièrenon-conforme. / Nowadays, two-dimensional quantumgravity can be studied in two differentapproaches, one involving discrete theories(triangulation, matrix model...), the othercontinuous ones, mainly based on the socalled Liouville action which universallydescribes the coupling of any conformal fieldtheory to gravity. While the Liouville action isrelatively well understood, the appropriatefunctional integral measure is however rathercomplicated. Nevertheless, a formula for thearea dependence of the quantum gravitypartition function in the presence of conformalmatter has been obtained, under thesimplifying assumption of a free-fieldmeasure. Notwithstanding its non-rigorousderivation, this formula, often referred to asthe KPZ formula, has since been verified inmany instances and has scored manysuccesses. Recent developments of efficient multiloopregularization methods on curved spacetimesopened the way for a precise and welldefinedperturbative computation of the fixedareapartition function in the Kählerformalism. In this work, a first-principlescomputation of the fixed-area partitionfunction in the Liouville theory is performed,up to three loops. Among other things, therole of the non-trivial quantum gravityintegration measure is highlighted.Renormalization is required and may beinterpreted as a renormalization of theintegration measure. This leads to a finite andregularization-independent result at two loops,that is more general than the KPZ result,although compatible. Finiteness andregularization-independence seem alsopossible at three loops. These results aregeneralized to the coupling to non-conformalmatter on the torus.

Cette thèse explore divers aspects des cartes aléatoires par l'étude de trois modèles. Dans un premier temps, nous examinons les propriétés d’une mesure définie sur l’ensemble des triangulations de Delaunay planaires comportant n sommets, qui est un modèle de cartes où les arêtes sont décorées par des angles. Nous montrons ainsi que la mesure est égale à la mesure de Weil-Petersson sur l’espace des modules des surfaces de Riemann planaires marquées. Sont aussi montrées deux propriétés de la mesures, premiers pas d'une étude de la limite continue de ce modèle. Dans un deuxième temps, nous définissons des fonctions de corrélations sur les graphes de Strebel planaires isopérimétriques à n faces, qui sont des cartes métriques trivalentes. Les périmètres des faces sont fixés. Nous recourons au théorème de Kontsevich pour calculer les fonctions de corrélations en termes de nombres d’intersection de classes de Chern sur l’espace des modules des surfaces de Riemann. Pour la fonction à une face marquée, la limite des grandes cartes est examinée via l’approximation du point-selle, pour différents régimes du périmètre de la face marquée, et nous déduisons le régime où le comportement de la fonction de corrélation n’est pas trivial. Les fonctions de corrélations peuvent être calculées de manière systématique par la récurrence topologique. Partant, nous calculons la courbe spectrale de notre modèle, ce qui nous permet de montrer qu’il existe une courbe spectrale critique. Nous déduisons de cette courbe critique que la limite continue des graphes de Strebel isopérimétriques est un modèle minimal de type (3,2), habillé par la théorie de Liouville. Cela correspond bien à la gravité pure. Enfin, nous abordons la question des symétries dans le modèle d’Ising sur cartes aléatoires. Certaines fonctions de corrélations de ce modèle comptent le nombre de cartes bicolores avec des faces marquées, les bords, ayant des conditions aux bords mixtes, calculées par récurrence à partir de la courbe spectrale du modèle. Nous prouvons ici que, pour des courbes spectrales génériques, les fonctions de corrélations des cartes à un bord mixte sont symétriques par rotation et par inversion du bord mixte. Nous décrivons ensuite les conséquences de telles symétries, suggérant une possible reformulation du modèle en termes de chaînes de spins. / This thesis explore several aspects of random maps through the study of three models. First, we examine the properties of a measure defined on the set of planar Delaunay triangulations with n vertices, a model in which the edges of the maps are decorated with angles. We show that the measure is the Weil-Petersson volume form on the moduli space of planar Riemann surfaces having n marked points. Two other properties, first steps toward the continuous limit study of the model, are also shown. Second, we define correlation functions on isoperimetric planar Strebel graphs with n faces, which are trivalent maps whose edges are decorated by positive lengths, and whose faces have a fixed perimeter. Kontsevich's theorem allows us to compute the correlation functions in terms of the intersection numbers of Chern classes of moduli space of Riemann surfaces. The continuous limit of the one-point function is computed in different regimes for the perimeter of the marked face via the saddle-point approximation. We identify the regime in which the behaviour of the one-point function is not trivial. The correlation functions can be computed in a systematic way by the Topological Recursion. To do so, we compute the spectral curve of the model, and show that there exists a critical spectral curve. We deduce from the latter that the continuous limit of isoperimetric Strebel graphs is a (3,2) minimal model dressed by Liouville theory: it corresponds to pure gravity. Last, we address the problem of symmetries in the Ising model on random maps. Some correlation functions of this model count the bi-colored maps with marked faces having mixed boundary conditions. They are computed via a recursive formula and the spectral curve of the model. We prove here that the correlation functions of maps with one mixed boundary, computed from the recursive relation with generic spectral curve, are invariant under rotation and inversion of the mixed boundary. We describe the consequences of such symmetries, suggesting a possible reformulation of the model in terms of spin chains.

In this work, we make a brief review of the Conformal Field Theory in two dimensions,in order to understand some basic definitions in the study of the Liouville Field Theory, which has many application in theoretical physics like string theory, general relativity and supersymmetric gauge field theories. In particular, we focus on the analytic continuation of the Liouville Field Theory, context in which an interesting relation with the Chern-Simons Theory arises as an extension of its well-known relation with the Wess-Zumino-Witten model. Thus, calculating correlation functions by using the complex solutions of the Liouville Theory will be crucial aim in this work in order to test the consistency of this analytic continuation. We will consider as an application the time-like version of the Liouville Theory, which has several applications in holographic quantum cosmology and in studying tachyon condensates. Finally, we calculate the three-point function for the Wess-Zumino-Witten model for the standard Kac-Moody level k > 2 and the particular case 0 < k < 2, the latter has an interpretation in time-dependent scenarios for string theory. Here we will find an analogue relation we find by comparing the correlation function of the time-like and space-like Liouville Field Theory. / Neste trabalho, nós fazemos uma breve revisão da Teoria de Campo Conforme em duas dimensões, a fim de entender algumas denições básicas do estudo da Teoria de Campo de Liouville, que tem muitas aplicações em física teórica como a teoria das cordas, a relatividade geral e teorias de campo de calibre supersimétricas. Em particular, vamos nos concentrar sobre a continuação analítica da Teoria de Campo de Liouville, contexto no qual uma interessante relação com a Teoria de Chern-Simons surge como uma extensão de sua relação conhecida com o modelo de Wess-Zumino-Witten. Assim, o cálculo das funções de correlação usando as soluções complexas da Teoria Liouville será o objectivo fundamental neste trabalho, a fim de testar a consistência da continuação analítica. Vamos considerar como uma aplicação a versão time-like da Teoria de Liouville, que tem várias aplicações em cosmologia quântica holográfica e no estudo de condensados de tachyon. Finalmente, calculamos a função de três pontos para o modelo de Wess-Zumino-Witten no nível de Kac-Moody k > 2 e o caso particular 0 < k < 2, este último tem uma interpretação em cenários dependentes do tempo para a teoria das cordas. Aqui nós vamos encontrar uma relação análoga ao que temos para a função de correlação do space-like e time-like na Teoria de Campo de Liouville.

In this work, we make a brief review of the Conformal Field Theory in two dimensions,in order to understand some basic definitions in the study of the Liouville Field Theory, which has many application in theoretical physics like string theory, general relativity and supersymmetric gauge field theories. In particular, we focus on the analytic continuation of the Liouville Field Theory, context in which an interesting relation with the Chern-Simons Theory arises as an extension of its well-known relation with the Wess-Zumino-Witten model. Thus, calculating correlation functions by using the complex solutions of the Liouville Theory will be crucial aim in this work in order to test the consistency of this analytic continuation. We will consider as an application the time-like version of the Liouville Theory, which has several applications in holographic quantum cosmology and in studying tachyon condensates. Finally, we calculate the three-point function for the Wess-Zumino-Witten model for the standard Kac-Moody level k > 2 and the particular case 0 < k < 2, the latter has an interpretation in time-dependent scenarios for string theory. Here we will find an analogue relation we find by comparing the correlation function of the time-like and space-like Liouville Field Theory. / Neste trabalho, nós fazemos uma breve revisão da Teoria de Campo Conforme em duas dimensões, a fim de entender algumas denições básicas do estudo da Teoria de Campo de Liouville, que tem muitas aplicações em física teórica como a teoria das cordas, a relatividade geral e teorias de campo de calibre supersimétricas. Em particular, vamos nos concentrar sobre a continuação analítica da Teoria de Campo de Liouville, contexto no qual uma interessante relação com a Teoria de Chern-Simons surge como uma extensão de sua relação conhecida com o modelo de Wess-Zumino-Witten. Assim, o cálculo das funções de correlação usando as soluções complexas da Teoria Liouville será o objectivo fundamental neste trabalho, a fim de testar a consistência da continuação analítica. Vamos considerar como uma aplicação a versão time-like da Teoria de Liouville, que tem várias aplicações em cosmologia quântica holográfica e no estudo de condensados de tachyon. Finalmente, calculamos a função de três pontos para o modelo de Wess-Zumino-Witten no nível de Kac-Moody k > 2 e o caso particular 0 < k < 2, este último tem uma interpretação em cenários dependentes do tempo para a teoria das cordas. Aqui nós vamos encontrar uma relação análoga ao que temos para a função de correlação do space-like e time-like na Teoria de Campo de Liouville.