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Zugl.: Göttingen, University, Diss., 2009.

A Kahler category axiomatizes the algebraic geometric theory of Kahler Differentials in an abstract categorical setting. To facilitate this, a Kahler category is equipped with an algebra modality, which endows each object in the image of a specified monad with an associative algebra structure; universal derivations are then required to exist naturally for each of these objects. Moreover, it can be demonstrated that for each T-algebra of said monad there is a natural associative algebra structure. In this paper I will show that under certain conditions on the Kahler category, the universal derivations for the algebras arising from T-algebras exist and arise via a coequalizer. Furthermore, this result is extended to provide an alternative construction for universal derivations for a more general class of algebras, including all algebras in a Kahler category. A prospective categorical formulation of the theory of noncommutative Kahler differentials is then given, and the above said results are shown to apply in this context. Finally, another class of algebras is constructed via a colimit, and the modules of differential forms for these algebras is computed.

A Kahler category axiomatizes the algebraic geometric theory of Kahler Differentials in an abstract categorical setting. To facilitate this, a Kahler category is equipped with an algebra modality, which endows each object in the image of a specified monad with an associative algebra structure; universal derivations are then required to exist naturally for each of these objects. Moreover, it can be demonstrated that for each T-algebra of said monad there is a natural associative algebra structure. In this paper I will show that under certain conditions on the Kahler category, the universal derivations for the algebras arising from T-algebras exist and arise via a coequalizer. Furthermore, this result is extended to provide an alternative construction for universal derivations for a more general class of algebras, including all algebras in a Kahler category. A prospective categorical formulation of the theory of noncommutative Kahler differentials is then given, and the above said results are shown to apply in this context. Finally, another class of algebras is constructed via a colimit, and the modules of differential forms for these algebras is computed.

Dans la première partie de la thèse, nous étudions le problème de l'équivalence quantique de modèles sigma reliés entre eux par la T-dualité non-abelienne. Nous prouvons que la renormalisabilité à une boucle de divers modèles initiaux implique la renormalisabilité à une boucle de leur partenaire dualisé, et qu'ils partagent les mêmes fonctions beta. Ceci est fait pour tous les modèles sigma principaux (G_L X G_R)/G_D, quelle que soit la brisure de G_R, ainsi que pour la large classe de métriques à quatre dimensions, inhomogènes et d'isométrie SU(2) X U(1). Pour l'exemple simple du modèle sigma T-dualisé SU(2), dont la non-renormalisabilité à deux boucles a été démontrée dans le schéma dimensionel minimal, nous prouvons qu'il est encore possible, à cet ordre, de définir une théorie quantique correcte en modifiant, à l'ordre \hbar, la métrique de l'espace cible de façon finie. Dans la seconde partie, nous construisons de façon explicite, grâce au super-espace harmonique et à l'approche du quotient quaternionique, une extension quaternion-Kähler de la métrique hyper-Kähler à deux centres la plus générale. Elle possède le groupe d'isométrie U(1) X U(1) et contient comme cas particuliers les extensions quaternion-Kähler des métriques de Taub-NUT et d'Eguchi-Hanson. Elle fait aussi apparaître un paramètre supplémentaire qui disparaît dans la limite hyper-Kähler.

Submitted by Maike Costa (maiksebas@gmail.com) on 2016-03-31T13:40:38Z No. of bitstreams: 1 arquivo total.pdf: 1581712 bytes, checksum: 55cfc2e330d11ed8545538014daa3873 (MD5) / Made available in DSpace on 2016-03-31T13:40:38Z (GMT). No. of bitstreams: 1 arquivo total.pdf: 1581712 bytes, checksum: 55cfc2e330d11ed8545538014daa3873 (MD5) Previous issue date: 2015-07-16 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this dissertation, we initially present an overview about the symmetric and the Rees algebras in the wide context of modules, and we consider particularly the special situation in which the given module possesses a linear presentation. In the sequel, the main goal is the study of such blowup algebras in the case where the module is the celebrated module of K ahler di erentials, the focus being given on the investigation of an interesting version of the long-standing Berger's Conjecture for the symmetric algebra, as well as on the study of fundamental properties such as: integrality, Cohen- Macaulayness and normality; these properties are also investigated in a special way in the case of the Rees algebra (of the di erential module), highlighting the connection to the so-called Fitting conditions. / Nesta disserta c~ao, inicialmente apresentamos no c~oes gerais sobre a algebra sim etrica e a algebra de Rees no contexto amplo de m odulos, e consideramos particularmente a situa c~ao especial na qual o dado m odulo possui apresenta c~ao linear. Na sequ^encia, o principal objetivo e o estudo de tais algebras de blowup no caso em que o m odulo e o celebrado m odulo de diferenciais de K ahler, tendo como foco a investiga c~ao de uma interessante vers~ao da persistente Conjectura de Berger para a algebra sim etrica, bem como o estudo de propriedades fundamentais como: integridade, Cohen-Macaulicidade e normalidade; tais propriedades s~ao tamb em investigadas de forma especial no caso da algebra de Rees (do m odulo de diferenciais), evidenciando inclusive a conex~ao com as chamadas condi c~oes de Fitting.

Submitted by Leonardo Cavalcante (leo.ocavalcante@gmail.com) on 2018-05-02T14:09:22Z No. of bitstreams: 1 Arquivototal.pdf: 978771 bytes, checksum: bf0c05c8da5e986a77b6215a2235ab5e (MD5) / Made available in DSpace on 2018-05-02T14:09:22Z (GMT). No. of bitstreams: 1 Arquivototal.pdf: 978771 bytes, checksum: bf0c05c8da5e986a77b6215a2235ab5e (MD5) Previous issue date: 2017-04-27 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / At the end of the 60s, Andr e and Quillen introduced a cohomology theory for commutative algebras, which today is called Andr e-Quillen's cohomology. In this work, we will study K ahler di erential functor, which is here seen as a derived functor (in a nonabelian context), which connects the categories: simpli ed R-algebras and simpli ed R-modules. In the rst, through simplicial resolutions, we will notice that they characterize certain objects and diagrams of this model category, which in turn, are preserved by K ahler di erential functor. In addition, we will approach the complex cotangent of a R-algebra, and through it, de ne the homology and cohomology of Andr e-Quilen, and of course, expose some properties of these. / No nal da década de 60, André e Quillen introduziram uma teoria de cohomologia para álgebras comutativas, que hoje recebe o nome de cohomologia de André-Quillen. Neste trabalho, estudaremos o funtor de diferenciais de K ahler, que aqui é visto como funtor derivado (em um contexto não abeliano), que conecta as categorias: R-álgebras simpliciais e R-m odulos simpliciais. Na primeira, atrav es das resolu c~oes simpliciais, notaremos que estas caracterizam certos objetos e diagramas desta categoria modelo, que por sua vez, s~ao preservados pelo funtor de diferenciais de K ahler. Al em disso, abordaremos o complexo cotangente de uma R- algebra, e atrav es dele, de nir a homologia e cohomologia de André-Quillen, e naturalmente, expor algumas propriedades destas.

This thesis comes in three main parts. In the first of these (comprising chapters 2 - 6), the basic theory of Riemannian and conformal submersions is described and the relevant geometric machinery explained. The necessary Clifford algebra is established and applied to understand the relationship between the spinor bundles of the base, the fibres and the total space of a submersion. O'Neill-type formulae relating the covariant derivatives of spinor fields on the base and fibres to the corresponding spinor field on the total space are derived. From these, formulae for the Dirac operators are obtained and applied to prove results on Dirac morphisms in cases so far unpublished. The second part (comprising chapters 7-9) contains the basic theory and known classifications of G2-structures and Spin+ 7 -structures in seven and eight dimensions. Formulae relating the covariant derivatives of the canonical forms and spinor fields are derived in each case. These are used to confirm the expected result that the form and spinorial classifications coincide. The mean curvature vector of associative and Cayley submanifolds of these spaces is calculated in terms of naturally-occurring tensor fields given by the structures. The final part of the thesis (comprising chapter 10) is an attempt to unify the first two parts. A certain `7-complex' quotient is described, which is analogous to the well-known hyper-Kahler quotient construction. This leads to insight into other possible interesting quotients which are correspondingly analogous to quaternionic-Kahler quotients, and these are speculated upon with a view to further research.

Dans cette thèse nous considérons des familles $pi : cc X to S$ de variétés compactes k"ahlerinnes au-dessus d'une base lisse $S$. Nous construisons un cône de K"ahler relatif $p : cc K to S$ au-dessus de la base de déformations. Ensuite nous démontrons l'existence des métriques hermitiennes naturelles sur les espaces totals $cc K$ et $cc X times_S cc K$ qui généralisent la métrique de Weil--Petersson classiuque associée aux familles polarisées de telles variétés. Nous obtenons aussi une métrique riemannienne sur le cône de K"ahler d'une variété compacte k"ahlerienne quelconque. Nous exprimons son tenseur de courbure à l'aide d'un plongement du cône de K"ahler dans l'espace de toutes métriques hermitiennes sur la variété. Nous démontrons aussi que si les variétés en question sont de fibré canonique trivial, alors notre métrique est la forme de courbure d'un fibré en droites holomorphe. Nous donnons ensuite quelques exemples et applications. / In this thesis we consider families $pi : cc X to S$ of compact K"ahler manifolds with zero first Chern class over a smooth base $S$. We construct a relative complexified K"ahler cone $p : cc K to S$ over the base of deformations. Then we prove the existence of natural hermitian metrics on the total spaces $cc K$ and $cc X times_S cc K$ that generalize the classical Weil--Petersson metrics associated to polarized families of such manifolds. As a byproduct we obtain a Riemannian metric on the K"ahler cone of any compact K"ahler manifold. We obtain an expression of its curvature tensor via an embedding of the K"ahler cone into the space of hermitian metrics on the manifold. We also prove that if the manifolds in our family have trivial canonical bundle, then our generalized Weil--Petersson metric is the curvature form of a positive holomorphic line bundle. We then give some examples and applications.

We discuss two topics in this talk. First we study compact Ricci-flat four dimensional manifolds without boundary and obtain point wise restrictions on curvature( not involving global quantities such as volume and diameter) which force the metric to be flat. We obtain the same conclusion for compact Ricci-flat K¨ahler surfaces with similar but weaker restrictions on holomorphic sectional curvature. Next we study the reaction ODE associated to the evolution of the Riemann curvature operator along the Ricci flow. We analyze the behavior of this ODE near algebraic curvature operators of certain special type that includes the Riemann curvature operators of various(locally) symmetric spaces. We explicitly show the existence of some solution curves to the ODE connecting the curvature operators of certain symmetric spaces. Although the results of these two themes are different, the underlying common feature is the reaction ODE which plays an important role in both.