8 search hits
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Computing canonical heights on Jacobians
(2010)
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Jan Steffen Müller
- The canonical height is an indispensable tool for the study of the arithmetic of abelian varieties. In this dissertation we investigate methods for the explicit computation of canonical heights on Jacobians of smooth projective curves. Building on an existing algorithm due to Flynn and Smart with modifications by Stoll we generalize efficient methods for the computation of canonical heights on elliptic curves to the case of Jacobian surfaces. The main tools are the explicit theory of the Kummer surface associated to a Jacobian surface, which we develop in full generality, building on earlier work due to Flynn, and a careful study of the local Néron models of the Jacobian. As a first step for a further generalization to Jacobian threefolds of hyperelliptic curves, we completely describe the associated Kummer threefold and conjecture formulas for explicit arithmetic on it, based on experimental data. Assuming the validity of this conjecture, many of the results for Jacobian surfaces can then be generalized. Finally, we use a theorem due to Faltings, Gross and Hriljac which expresses the canonical height on the Jacobian in terms of arithmetic intersection theory on the curve to develop an algorithm for the computation of the canonical height which is applicable in principle to any Jacobian. However, it uses several subroutines and some of these are currently only implemented in the hyperelliptic case, although the theory is available in general. Among the possible applications of the computation of canonical heights are the determination of generators for the Mordell-Weil group of the Jacobian and the computation of its regulator, appearing for instance in the famous Birch and Swinnerton-Dyer conjecture. We illustrate our algorithm with two examples: The regulator of a finite index subgroup of the Mordell-Weil group of the Jacobian of a genus 3 hyperelliptic curve and the non-archimedean part of theregulator computation for the Jacobian of a non-hyperelliptic genus 4 curve, where the remaining computations can be done immediately once the above-mentioned implementations are available.
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Algebraische Approximation von Kählermannigfaltigkeiten
(2010)
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Florian Schrack
- Eine kompakte komplexe Mannigfaltigkeit heißt algebraisch approximierbar, wenn sie beliebig kleine projektive Deformationen besitzt. Eine natürliche Fragestellung ist, ob jede kompakte Kählermannigfaltigkeit algebraisch approximierbar ist. Während dies in Dimension 2 nach den Arbeiten von Kodaira richtig ist, hat Voisin vierdimensionale Gegenbeispiele gefunden. In Dimension 3 ist die Frage noch offen. Ziel der vorliegenden Arbeit ist es, den dreidimensionalen Fall etwas näher zu beleuchten. Dazu wird algebraische Approximierbarkeit zunächst von einem allgemeinen Standpunkt aus betrachtet. Es werden Funktorialitätsfragen untersucht, also der Zusammenhang zwischen algebraischer Approximierbarkeit der Quelle und des Ziels gewisser holomorpher Abbildungen, und Ergebnisse für verschiedene Klassen von Abbildungen erzielt, wie etwa Aufblasungen, endliche Abbildungen, Faserungen und Morikontraktionen. Als Fallstudie einer konkreten Klasse von Kählerdreifaltigkeiten werden anschließend Konikbündel über Kählerflächen untersucht, die in natürlicher Weise in der Moritheorie auftreten. Nach dem Beweis einiger grundlegender Tatsachen über Konikbündel werden ihre Diskriminantenorte genauer untersucht und damit Chernklassenabschätzungen für Konikbündel mit relativer Picardzahl 1 über nichtalgebraischen kompakten Kählerflächen hergeleitet. Unter Verwendung dieser Abschätzungen wird die Existenz infinitesimaler Deformationen solcher Konikbündel gezeigt, was einen wichtigen ersten Schritt zum Beweis der algebraischen Approximierbarkeit darstellt. Ein spezieller Typ von Konikbündeln sind die projektivierten Rang-2-Bündel. Die Periodenabbildung verhilft zu einem guten Verständnis der Deformationstheorie solcher Bündel über K3-Flächen und zweidimensionalen Tori. Konkret werden Fortsetzungssätze für Geradenbündel und Rang-2-Bündel bewiesen, die implizieren, dass jedes projektivierte Rang-2-Bündel über einer K3-Fläche oder einem zweidimensionalen Torus algebraisch approximierbar ist. Durch Untersuchung von Aufblasungen solcher Flächen wird dieses Resultat auf projektivierte Rang-2-Bündel über beliebigen kompakten Kählerflächen mit Kodairadimension 0 ausgedehnt. Schließlich wird die zuvor entwickelte Deformationstheorie für Vektorbündel verwendet, um weitere Approximierbarkeitsergebnisse für Konikbündel über elliptischen Flächen und K3-Flächen zu bekommen.
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Exploiting combinatorial relaxations to solve a routing & scheduling problem in car body manufacturing
(2010)
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Jörg Rambau
Cornelius Schwarz
- Motivated by the laser sharing problem (LSP) in car body manufacturing, we define the new general routing and scheduling problem (RSP). In the RSP, multiple servers have to visit and process jobs; renewable resources are shared among them. The goal is to find a makespan-minimal scheduled dispatch. We present complexity results as well as a branch-and-bound algorithm for the RSP. This is the first algorithm that is able to solve the LSP for industrially relevant problem scales.
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How to avoid collisions in scheduling industrial robots?
(2010)
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Jörg Rambau
Cornelius Schwarz
- In modern production facilities industrial robots play an important role. When two ore more of them are moving in the same area, care must be taken to avoid collisions between them. Due to expensive equipment costs our approach to handle this is very conservative: Each critical area is modeled as a shared resource where only one robot is allowed to use it at a time. We studied collision avoidance in the context of arc welding robots in car manufacture industry. Here another shared resource comes into place. When using laser welding technology every robot needs to be connected to a laser source supplying it with the necessary energy. Each laser source can be connected to up to six robots but serve only one at a time. An instance of the problem consists of a set of robots, a set of welding task, a number of laser sources, a distance table, collision information and a production cycle time. The goal is to design robot tours covering all task and schedule them resource conflict free such that the makespan does not exceed the cycle time. We propose a general model for integrated routing and scheduling including collision avoidance as well as a branch-and-bound algorithm for it. Computational results on data generated with the robot simulation software KuKa Sim Pro are also provided showing that our algorithm outperforms standard mixed-integer models for our application.
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The classification of isotrivially fibred surfaces with p_g=q=2, and topics on Beauville surfaces
(2010)
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Matteo Penegini
- In our thesis we treat mainly two topics: the classification of isotrivially fibred surfaces with p_g=q=2, and the construction of new Beauville surfaces. An isotrivially fibred surface is a smooth projective surface endowed with a morphism onto a smooth curve such that all the smooth fibres are isomorphic to each other. The first goal of this thesis is to classify the isotrivially fibred surfaces with p_g=q=2 completing and extending a result by Zucconi. As an important byproduct, we provide new examples of minimal surfaces of general type with p_g=q=2 and K^2=4,5 and the first example with K^2=6. We say that a surface S is isogenous to a product of curves if S = (C times F )/G, for C and F smooth curves and G a finite group acting freely on C times F. Beauville surfaces are a special case of surfaces isogenous to a product. In this thesis we include part of a joint work with Shelly Garion, in which we construct new Beauville surfaces with group G either PSL(2,p^e), or A_n, or S_n, proving a conjecture of Bauer, Catanese and Grunewald. The proofs rely on probabilistic group theoretical results of Liebeck and Shalev, and on classical results of Macbeath. The thesis is divided into three chapters, which are subdivided in several sections. In the first chapter we treat the problem of the classification of isotrivially fibred surfaces with p_g=q=2. We start by recalling some basic facts and theorems about fibred surfaces and surfaces isogenous to a higher product of curves. Then we solve the classification problem using techniques coming from both geometry and combinatorial group theory. In the second chapter we deal with Beauville surfaces. First we give a group theoretical characterization of them. Then we enunciate a theorem of Liebeck and Shalev that we use for the construction of Beauville surfaces with group A_n or S_n. Afterwards we also study Beauville surfaces with group PSL(2,p^e). In the third chapter we give a description of the locus, in the moduli space of surfaces of general type, corresponding to the surfaces isogenous to a product with p_g=q=2 described in the first chapter. Indeed, by the results proven by Catanese, this locus is a union of connected components, whose number can be computed using a theorem of Bauer and Catanese. In the same way we are able to provide an asymptotic result about the number of connected components of the moduli space corresponding to certain families of Beauville surfaces with group either PSL(2,p^e), or A_n, or (mathbb{Z}/nmathbb{Z})^2 as p and n go to infinity.
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Optimal Control Problems Governed by Nonlinear Partial Differential Equations and Inclusions
(2010)
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Julia Fischer
- The focus of this thesis lies on examining the solvability of optimal control problems constrained by nonlinear partial differential equations (PDE) and inclusions (PDI). There exist statements on the existence of solutions for optimal control problems with linear and semi-linear PDEs with monotone parts. The theory for non-monotone PDEs resp. the related optimal control problems is, to the author’s knowledge, incomplete regarding important issues. This concerns particularly the case of PDEs containing mappings, which only satisfy boundedness conditions on restricted sets. At first an optimal control problem is considered, which is characterized by a Laplace equation with Dirichlet boundary conditions and a nonlinear non-monotone Nemytskii operator. Under the decisive assumption of the existence of so called sub- and supersolutions for this differential equation and by introducing a truncation operator we can define an auxiliary problem which is characterized by a pseudomonotone operator. Thereby the solution theory for pseudomonotone operators of Brézis (1968) is applicable. Moreover, starting with the definitions of sub- und supersolution it can be shown, that every solution of the auxiliary problem is a solution of the original problem. The choice of a new optimal control problem which substitutes the original optimal control problem is again governed by the properties of the auxiliary operator. The equivalence of the auxiliary problem to the original problem and the existence of at least one solution can be shown. The technique of applying the Theorem of Lax-Milgram on a linearized problem can be adapted to the semi-linear non-monotone case. This procedure is already known from the theory of semi-linear monotone problems. For optimal control problems with quasi-linear differential equations, different methods are required. As in the semi-linear case, the property of pseudomonotonicity plays a key role in proving the existence of a solution of the quasi-linear PDE. In the proof of the existence of a solution for the optimal control problem other properties of the auxiliary operator are exploited. In the elliptic case operators which satisfy the S+ -property are important. In order to utilize this property, a transformation of the operator to some coercive auxiliary operator is necessary. For this reason a term is added, which penalizes the deviation from the admissible set of states. This term is characterized by a factor, which is derived explicitly in this work. The proof of the existence of a solution of the optimal control problem with parabolic equations is based on the definition of an auxiliary operator, coercivity and the S+ -property of operators. The set of solutions of the considered PDE is compact, but the number of solutions and the situation to each other is unknown. This leads to difficulties in deriving necessary optimality conditions. For this reason a direct approach to solve the optimal control problem with semi-linear PDEs is introduced. It is assumed, that the state constraints coincide with the sub- and the supersolution of the PDE with the upper and lower boundary of the control variable. Using an auxiliary operator, this assumption allows the formulation of an equivalent optimal control problem without pointwise state constraints. Through semi-discretization we can define a family of optimal control problems on a finite dimensional state-space. Existence of a subsequence of solutions of these optimal control problems which converges to a solution of the original problem is shown. Another important class of optimal control problems include differential inclusions which are described by multivalued operators. Quasi-linear elliptic inclusions are examined under global as well as local boundedness conditions. Under the assumption of global boundedness the properties of pseudomonotonicity and coercivity for a multivalued auxiliary operator are proven. The existence of at least one solution for the original inclusion follows from the application of a result from Hu and Papageorgiou (1997) on the auxiliary problem. The existence of at least one solution of the optimal control problem is proven by exploiting the coercivity of the multivalued auxiliary operator and the S+ -property of the non-multivalued part of this mapping. In the case of multivalued mappings of Clarke’s gradient type, the existence of at least one solution of the optimal control problem can be shown under local boundedness conditions. Elliptic as well as parabolic quasi-linear inclusions are considered. The proof is again based on coercivity and the S+ -property of the related auxiliary operators and the embedding properties of the spaces.
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Cosserat Operators of Higher Order and Applications
(2010)
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Thorsten Riedl
- We take a look at certain operators called Cosserat operators and get a compactness result for them leading to several interesting applications. For a more detailed abstract, see the actual abstract at the beginning of the work.
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Surfaces Isogenous to a Product: Their Automorphisms and Degenerations
(2010)
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Wenfei Liu
- In this thesis, I consider the automorphisms and stable degenerations of surfaces isogenous to a product. First I consider the action of the automorphisms on cohomology in the case where the group G is abelian. It is shown that, if the irregularity of the surface is larger than 1, the action of G on the second cohomology is mostly faithful. For surfaces with irregularity 0 or 1, examples are given. Then I consider the stable degenerations of surfaces isogenous to a product and classify the possible singularities on them. As a result, I show that the Q-Gorenstein deformations of the degenerations with certain singuarities are unobstructed and get some connected components of the moduli space of stable surfaces.
