Proefschrift Universiteit van Amsterdam. / Met bibliogr., lit. opg. - Met samenvatting in het Nederlands.

No description available.

No description available.

We consider linear perturbations of the background type IIB SUGRA solutions and find the equations of motion for the moduli. In particular, we allow for spacetime fluctuations of the positions of D3-branes in the compact dimensions. We postulate an ansatz for the 5-form flux due to the motion of the D3-branes, and a corresponding first-order part of the metric. The movement of the D3-branes is then shown to affect the warp factor at linear order. Using the equations of motion for the D3-branes, the universal volume modulus, and the universal axion, we construct a second-order, effective action. Finally, based on the form of the effective action, we examine a Kahler potential for the moduli space.

CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior / Consideramos o problema de representaÃÃo de superfÃcies imersas em grupos de Lie tridimensionais.Especificamente, nos espaÃos HiperbÃlico, de Sitter, Heisenberg (Riemanniano e pseudo-Riemanniano), nas esferas de Berger e em espaÃos Anti de Sitter exÃticos. Estabelecemos como condiÃÃes de integrabilidade para a existÃncia de uma imersÃo conforme de uma superfÃcie de Riemann nos espaÃos HiperbÃlico, de Sitter, Heisenberg(Riemannianoe pseudo-Riemanniano) as equaÃÃes de compatibilidade para um sistema deprimeira ordem,envolvendo uma equaÃÃo de Dirac com potenciais geomÃtricos. Nas esferas de Berger e nos espaÃos Anti de Sitter exÃticos,demonstra-se que a harmonicidade de uma dada aplicaÃÃo, definida na superfÃcie com valores em abertos da esfera,Ã condiÃÃo suficiente para a existÃncia de uma imersÃo conforme mÃnima. / We considered the problem of representation of immersed surfaces in three-dimensional Lie groups. We search for integrability conditions assuring the existence of a conformal immersion of a given Riemann surface in some Lie group with left-invariant metric. Such compatibility conditions are found to be a first order system, consisting of a Dirac equation with geometric potentials and an extra pair of equations relating the metric and the Hopf differential. In many cases, we proved that the harmonicity of a map, defined in an open of the sphere is a sufficient condition for the existence of a conformal minimal or constant mean curvature immersion.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior / Subvariedades analÃticas complexas e totalmente reais sÃo duas classes tÃpicas dentre todas as subvariedades de uma variedade quase Hermitiana. Neste trabalho procuramos dar algumas caracterizaÃÃes de subvariedades totalmente reais. AlÃm disso algumas classificaÃÃes de subvariedades totalmente reais em formas espaciais complexas sÃo obtidas. / Complex analytic submanifolds and totally real submanifolds are two typical classes among all submanifolds of an almost Hermitian manifolds. In this work, some characterizations of totally real submanifolds are given. Moreover some classifications of totally real submanifolds in complex space forms are obtained.

The global log canonical threshold, algebraic counterpart to Tian's alpha-invariant, plays an important role when studying the geometry of Fano varieties. In particular, Tian showed that Fano manifolds with big alpha-invariant can be equipped with a Kahler-Einstein metric. In recent years Donaldson drafted a programme to precisely determine when a smooth Fano variety X admits a Kahler-Einstein metric. It was conjectured that the existence of such a metric is equivalent to X being K-stable, an algebraic-geometric property. A crucial step in Donaldson's programme consists on finding a Kahler-Einstein metric with edge singularities of small angle along a smooth anticanonical boundary. Jeffres, Mazzeo and Rubinstein showed that a dynamic version of the alpha-invariant could be used to find such metrics. The global log canonical threshold measures how anticanonical pairs fail to be log canonical. In this thesis we compute the global log canonical threshold of del Pezzo surfaces in various settings. First we extend Cheltsov's computation of the global log canonical threshold of complex del Pezzo surfaces to non-singular del Pezzo surfaces over a ground field which is algebraically closed and has arbitrary characteristic. Then we study which anticanonical pairs fail to be log canonical. In particular, we give a very explicit classifiation of very singular anticanonical pairs for del Pezzo surfaces of degree smaller or equal than 3. We conjecture under which circumstances such a classifcation is plausible for an arbitrary Fano variety and derive several consequences. As an application, we compute the dynamic alpha-invariant on smooth del Pezzo surfaces of small degree, where the boundary is any smooth elliptic curve C. Our main result is a computation of the dynamic alpha-invariant on all smooth del Pezzo surfaces with boundary any smooth elliptic curve C. The values of the alpha-invariant depend on the choice of C. We apply our computation to find Kahler-Einstein metrics with edge singularities of angle β along C.

Toric geometry studies manifolds M2n acted on effectively by a torus of half their dimension, Tn. Joyce shows that for such a 4-manifold sufficient conditions for a conformal class of metrics on the free part of the action to be self-dual can be given by a pair of linear ODEs and gives criteria for a metric in this class to extend to the degenerate orbits. Joyce and Calderbank-Pedersen use this result to find representatives which are scalar flat K¨ahler and self-dual Einstein respectively. We review some results concerning the topology of toric manifolds and the construction of Joyce metrics. We then extend this construction to give explicit complete scalar-flat K¨ahler and self-dual Einstein metrics on manifolds of infinite topological type, and to find a new family of Joyce metrics on open submanifolds of toric spaces. We then give two applications of these extensions — first, to give a large family of scalar flat K¨ahler perturbations of the Ooguri-Vafa metric, and second to search for a toric scalar flat K¨ahler metric on a neighbourhood of the origin in C2 whose restriction to an annulus on the degenerate hyperboloid {(z1, z2)|z1z2 = 0} is the cusp metric.

Let (M,g) be a quasi-Sasaki manifold with Reeb vector field xi. Our goal is to understand the structure of M when g is an Einstein metric. Assuming that the S^1 action induced by xi is locally free or assuming a certain non-negativity condition on the transverse curvature, we prove some rigidity results on the structure of (M,g). Naturally associated to a quasi-Sasaki metric g is a transverse Kahler metric g^T. The transverse Kahler-Ricci flow of g^T is the normalized Ricci flow of the transverse metric. Exploiting the transverse Kahler geometry of (M,g), we can extend results in Kahler-Ricci flow to our transverse version. In particular, we show that a deep and beautiful theorem due to Perleman has its counterpart in the quasi-Sasaki setting. We also consider evolving a Sasaki metric g by Ricci flow. Unfortunately, if g(0) is Sasaki then g(t) is not Sasaki for t>0. However, in some instances g(t) is quasi-Sasaki. We examine this and give some qualitative results and examples in the special case that the initial metric is eta-Einstein.

In the first part of my thesis we provide infinitely many examples of pairs of diffeomorphic, non simply connected Kahler manifolds of complex dimension 3 with different Kodaira dimensions. Also, in any allowed Kodaira dimension we find infinitely many pairs of non deformation equivalent, diffeomorphic Kahler threefolds. In the second part we study the existence of Kahler metrics of positive total scalar curvature on 3-folds of negative Kodaira dimension. We give a positive answer for rationally connected threefolds. The proof relies on the Mori theory of minimal models, the weak factorization theorem and on a specialization technique.