A conjectura de Makar-Limanov arma que se um anel de divisão D e finitamente gerado e de dimensão infinita sobre seu centro k, então D contém uma k-subálgebra livre de posto 2. Neste trabalho, investigaremos a existência de tais estruturas no anel de divisão de frações do anel de polinômios skew L[t; \'\\sigma\' ], onde t é uma variável e \'\\sigma\' é um k-automorfismo de L. Mais especificamente, assumindo o que chamamos de Hipótese do Delta 3.3.1, provaremos esse resultado para L / k uma extensão de corpos, mesmo quando L não é finitamente gerado sobre k. Finalmente, provaremos a Hipótese do Delta e a conjectura, quando L é o corpo de funções de uma variedade abeliana ou o corpo de funções do espaço projetivo n-dimensional / Makar-Limanov\'s conjecture states that if a division ring D is finitely generated and infinite dimensional over its center k then D contains a free k-subalgebra of rank 2. In this work, we will investigate the existence of such structures in the division ring of fractions of the skew polynomial ring L[t; \'\\sigma\' ], where t is a variable and \'\\sigma\' is an k-automorphism of L. More specifically, assuming what we called Delta\'s Hipothesis 3.3.1, we prove this result for L / k a field extension, even when L isn\'t finitely generated over k. Finally, we prove Delta\'s Hipothesis and the conjecture when either L is the function field of an abelian variety or the function field of the n-dimensional projective space

A conjectura de Makar-Limanov arma que se um anel de divisão D e finitamente gerado e de dimensão infinita sobre seu centro k, então D contém uma k-subálgebra livre de posto 2. Neste trabalho, investigaremos a existência de tais estruturas no anel de divisão de frações do anel de polinômios skew L[t; \'\\sigma\' ], onde t é uma variável e \'\\sigma\' é um k-automorfismo de L. Mais especificamente, assumindo o que chamamos de Hipótese do Delta 3.3.1, provaremos esse resultado para L / k uma extensão de corpos, mesmo quando L não é finitamente gerado sobre k. Finalmente, provaremos a Hipótese do Delta e a conjectura, quando L é o corpo de funções de uma variedade abeliana ou o corpo de funções do espaço projetivo n-dimensional / Makar-Limanov\'s conjecture states that if a division ring D is finitely generated and infinite dimensional over its center k then D contains a free k-subalgebra of rank 2. In this work, we will investigate the existence of such structures in the division ring of fractions of the skew polynomial ring L[t; \'\\sigma\' ], where t is a variable and \'\\sigma\' is an k-automorphism of L. More specifically, assuming what we called Delta\'s Hipothesis 3.3.1, we prove this result for L / k a field extension, even when L isn\'t finitely generated over k. Finally, we prove Delta\'s Hipothesis and the conjecture when either L is the function field of an abelian variety or the function field of the n-dimensional projective space

In spite of its 150 years history, the problem of classifying all finite-dimensional division algebras over a field k is still unsolved whenever k is not algebraically closed. The present thesis concerns some different aspects of this problem, and the related problems of classifying all composition and absolute valued algebras. A tripartition of the class of all fields is given, based on the dimensions in which division algebras over a field exist. Moreover, all finite-dimensional flexible real division algebras are classified. This class includes in particular all finite-dimensional commutative real division algebras, of which two different classifications, along different lines, are presented. It is shown that every vector product algebra has dimension zero, one, three or seven, and that its isomorphism type is determined by its adherent quadratic form. This yields a new and elementary proof for the corresponding, classical result for unital composition algebras. A rotation in a Euclidean space is an orthogonal map that locally acts as a plane rotation with a fixed angle. All pairs of rotations in finite-dimensional Euclidean spaces are classified up to orthogonal similarity. A description of all composition algebras having an LR-bijective idempotent is given. On the basis of this description, all absolute valued algebras having a one-sided unity or a non-zero central idempotent are classified.

For a given field F we seek all division algebras over F up to isomorphism. This question was first investigated for division algebras of finite dimension over F by Richard Brauer. We discuss the construction of the Brauer group and some examples. Crossed products and PI algebras are then introduced with a focus on Amitsur's non-crossed product algebra. Finally, we look at some modern results of Bell on the Gelfand-Kirillov dimension of finitely generated algebras over F and the classification of their division subalgebras.

For a given field F we seek all division algebras over F up to isomorphism. This question was first investigated for division algebras of finite dimension over F by Richard Brauer. We discuss the construction of the Brauer group and some examples. Crossed products and PI algebras are then introduced with a focus on Amitsur's non-crossed product algebra. Finally, we look at some modern results of Bell on the Gelfand-Kirillov dimension of finitely generated algebras over F and the classification of their division subalgebras.

We first introduce the octonions as an eight dimensional vector space over a field of characteristic zero with a multiplication defined using a table. We also show that the multiplication rules for octonions can be derived from a special graph with seven vertices call the Fano Plane. Next we explain the Cayley-Dickson construction, which exhibits the octonions as the set of ordered pairs of quaternions. This approach parallels the realization of the complex numbers as ordered pairs of real numbers. The rest of the thesis is devoted to following a paper by N. Jacobson written in 1939 entitled "Cayley Numbers and Normal Simple Lie Algebras of Type G". We prove that the algebra of derivations on the octonions is a Lie algebra of type G_2. The proof proceeds by showing the set of derivations on the octonions is a Lie algebra, has dimension fourteen, and is semisimple. Next, we complexify the algebra of derivations on the octonions and show the complexification is simple. This suffices to show the complexification of the algebra of derivations is isomorphic to g_2 since g_2 is the only semisimple complex Lie algebra of dimension fourteen. Finally, we conclude the algebra of derivations on the octonions is a simple Lie algebra of type G_2. / Master of Science

In an attempt to prove Amitsur's Conjecture for cyclic subgroups of the Brauer group, we look at rational embeddings of the Severi Brauer variety of an algebra into its norm hypersurface. We enlarge the collection of such embeddings, and generalize them to embeddings of generalized Severi Brauer varieties into determinantal varieties. / text

A composition algebra is a non-zero algebra endowed with a strictly non-degenerate, multiplicative quadratic form. Finite-dimensional composition algebras exist only in dimension 1, 2, 4 and 8 and are in general not associative or unital. Over the real numbers, such algebras are division algebras if and only if they are absolute valued, i.e. equipped with a multiplicative norm. The problem of classifying all absolute valued algebras and, more generally, all composition algebras of finite dimension remains unsolved. In dimension eight, this is related to the triality phenomenon. We approach this problem using a categorical language and tools from representation theory and the theory of algebraic groups. We begin by considering the category of absolute valued algebras of dimension at most four. In Paper I we determine the morphisms of this category completely, and describe their irreducibility and behaviour under the actions of the automorphism groups of the algebras. We then consider the category of eight-dimensional absolute valued algebras, for which we provide a description in Paper II in terms of a group action involving triality. Then we establish general criteria for subcategories of group action groupoids to be full, and applying this to the present setting, we obtain hitherto unstudied subcategories determined by reflections. The reflection approach is further systematized in Paper III, where we obtain a coproduct decomposition of the category of finite-dimensional absolute valued algebras into blocks, for several of which the classification problem does not involve triality. We study these in detail, reducing the problem to that of certain group actions, which we express geometrically. In Paper IV, we use representation theory of Lie algebras to completely classify all finite-dimensional absolute valued algebras having a non-abelian derivation algebra. Introducing the notion of quasi-descriptions, we reduce the problem to the study of actions of rotation groups on products of spheres. We conclude by considering composition algebras over arbitrary fields of characteristic not two in Paper V. We establish an equivalence of categories between the category of eight-dimensional composition algebras with a given quadratic form and a groupoid arising from a group action on certain pairs of outer automorphisms of affine group schemes

Cette thèse concerne les codes utilisés pour les télécommunications sans-fil multi-antennes. Les résultats portent notamment sur des constructions explicites ainsi que sur des bornes numériques et théoriques pour les cardinaux de ces codes. Le premier chapitre introduit brièvement les différents contextes multi-antennes et les modélisations qui leur sont associées. Les chapitres 2,3 et 4 traitent respectivement des codes dans les espaces grassmanniens, des codes dans les matrices unitaires et des codes dans les algèbres à division. / This thesis deals with codes used for multi-antennas wireless telecommunications. The results concern explicit constructions and bounds on the cardinalities of such codes (analytical and numerical bounds) . The first chapter introduce various modelisations of the multi-antennas wireless system and the related mathematical problems. Chapters 2,3,4 deal respectively with codes in Grassmannian spaces, code in unitary matrices and code in division algebras.

Les algèbres centrales simples ont de nombreuses applications en théorie des nombres, mais leur algorithmique est encore peu développée. Dans cette thèse, j’apporte une contribution dans deux directions. Premièrement, je présente des algorithmes de complexité prouvée, ce qui est nouveau dans la plupart des cas. D’autre part, je développe des algorithmes heuristiques mais très efficaces dans la pratique pour les exemples qui nous intéressent le plus, comme en témoignent mes implantations. Les algorithmes sont à la fois plus rapides et plus généraux que les algorithmes existants. Plus spécifiquement, je m’intéresse aux problèmes suivants : calcul du groupe des unités d’un ordre et problème de l’idéal principal. Je commence par étudier le diamètre du domaine fondamental de certains groupes d’unités grâce à la théorie des représentations. Je décris ensuite un algorithme prouvé pour calculer des générateurs et une présentation du groupe des unités d’un ordre maximal dans une algèbre à division, puis un algorithme efficace qui calcule également un domaine fondamental dans le cas où le groupe des unités est un groupe kleinéen. Je donne en outre un algorithme de complexité prouvée qui détermine si un idéal d’un tel ordre est principal, et qui en calcule un générateur le cas échéant, puis je décris un algorithme heuristiquement sous-exponentiel pour résoudre le même problème dans le cas d’une algèbre de quaternions indéfinie. / Central simple algebras have many applications in number theory, but their algorithmic theory is not yet fully developed. I present algorithms to compute effectively with central simple algebras that are both faster and more general than existing ones. Some of these algorithms have proven complexity estimates, a new contribution in this area; others rely on heuristic assumptions but perform very efficiently in practice.Precisely, I consider the following problems: computation of the unit group of an order and principal ideal problem. I start by studying the diameter of fundamental domains of some unit groups using representation theory. Then I describe an algorithm with proved complexity for computing generators and a presentation of the unit group of a maximal order in a division algebra, and then an efficient algorithm that also computes a fundamental domain in the case where the unit group is a Kleinian group. Similarly, I present an algorithm with proved complexity that decides whether an ideal of such an order is principal and that computes a generator when it is. Then I describe a heuristically subexponential algorithm that solves the same problem in indefinite quaternion algebras.