Neste trabalho mostramos que hipersuperfícies completas da esfera Euclidiana S^4, com curvatura média constante e curvatura de Gauss-Kronecker nula são mínimas, sempre que o quadrado da norma da segunda forma fundamental for limitado superiormente. Além disso apresentamos uma descrisão local das hipersuperfícies mínimas e completas em S^5 com curvatura de Gauss- Kronecker nula e algumas hipóteses adicionais sobre as funções simétricas das curvaturas principais. / In this work we show that a complete hipersurface of the unitary sphere S^4, with constant mean curvature and zero Gauss-Kronecker curvature must be minimal, if the squared norm of the second fundamental form is bounded from above. Also, we present a local description for complete minimal hipersurfaces in S^5 with zero Gauss-Kronecker curvature, and some restrictions for the symmetric functions of the principal curvatures.

Neste trabalho mostramos que hipersuperfícies completas da esfera Euclidiana S^4, com curvatura média constante e curvatura de Gauss-Kronecker nula são mínimas, sempre que o quadrado da norma da segunda forma fundamental for limitado superiormente. Além disso apresentamos uma descrisão local das hipersuperfícies mínimas e completas em S^5 com curvatura de Gauss- Kronecker nula e algumas hipóteses adicionais sobre as funções simétricas das curvaturas principais. / In this work we show that a complete hipersurface of the unitary sphere S^4, with constant mean curvature and zero Gauss-Kronecker curvature must be minimal, if the squared norm of the second fundamental form is bounded from above. Also, we present a local description for complete minimal hipersurfaces in S^5 with zero Gauss-Kronecker curvature, and some restrictions for the symmetric functions of the principal curvatures.

Neste trabalho, classificamos localmente as hipersuperfcies dos espaços produto S n × R e H n × R, n 6 = 3, com g curvaturas principais constantes e distintas, g {1, 2, 3}. Verifi- camos que tais hipersuperfcies são isoparamétricas de Q nc × R. Além disso, encontramos uma condição necessária e suficiente para que uma hipersuperfcie isoparamétrica de Q nc × R que possui fibrado normal plano, quando observada como uma subvariedade de codimensão dois de R n+2 contendo S n × R e de L n+2 contendo H n × R, tenha curvaturas principais constantes. / In this work, we classify locally the hypersurfaces in product spaces S n × R and H n × R, n 6 = 3, with g distinct constant principal curvatures, g {1, 2, 3}. We verify that such hy- persurfaces are isoparametric in Q nc × R. Furthermore, we find a necessary and sufficient condition for an isoparametric hypersurface in Q nc × R with flat normal bundle, when re- garded as a submanifold with codimension two of the flat spaces R n+2 containing S n × R and L n+2 containing H n × R, having constant principal curvatures.

Cette thèse s'intéresse à la factorisation des fonctions zêta des hypersurfaces de Dwork. Candelas, de la Ossa et Rodriguez-Villegas ont mis en évidence, dans le cas de la quintique, un facteur provenant de la symétrie miroir et deux facteurs provenant de courbes de type hypergéométrique. Wan a établit le lien avec la symétrie miroir dans le cas général, mais les facteurs complémentaires n'ont pas été étudiés avec le même niveau de détail que dans le cas de la quintique, et c'est sur eux que se concentre cette thèse. Après un premier chapitre de rappels sur les hypersurfaces de Dwork, on détermine, dans le chapitre 2, une factorisation explicite des fonctions zêta en terme de facteurs provenant d'hypersurfaces de type hypergéométrique. Dans le chapitre 3, on déduit une factorisation à partir d'une décomposition isotypique de la cohomologie des hypersurfaces de Dwork. Finalement, dans le chapitre 4, on relie les deux factorisations précédentes.

Das erste Thema dieser Dissertation ist der Defekt projektiver Hyperflächen. Es scheint, dass Hyperflächen mit Defekt einen verhältnismäßig großen singulären Ort besitzen. Diese Aussage wird im ersten Kapitel der Dissertation präzisiert und für Hyperflächen mit beliebigen isolierten Singularitäten über einem Körper der Charakteristik null, sowie für gewisse Klassen von Hyperflächen in positiver Charakteristik bewiesen. Darüber hinaus lässt sich die Dichte von Hyperflächen ohne Defekt über einem endlichen Körper abschätzen. Schließlich wird gezeigt, dass eine nicht-faktorielle Hyperfläche der Dimension drei mit isolierten Singularitäten stets Defekt besitzt. Das zweite Kapitel der Dissertation behandelt Bertini-Sätze über endlichen Körpern, aufbauend auf Poonens Formel für die Dichte glatter Hyperflächenschnitte in einer glatten Umgebungsvarietät. Diese wird auf quasiglatte Hyperflächen in simpliziellen torischen Varietäten verallgemeinert. Die Hauptanwendung ist zu zeigen, dass Hyperflächen mit einem in Relation zum Grad großen singulären Ort die Dichte null haben. Weiterhin enthält das Kapitel einen Bertini-Irreduzibilitätssatz, der auf einer Arbeit von Charles und Poonen beruht. Im dritten Kapitel werden ebenfalls Dichten über endlichen Körpern untersucht. Zunächst werden gewisse Faserungen über glatten projektiven Basisvarietäten in einem gewichteten projektiven Raum betrachtet. Das erste Resultat ist ein Bertini-Satz für glatte Faserungen, der Poonens Formel über glatte Hyperflächen impliziert. Der letzte Abschnitt behandelt elliptische Kurven über einem Funktionskörper einer Varietät der Dimension mindestens zwei. Die zuvor entwickelten Techniken ermöglichen es, eine untere Schranke für die Dichte solcher Kurven mit Mordell-Weil-Rang null anzugeben. Dies verbessert ein Ergebnis von Kloosterman. / The first topic of this dissertation is the defect of projective hypersurfaces. It is indicated that hypersurfaces with defect have a rather large singular locus. In the first chapter of this thesis, this will be made precise and proven for hypersurfaces with arbitrary isolated singularities over a field of characteristic zero, and for certain classes of hypersurfaces in positive characteristic. Moreover, over a finite field, an estimate on the density of hypersurfaces without defect is given. Finally, it is shown that a non-factorial threefold hypersurface with isolated singularities always has defect. The second chapter of this dissertation deals with Bertini theorems over finite fields building upon Poonen’s formula for the density of smooth hypersurface sections in a smooth ambient variety. This will be extended to quasismooth hypersurfaces in simplicial toric varieties. The main application is to show that hypersurfaces admitting a large singular locus compared to their degree have density zero. Furthermore, the chapter contains a Bertini irreducibility theorem for simplicial toric varieties generalizing work of Charles and Poonen. The third chapter continues with density questions over finite fields. In the beginning, certain fibrations over smooth projective bases living in a weighted projective space are considered. The first result is a Bertini-type theorem for smooth fibrations, giving back Poonen’s formula on smooth hypersurfaces. The final section deals with elliptic curves over a function field of a variety of dimension at least two. The techniques developed in the first two sections allow to produce a lower bound on the density of such curves with Mordell-Weil rank zero, improving an estimate of Kloosterman.

Dans cette thèse, nous interprétons géométriquement la torsion de l'algèbre symétrique d'un faisceau d'idéaux I_Z d'un schéma Z défini par n+1 équations dans une variété n-dimensionnelle. Ceci revient à étudier la géométrie de la projectivisation de I_Z. Les applications de ce point de vue concernent en particulier le domaine des transformations birationnelles de l'espace projectif de dimension 3 au sujet duquel nous construisons des transformations birationnelles explicites qui ont le même degré algébrique que leur inverse, le domaine des courbes libres et presque-libres au sujet duquel nous généralisons une caractérisation des courbes libres en étendant les notions de nombre de Milnor et de nombre de Tjurina. Nous abordons aussi le sujet des hypersurfaces homaloides, notre motivation initiale, au sujet duquel nous exhibons en particulier une courbe homaloide de degré 5 en caractéristique 3. La dernière application concerne le calcul de l'inverse d'une transformation birationnelle. / In this thesis, we interpret geometrically the torsion of the symmetric algebra of the ideal sheaf I_Z of a scheme Z defined by n+1 equations in an n-dimensional variety. This is equivalent to study the geometry of the projectivization of I_Z. The applications of this point of view concern, in particular, the topic of birational maps of the projective space of dimension 3 for which we construct explicit birational maps that have the same algebraic degree as their inverse, free and nearly-free curves for which we generalise a characterization of free curves by extending the notion of Milnor and Tjurina numbers. We tackle also the topic of homaloidal hypersurfaces, our original motivation, for which we produce in particular a homaloidal curve of degree 5 in characteristic 3. The last application concerns the computation of the inverse of a birational map.

Une variété algébrique est dite unirationnelle si elle est dominée par un espace projectif ; elle est dite séparablement unirationnelle si on peut prendre le morphisme précédent séparable. Cette dernière propriété n'a d'intérêt qu'en caractéristique positive. En reprenant la démonstration de Paranjape et Srinivas de l'unirationalité des hypersurfaces de degré très petit devant la dimension, nous remarquons qu'elle montre en fait l'unirationalité séparable. Nous nous intéressons aussi à la séparabilité des morphismes fournis par différentes constructions classiques de l'unirationalité des hypersurfaces cubiques.<br /><br />Dans la troisième partie, nous étudions la connexité rationnelle séparable : une variété projective lisse X sur un corps algébriquement clos est dite séparablement rationnellement connexe s'il existe une courbe rationnelle très libre (c'est-à-dire à fibré normal ample) sur X. Nous testons sur les hypersurfaces de Fermat de dimension N-1 et de degré q+1, où q est une puissance de la caractéristique du corps de base, la conjecture que toutes les hypersurfaces lisses de dimension N-1 et de degré plus petit que N sont séparablement rationnellement connexes. Nous montrons que pour N plus grand que 2q-1, l'hypersurface de Fermat de degré q+1 contient une courbe rationnelle très libre définie sur le sous-corps premier ; elle est donc séparablement rationnellement connexe.

Submitted by Aelson Maciera (aelsoncm@terra.com.br) on 2017-05-31T16:42:01Z No. of bitstreams: 1 TeseCGRF.pdf: 1149604 bytes, checksum: 8b0a42d65883e0af42693ac90b36059a (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-05-31T18:41:10Z (GMT) No. of bitstreams: 1 TeseCGRF.pdf: 1149604 bytes, checksum: 8b0a42d65883e0af42693ac90b36059a (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-05-31T18:41:18Z (GMT) No. of bitstreams: 1 TeseCGRF.pdf: 1149604 bytes, checksum: 8b0a42d65883e0af42693ac90b36059a (MD5) / Made available in DSpace on 2017-05-31T19:42:46Z (GMT). No. of bitstreams: 1 TeseCGRF.pdf: 1149604 bytes, checksum: 8b0a42d65883e0af42693ac90b36059a (MD5) Previous issue date: 2016-11-10 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / In this work we study conformally flat hypersurfaces f: M3 ^ Q4(c) with three distinct principal curvatures in a space form with constant sectional curvature c, under the assumption that either its mean curvature H or its scalar curvature S is constant. In case H is constant, first we extend to any c G R a theorem due to Defever when c = 0 and show that there is no such hypersurface if H = 0. Our main results are for the minimal case H = 0. If c = 0, we prove that f (M3) is an open subset of a generalized cone over a Clifford torus in an umbilical hypersurface Q4(c) C Q4(c), c > 0, with c > c if c > 0. For c = 0, we show that, besides the cone over the Clifford torus in S3 C R4, there exists precisely a one-parameter family of (congruence classes of) minimal isometric immersions f: M3 ^ R4 with three distinct principal curvatures of simply-connected conformally flat Riemannian manifolds. Assuming S to be constant, we only study the case c = 0. We prove that f (M3) is an open subset of a cylinder over a surface of nonzero constant Gauss curvature in R3. / Nesta tese estudamos hipersuperfícies conformemente euclidianas f : M3 ^ Q4(c), com três curvaturas principais distintas e curvatura média H ou curvatura escalar S constante, em formas espaciais com curvatura seccional c. No caso em que a curvatura média H é constante, inicialmente estendemos para c arbitrário um resultado provado por Defever [10] quando c =0 e mostramos que uma tal hipersuperfície não existe se H = 0. Nossos principais resultados são para o caso mínimo H = 0. Se c = 0, mostramos que f (M3) é um subconjunto aberto de um cone generalizado sobre um toro de Clifford em uma hipersuperfície umbílica Q3(c) C Q4(c), c > 0, com c > c se c > 0. Para c = 0, mostramos que, além do cone sobre o toro de Clifford em S3 C R4, existe precisamente uma família a 1-parâmetro de hipersuperfícies conformemente euclidianas com três curvaturas principais distintas duas a duas não congruentes, sendo o cone sobre o toro de Clifford o elemento singular da família. No caso em que a curvatura escalar é constante, estudamos apenas o caso c = 0. Mostramos, nesse caso, que f (M3) é um subconjunto aberto de um cilindro sobre uma superfície de curvatura Gaussiana constante do espaço euclidiano R3.

Darbas skirtas hiperbolinio tipo I rūšies beveik kontaktinėms metrinėms struktūroms, arba -struktūroms, egzistuojančioms hiperbolinio tipo A-erdvės hiperpaviršiuose. / Working for the hyperbolic type of class I almost contact metric structures existing in the hypersurfaces of hyperbolic type A-space.

[pt] Neste trabalho estudamos hipersuperfícies equivariantes mínimas ou com curvatura média constante imersas em S(n) e H(n). Tais hipersuperfícies são construídas a partir de uma curva em S(2) e em H(2) respectivamente, chamada de curva geratriz. A equação da curvatura média constante reduz-se a um sistema de EDO sobre a curva geratriz, e graças à simetria do problema, podemos eliminar uma variável desse sistema. O sistema simplificado, por sua vez, admite uma integral primeira. No caso esférico, encontramos condições para obter curvas soluções fechadas, produzindo assim exemplos de hipersuperfícies compactas mínimas ou com curvatura média constante em S(n). Discutimos também a questão do mergulho dessas hipersuperfícies. No caso hiperbólico, nos limitamos ao caso das hipersuperfícies mínimas; observamos que as curvas soluções não são fechadas e tratamos da questão do mergulho. / [en] In this work we study equivariant hypersurfaces in S(n) and H(n) which are minimal or have constant mean curvature. These hypersurfaces are described via a curve in S(2) and H(2) respectively, called the generating curve. In the equivariant case, the constant mean curvature equation reduces to an ODE on the generating curve, which can be reduced by one variable using the symmetry of the problem. It then turns out that this reduced system admits a first integral. In the spherical case, we find conditions insuring closedness of the integral curves, and we deduce the existence of compact hypersurfaces which are minimal or have constant mean curvature. We also discuss the question of embeddedness of these hypersurfaces. In the hyperbolic case, we limit ourselves to the minimal case. We observe that the curves are no longer closed and again we discuss embededdness.