Let K=Q(theta) be the algebraic number field formed by adjoining theta to the rationals where theta is a real root of an irreducible monic cubic polynomial f(x) in Z[x]. If theta is not the cube root of a rational integer, we call the field K a non-pure cubic field, and if K doesn't contain the conjugates of theta, we call K a non-normal cubic field. A method described by Martinet and Payan allows us to construct such fields from elements of a quadratic field. In this work, we examine such non-normal, non-pure cubic fields and their normal closures, using algorithms in Mathematica to compute various invariants of these fields. In addition, we prove general results relating the ranks of the ideal class groups of the rings of integers of these cubic fields to those of their normal closures. / Ph. D.

If we adjoin the cube root of a cube free rational integer <i>m</i> to the rational numbers we construct a cubic field. If we adjoin the cube roots of distinct cube free rational integers <i>m</i> and <i>n</i> to the rational numbers we construct a bicubic field. The number theoretic invariants for the cubic fields and their normal closures are well known. Some work has been done on the units, classnumbers and other invariants of the bicubic fields and their normal closures by Parry but no method is available for calculating those invariants. This dissertation provides an algorithm for calculating the number theoretic invariants of the bicubic fields and their normal closure. Among these invariants are the discriminant, an integral basis, a set of fundamental units, the class number and the rank of the 3-class group. / Ph. D.

The thesis is concerned with the theory of binary quadratic forms with coefficients in the ring of algebraic integers of a number field. Under the assumption that the number field is of narrow class number one, there is developed a theory of composition of such quadratic forms. For a given discriminant, the composition is determined by a bijection between classes of quadratic forms and a so-called relative oriented class group (a group closely related to the class group). Furthermore, Bhargava cubes are generalized to cubes with entries from the ring of algebraic integers; by using the composition of quadratic forms, the composition of Bhargava cubes is proved in the generalized case. 1

Deligne and Howe described polarized abelian varieties over finite fields in terms of finitely generated free Z-modules satisfying a list of easy to state axioms. In this thesis we address the problem of developing an effective algorithm to compute isomorphism classes of (principally) polarized abelian varieties over a finite field, together with their automorphism groups. Performing such computations requires the knowledge of the ideal classes (both invertible and non-invertible) of certain orders in number fields. Hence we describe a method to compute the ideal class monoid of an order and we produce concrete computations in dimension 2, 3 and 4.

La recherche d’annulateurs du groupe des classes d’idéaux d’une extension abélienne de Q est un sujet classique et remonte à des travaux de Kummer et Stickelberger. La conjecture de Brumer-Stark porte sur les extensions abéliennes de corps de nombres et prédit qu’un élément de l’anneau de groupe du groupe de Galois, appelé élément de Brumer-Stickelberger, est un annulateur du groupe des classes de l’extension. De plus, elle stipule que les générateurs des idéaux principaux obtenus possèdent des propriétés bien particulières. Cette thèse est dédiée à la généralisation de cette conjecture aux extensions de corps de nombres galoisiennes mais non abéliennes. Dans un premier temps, nous nous focalisons sur l’étude de l’analogue non abélien de l’élément de Brumer, nécessaire à l’établissement d’une conjecture non abélienne. La seconde partie est consacrée à l’énoncé de la conjecture de Brumer-Stark non abélienne et à ses reformulations, ainsi qu’aux propriétés qu’elle vérifie. Nous nous intéressons notamment aux propriétés de changement d’extension. Nous étudions ensuite le cas spécifique des extensions dont le groupe de Galois possède un sous-groupe abélien H distingué d’indice premier. Sous la validité de la conjecture de Brumer-Stark associée à certaines extensions abéliennes, nous en déduisons deux résultats suivant la parité du cardinal de H : dans le cas impair, nous démontrons la conjecture de Brumer-Stark non abélienne, et dans le cas pair, nous établissons un résultat d’abélianité permettant d’obtenir, sous des hypothèses supplémentaires, la conjecture non abélienne. Enfin nous effectuons des vérifications numériques de la conjecture non abélienne permettant de démontrer cette conjecture dans les exemples testés. / Finding annihilators of the ideal class group of an abelian extension of Q is a classical subject which goes back to work of Kummer and Stickelberger. The Brumer-Stark conjecture deals with abelian extensions of number fields and predicts that a group ring element, called the Brumer-Stickelberger element, annihilates the ideal class group of the extension under consideration. Moreover it specifies that the generators thus obtained have special properties. The aim of this work is to generalize this conjecture to non-abelian Galois extensions. We first focus on the study of a non-abelian analogue of the Brumer element, necessary to establish a non-abelian generalization of the conjecture. The second part is devoted to the statement of our non-abelian conjecture, and the properties it satisfies. We are particularly interested in extension change properties. We then study the specific case of extensions whose Galois group has an abelian normal subgroup H of prime index. If the Brumer-Stark conjecture associated to certain abelian subextensions holds, we prove two results according to the parity of the cardinal of H : in the odd case, we get the non-abelian Brumer-Stark conjecture, and in the even case, we establish an abelianity result implying under additional hypotheses the proof of the non-abelian conjecture. Thanks to PARI-GP, we finally do some numerical verifications of the nonabelian conjecture, proving its validity in the tested examples.