## Fachbereich Mathematik

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- 2011 (14) (remove)

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- Doctoral Thesis (14) (remove)

#### Keywords

- Chow Quotient (1)
- Copula (1)
- Credit Default Swap (1)
- Finite Pointset Method (1)
- Local smoothing (1)
- Markov Chain (1)
- Markov Kette (1)
- Mathematik (1)
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- Multi Primary and One Second Particle Method (1)

- Tropical orbit spaces and moduli spaces of tropical curves (2011)
- A main result of this thesis is a conceptual proof of the fact that the weighted number of tropical curves of given degree and genus, which pass through the right number of general points in the plane (resp., which pass through general points in R^r and represent a given point in the moduli space of genus g curves) is independent of the choices of points. Another main result is a new correspondence theorem between plane tropical cycles and plane elliptic algebraic curves.

- A Multi-Phase Flow Model Incorporated with Population Balance Equation in a Meshfree Framework (2011)
- This study deals with the numerical solution of a meshfree coupled model of Computational Fluid Dynamics (CFD) and Population Balance Equation (PBE) for liquid-liquid extraction columns. In modeling the coupled hydrodynamics and mass transfer in liquid extraction columns one encounters multidimensional population balance equation that could not be fully resolved numerically within a reasonable time necessary for steady state or dynamic simulations. For this reason, there is an obvious need for a new liquid extraction model that captures all the essential physical phenomena and still tractable from computational point of view. This thesis discusses a new model which focuses on discretization of the external (spatial) and internal coordinates such that the computational time is drastically reduced. For the internal coordinates, the concept of the multi-primary particle method; as a special case of the Sectional Quadrature Method of Moments (SQMOM) is used to represent the droplet internal properties. This model is capable of conserving the most important integral properties of the distribution; namely: the total number, solute and volume concentrations and reduces the computational time when compared to the classical finite difference methods, which require many grid points to conserve the desired physical quantities. On the other hand, due to the discrete nature of the dispersed phase, a meshfree Lagrangian particle method is used to discretize the spatial domain (extraction column height) using the Finite Pointset Method (FPM). This method avoids the extremely difficult convective term discretization using the classical finite volume methods, which require a lot of grid points to capture the moving fronts propagating along column height.

- Valuation of Credit Derivatives (2011)
- In this work two main approaches for the evaluation of credit derivatives are analyzed: the copula based approach and the Markov Chain based approach. This work gives the opportunity to use the advantages and avoid disadvantages of both approaches. For example, modeling of contagion effects, i.e. modeling dependencies between counterparty defaults, is complicated under the copula approach. One remedy is to use Markov Chain, where it can be done directly. The work consists of five chapters. The first chapter of this work extends the model for the pricing of CDS contracts presented in the paper by Kraft and Steffensen (2007). In the widely used models for CDS pricing it is assumed that only borrower can default. In our model we assume that each of the counterparties involved in the contract may default. Calculated contract prices are compared with those calculated under usual assumptions. All results are summarized in the form of numerical examples and plots. In the second chapter the copula and its main properties are described. The methods of constructing copulas as well as most common copulas families and its properties are introduced. In the third chapter the method of constructing a copula for the existing Markov Chain is introduced. The cases with two and three counterparties are considered. Necessary relations between the transition intensities are derived to directly find some copula functions. The formulae for default dependencies like Spearman's rho and Kendall's tau for defined copulas are derived. Several numerical examples are presented in which the copulas are built for given Markov Chains. The fourth chapter deals with the approximation of copulas if for a given Markov Chain a copula cannot be provided explicitly. The fifth chapter concludes this thesis.

- Intersection Theory on Tropical Toric Varieties and Compactifications of Tropical Parameter Spaces (2011)
- We study toric varieties over the tropical semifield. We define tropical cycles inside these toric varieties and extend the stable intersection of tropical cycles in R^n to these toric varieties. In particular, we show that every tropical cycle can be degenerated into a sum of torus-invariant cycles. This allows us to tropicalize algebraic cycles of toric varieties over an algebraically closed field with non-Archimedean valuation. We see that the tropicalization map is a homomorphism on cycles and an isomorphism on cycle classes. Furthermore, we can use projective toric varieties to compactify known tropical varieties and study their combinatorics. We do this for the tropical Grassmannian in the Plücker embedding and compactify the tropical parameter space of rational degree d curves in tropical projective space using Chow quotients of the tropical Grassmannian.

- Some Steps towards Experimental Design for Neural Network Regression (2011)
- We discuss some first steps towards experimental design for neural network regression which, at present, is too complex to treat fully in general. We encounter two difficulties: the nonlinearity of the models together with the high parameter dimension on one hand, and the common misspecification of the models on the other hand. Regarding the first problem, we restrict our consideration to neural networks with only one and two neurons in the hidden layer and a univariate input variable. We prove some results regarding locally D-optimal designs, and present a numerical study using the concept of maximin optimal designs. In respect of the second problem, we have a look at the effects of misspecification on optimal experimental designs.

- Mathematical Programming Approaches for Decoding of Binary Linear Codes (2011)
- In this thesis, we aim at finding appropriate integer programming models and associated solution approaches for the maximum likelihood decoding problem of several binary linear code classes.

- Graded commutative algebra and related structures in Singular with applications (2011)
- This thesis is devoted to constructive module theory of polynomial graded commutative algebras over a field. It treats the theory of Groebner bases (GB), standard bases (SB) and syzygies as well as algorithms and their implementations. Graded commutative algebras naturally unify exterior and commutative polynomial algebras. They are graded non-commutative, associative unital algebras over fields and may contain zero-divisors. In this thesis we try to make the most use out of _a priori_ knowledge about their characteristic (super-commutative) structure in developing direct symbolic methods, algorithms and implementations, which are intrinsic to graded commutative algebras and practically efficient. For our symbolic treatment we represent them as polynomial algebras and redefine the product rule in order to allow super-commutative structures and, in particular, to allow zero-divisors. Using this representation we give a nice characterization of a GB and an algorithm for its computation. We can also tackle central localizations of graded commutative algebras by allowing commutative variables to be _local_, generalizing Mora algorithm (in a similar fashion as G.M.Greuel and G.Pfister by allowing local or mixed monomial orderings) and working with SBs. In this general setting we prove a generalized Buchberger's criterion, which shows that syzygies of leading terms play the utmost important role in SB and syzygy module computations. Furthermore, we develop a variation of the La Scala-Stillman free resolution algorithm, which we can formulate particularly close to our implementation. On the implementation side we have further developed the Singular non-commutative subsystem Plural in order to allow polynomial arithmetic and more involved non-commutative basic Computer Algebra computations (e.g. S-polynomial, GB) to be easily implementable for specific algebras. At the moment graded commutative algebra-related algorithms are implemented in this framework. Benchmarks show that our new algorithms and implementation are practically efficient. The developed framework has a lot of applications in various branches of mathematics and theoretical physics. They include computation of sheaf cohomology, coordinate-free verification of affine geometry theorems and computation of cohomology rings of p-groups, which are partially described in this thesis.

- A Tree Algorithm for Helmholtz Potential Wavelets on Non-Smooth Surfaces: Theoretical Background und Application to Seismic Data Postprocessing (2011)
- The interest of the exploration of new hydrocarbon fields as well as deep geothermal reservoirs is permanently growing. The analysis of seismic data specific for such exploration projects is very complex and requires the deep knowledge in geology, geophysics, petrology, etc from interpreters, as well as the ability of advanced tools that are able to recover some particular properties. There again the existing wavelet techniques have a huge success in signal processing, data compression, noise reduction, etc. They enable to break complicate functions into many simple pieces at different scales and positions that makes detection and interpretation of local events significantly easier. In this thesis mathematical methods and tools are presented which are applicable to the seismic data postprocessing in regions with non-smooth boundaries. We provide wavelet techniques that relate to the solutions of the Helmholtz equation. As application we are interested in seismic data analysis. A similar idea to construct wavelet functions from the limit and jump relations of the layer potentials was first suggested by Freeden and his Geomathematics Group. The particular difficulty in such approaches is the formulation of limit and jump relations for surfaces used in seismic data processing, i.e., non-smooth surfaces in various topologies (for example, uniform and quadratic). The essential idea is to replace the concept of parallel surfaces known for a smooth regular surface by certain appropriate substitutes for non-smooth surfaces. By using the jump and limit relations formulated for regular surfaces, Helmholtz wavelets can be introduced that recursively approximate functions on surfaces with edges and corners. The exceptional point is that the construction of wavelets allows the efficient implementation in form of a tree algorithm for the fast numerical computation of functions on the boundary. In order to demonstrate the applicability of the Helmholtz FWT, we study a seismic image obtained by the reverse time migration which is based on a finite-difference implementation. In fact, regarding the requirements of such migration algorithms in filtering and denoising the wavelet decomposition is successfully applied to this image for the attenuation of low-frequency artifacts and noise. Essential feature is the space localization property of Helmholtz wavelets which numerically enables to discuss the velocity field in pointwise dependence. Moreover, the multiscale analysis leads us to reveal additional geological information from optical features.

- Local Smoothing Methods with Regularization in Nonparametric Regression Models (2011)
- Mrázek et al. [14] proposed a unified approach to curve estimation which combines localization and regularization. In this thesis we will use their approach to study some asymptotic properties of local smoothers with regularization. In Particular, we shall discuss the regularized local least squares (RLLS) estimate with correlated errors (more precisely with stationary time series errors), and then based on this approach we will discuss the case when the kernel function is dirac function and compare our smoother with the spline smoother. Finally, we will do some simulation study.

- Algorithms for Symbolic Computation and their Applications - Standard Bases over Rings and Rank Tests in Statistics (2011)
- In the first part of the thesis we develop the theory of standard bases in free modules over (localized) polynomial rings. Given that linear equations are solvable in the coefficients of the polynomials, we introduce an algorithm to compute standard bases with respect to arbitrary (module) monomial orderings. Moreover, we take special care to principal ideal rings, allowing zero divisors. For these rings we design modified algorithms which are new and much faster than the general ones. These algorithms were motivated by current limitations in formal verification of microelectronic System-on-Chip designs. We show that our novel approach using computational algebra is able to overcome these limitations in important classes of applications coming from industrial challenges. The second part is based on research in collaboration with Jason Morton, Bernd Sturmfels and Anne Shiu. We devise a general method to describe and compute a certain class of rank tests motivated by statistics. The class of rank tests may loosely be described as being based on computing the number of linear extensions to given partial orders. In order to apply these tests to actual data we developed two algorithms and used our implementations to apply the methodology to gene expression data created at the Stowers Institute for Medical Research. The dataset is concerned with the development of the vertebra. Our rankings proved valuable to the biologists.