2010 -2009 - 2008 - 2007 - 2006 - 2005 - 2004 - 2003 - 2002 - 2001 - 2000 - 1999 - 1998 - 1997

2010

Prof. Dr. Gabriele Steidl (Universität Mannheim)

''Operator Splittings in Image Processing''

Abstract: Operator splitting techniques were developed in the 60th for the efficient solution of linear systems of equations appearing in connection with the numerical solution of partial differential equations. More than 20 years later these methods were generalized to nonlinear, set-valued, monotone operators and were recently successfully applied in image precessing. After motivating why methods of convex analysis/non-smooth convex optimization are useful for many image restoration tasks we give a brief introduction into the relevant operator splitting techniques. We point out that for special functionals there exist equivalent derivations via averaged operatoren, (Bregman) proximal point methods or augmented Lagrangian methods. Then we present applications of operator splitting methods in direction-steered inpainting (interpolation) and for denoising of images corruped with non-additive noise (and possibly some blur) which appear for example in Synthetic Aperture Radar and electronic microscopy.

Mittwoch, 07.07.2010, 17 Uhr c.t., 69/125

 

Prof. Dr. Elmar Vogt (Freie Universität Berlin)

''On the Search of Equilibira - or How Many Critical Points are there? A Topological Approach''

Abstract: Equilibria or rest points of a ball on a curved surface are exactly the critical points of the height function of the surface, i. e. points where all first derivatives of the height function vanish. More generally, critical points of differentiable maps play an important rôle in a variety of problems in mathematics and its applications. Just think of variational and (finite or infinite dimensional) minimax problems. In the early 1930s two Russian mathematicians, L.A. Lusternik and L.G. Schnirelmann, associated to any space X a positive integer, called in honor of its discoverers LS(X), and proved (under some hypotheses on X) that LS(X)is a lower bound for the number of critical points of any differentiable function on X. LS(X) is easy to define but hard to compute, and is to this day a much investigated invariant of topological spaces. After explaining the necessary prerequisites from topology and analysis we define LS(X), describe some of its properties and present some examples. We also discuss briefly some generalizations of LS to families of spaces, which were investigated in the last few years. No particular knowledge of topology will be assumed.

Mittwoch, 23.06.2010, 17 Uhr c.t., 69/125

 

Prof. Dr. Hans Daduna (Universität Hamburg)

''Stochastic Networks: Equlibrium Analysis and Asymptotics''

Abstract: TBA

Mittwoch, 16.06.2010, 17 Uhr c.t., 69/125

 

Prof. Dr. Wolfgang Lück (Universität Münster)

''On Hyperbolic Groups with Spheres as Boundary ''

Abstract: Let G be a torsion-free hyperbolic group and let n be an integer greater or equal to six. We prove that G is the fundamental group of a closed aspherical manifold if the boundary of G is homeomorphic to an (n-1)-dimensional sphere. This answers a question of Gromov.

Mittwoch, 09.06.2010, 17 Uhr c.t., 69/125

 

Prof. Dr. Bernd Siebert (Universität Hamburg)

''Mirror Symmetry and Tropical Geometry''

Abstract: "Mirror symmetry" is a phenomenon discovered by physicists around 1990 in the context of superconformal field theories. It claims a deep connection between the complex geometry of a space with the symplectic geometry of another space, its mirror partner. This correspondence works best for pairs of so-called Calabi-Yau manifolds, a class of Kähler manifolds admitting a holomorphic volume form. In the talk I want to give a view on mirror symmetry via the approach pursued jointly with Mark Gross (UCSD). It is based on degenerations of algebraic varieties and their description by discrete data, fashionably called "tropical geometry".

Mittwoch, 02.06.2010, 17 Uhr c.t., 69/125

 

Prof. Dr. Rolf Schneider (Universität Freiburg)

''Random Mosaics and Convex Geometry''

Abstract:  Random mosaics, that is, randomly generated tessellations of Euclidean space into polytopes, present one of the major topics of stochastic geometry. Typical cells or faces of such a mosaic give rise to interesting classes of random polytopes. The geometric investigation of these random polytopes can utilize, sometimes in a surprising way, some results from the classical geometry of convex bodies, like geometric inequalities, existence and uniqueness theorems, and corresponding stability results. Particularly useful are some auxiliary convex bodies, constructed from data of the tessellation.

 Mittwoch, den 26.05.2010, 17 Uhr c.t., 69/125

 

Prof. Anthony Geramita, PhD (Universität Genua)

''Sums of Squares: Evolution of an Idea''

Abstract: Beginning with Fermat's characterization of primes which are the sum of two squares, I would like to show how this naturally leads to Waring's Problem for Integers and then to Waring's Problem for Homogeneous Polynomials. One half of this latter problem has been solved in recent years and I will explain the nature of the approach to that solution through the study of the Higher Secant Varieties of Veronese Varieties and the study of non-reduced subschemes of projective n-space.

 Mittwoch, den 19.05.2010, 17 Uhr c.t., 69/125

 

Prof. Dr. Massimo Fornasier (RICAM, Linz)

''Innovative Sparse Recovery Methods for PDEs''

Abstract: Solutions of certain PDEs and variational problems may be characterized by "a few significant degrees of freedom", and one may want to take advantage of this feature in order to design efficient numerical solutions. Examples of such situations are ubiquitous: adaptive solution of PDEs, degenerate PDEs for image processing, crack modelling and free-discontinuity problems, viscosity solutions of Hamilton-Jacobi equations, digital signal coding/decoding, and compressed sensing. In this talk we focus on methods for the compressed solution of PDEs and we address three different topics.
1) Adaptive numerical methods for elliptic PDEs by means of redundant frame discretizations;
2) Numerical methods for 1-Laplacian equations associated to total variation minimization;
3) Inverse free-discontinuity problems.

 Mittwoch, den 12.05.2010, 17 Uhr c.t., 69/125

 

Prof. Günter M. Ziegler (Technische Universität Berlin)

''A Sharp Colored Tverberg Theorem''

Abstract: More than 50 years ago, the Cambridge undergraduate Bryan Birch showed that "3N points in a plane" can be split into N triples that span triangles with a non-empty intersection. He also conjectured a sharp, higher-dimensional version of this, which was proved by Helge Tverberg in 1964 (freezing, in a hotel room in Manchester). In a 1988 Computational Geometry paper, Bárány, Füredi & Lovász noted that they needed a "colored version of Tverberg's theorem". Soon Bárány & Larman proved such a theorem for 3N colored points in a plane. A d-dimensional version was obtained in a remarkable 1992 paper by Zivaljevic & Vrecica obtained this, though not with a tight bound on the number of points. We propose a new "colored Tverberg theorem", which is tight, and which generalizes Tverberg's original theorem. The proof uses a "configuration space/test map" scheme, the combinatorics of special chessboard complexes, and finishes using (your choice) either equivariant obstruction theory, or a degree argument. (Joint work with Pavle V. M. Blagojevic und Benjamin Matschke).

 Mittwoch, den 05.05.2010, 17 Uhr c.t., 69/125

 

Prof. Dr. Martin Schlather (Universität Göttingen)

''The Use of Positive Definite Functions in Spatial Statistics''

Abstract: Positive definite functions appeared in the literature for the first time at the beginning of the 20th century in the context of integral equations (Hilbert, 1904; Mercer, 1906) and power series (Caratheodory, 1907). Also in stochastics, the class of positive definite funcions plays a crucial role, namely as the covariance function of time series and spatial processes of second order. In the talk, an introduction to positive definite functions will be given from the perspective of stochastics. Important properties and construction principles will be given. The talk will be illustrated by realisations of stochastic processes with a given covariance function.

 Mittwoch, den 28.04.2010, 17 Uhr c.t., 69/125

 

Prof. Marcin Bownik, PhD (University of Oregon)

''The Pythagorean Theorem for Orthogonal Projections''

Abstract: The Pythagorean Theorem should be known to everyone after completing  elementary school. However, its generalization for orthogonal  projections might not be widely known among professional  mathematicians due to the fact that it was proved only a few years ago. This theorem characterizes diagonals of orthogonal projections in  Hilbert spaces and it was shown in 2002 by Kadison. For finitely  dimensional spaces this result is a consequence of a more general  Schur-Horn Theorem characterizing diagonals of Hermitian matrices with  prescribed eigenvalues. In the finite dimensional case diagonals of a  projection are numbers between 0 and 1 summing up to an integer.  However, the infinite dimensional case, though simple in formulation,  is a much deeper result. In this talk we shall describe Kadison's theorem and some newer  results characterizing diagonals of self-adjoint operators with  prescribed spectrum.

Mittwoch, den 21.04.2010, 17 Uhr c.t., 69/125

 

Prof. Dr. Stefan Bauer (Universität Bielefeld)

''Four Dimensional Manifolds''

Abstract: The world we live in is four dimensional: There are three space dimensions, complemented by time. Manifolds of dimension three  and four are mathematical models of our universe, considered either as a space at a fixed time or in its a totality, from beginning to end of time. Of course we don't stand a chance to ever know how our universe looks like as a whole. Nevertheless, we may pose the question, which models there are and how to distinguish them. During the past decades, ideas and methods from topology, geometry and physics revealed phenomena unknown of in other dimensions. Indeed, amongst all dimensions, geometry is most bizarre and least understood in dimension four. The talk aims to elucidate some aspects of the puzzle.

Mittwoch, den 14.04.2010, 17 Uhr c.t., 69/125

 

Prof. Dr. Gert-Martin Greuel (Universität Kaiserslautern)

''Asymptotic Bounds for Singularities on Algebraic Curves''

Abstract: . The most important basic questions about families of algebraic curves with a fixed number of singularities of given type are necessary and sufficient conditions for - existence, - regularity, - irreducibility of such families. In the last 12 years substantial progress has been made on these nearly 100 years old questions by indicating asymptotically correct or even optimal conditions. Starting from classical facts, the lecture will give an overview of new results, open questions and conjectures.

Mittwoch, 20.01.2010, 17 Uhr c.t., 69/125

 

Prof. Dr. Henning Krause (Universität Paderborn)

''Stratifying Modular Representations of Finite Groups''

Abstract: The problem of classifying the modular representations of a finite group has a long history, going back at least 50 years. I will give a quick survey, discussing the representation type of a group algebra. Then I will present a stratification of the category of modular representations which builds on homological and geometric methods. This is based on joint work with Dave Benson and Srikanth Iyengar.

Mittwoch, 13.01.2010, 17 Uhr c.t., 69/125

 

Prof. Dr. Thomas Schick (Georg-August-Universität Göttingen)

''Positive Scalar Curvature and the Novikov Conjecture''

Abstract: An important global problem in differential geometry asks for the type of Riemannian geometries which exist on a given smooth manifold M. In this talk, we will in particular discuss the question whether a given M does admit a metric of positive scalar curvature. We will describe powerful obstructions coming from the Dirac operator and the Schrödinger-Lichnerowicz formula. Refinements of these obstructions take the theory of operator algebras into account and are related to the strong Novikov conjecture (which claims that the Baum-Connes assembly map is injective). We will discuss special cases in which we can give answers to both questions, using a blend of methods from topology and index theory. This is partly joint work with Hanke, Kotschick and Roe.

Mittwoch, 06.01.2010, 17 Uhr c.t., 69/125

 

2009

Prof. Dr. John M. Sullivan (Technische Universität Berlin)

''Geometric Knot Theory''

Abstract: Geometric knot theory is the study of geometric properties of space curves that derive from their topological knottedness. Perhaps the most famous result is the Fary/Milnor theorem relating total curvature to bridge number, but the field can be dated back to work of Pannwitz on quadrisecant lines for knots. There has been a surge of interest in geometric knot theory over the past 15 years, partly due to biophysical applications to the shapes of knotted polymers like DNA molecules. One interesting problem with some surprising answers asks for the shapes of knots and links tied tight in rope of fixed thickness. We will survey recent results on this so-called ropelength problem, as well as some new strengthened versions of the theorems of Pannwitz and Fary/Milnor. We will also give a partial answer to a question of Gromov on distortion of knots.

Mittwoch, 16.12.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Ilya Molchanov (Universität Bern)

''Stability for Random Measures, Point Processes and Discrete Semi-Groups'' (Joint work with Yu. Davydov (Lille) and S. Zuyev (Goeteborg))

Abstract: .A scaling operation on non-negative integers can be defined in a randomised way by transforming an integer into the corresponding binomial distribution with success probability being the scaling factor. We explore a similar (thinning) operation defined on counting measures and characterise the corresponding discrete stablility property of point processes. It is shown that these processes are exactly Cox (doubly stochastic Poisson) processes with strictly stable random intensity measures. The paper contains spectral and LePage representations for strictly stable measures and characterises some special cases, e.g. independently scattered measures. As consequence, spectral representations are provided for the probability generating functional and void probabilities of discrete stable processes. An alternative cluster representation for discrete stable processes is also derived using the so-called Sibuya point processes that constitute a new family of purely random point processes. The obtained results are then applied to explore stable random elements in discrete semigroups, where the scaling is defined by means of thinning of a point process on the basis of the semigroup. Particular examples include discrete stable vectors that generalise the one-dimensional case of discrete random variables studied by Steutel and van Harn (1979) and the family of natural numbers with the multiplication operation, where the primes form the basis.

Mittwoch, 09.12.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Tomas Sauer (Justus-Liebig-Universität Gießen)

''Subdivision, Factorization and Algebra''

Abstract: Subdision schemes generate curves or surfaces from discrete data by means of repeated local and stationary refinement of that data, based on very simple rules. The initial data can be scalar or vector valued and the vector components can represent multiple data or derivative information. Any investigation of approximation order or smoothness (in the sense of derivatives) of the resulting limit functions naturally leads to factorization properties of certain Laurent polynomials. While in one variable things are still quite simple and all reduces to a linear factor of the form 1+z, the multivariate situation becomes more exciting as now quotient ideals in the ring of Laurent polynomials are involved, a ring that is fundamentally different from the ring of polynomials. In this talk I want to introduce some of these less freightening than natural connections and point out some consequences of this nice connection between analysis and algebra.

Mittwoch, 02.12.2009, 17 Uhr c.t., 69/125

 

Prof. Guiseppe Valla (Universität Genua)

''The Equations Defining a Projective Variety: The Case of Rational Normal Scrolls''

Abstract.

Mittwoch, 25.11.2009, 17 Uhr c.t., 69/125

 

Prof. Harry Gingold PhD (West Virginia University Morgantown)

''Compactifications and Applications''

Abstract:  The properties of the parabolic compactification are described and compared to Riemann's and Poincare's compactifications. Its applications in various fields of mathematics is discussed. In approximation theory we show how to use it as a tool for rational approximations of unbounded functions. Extensions to the celebrated theorems of Weierstrass and Fourier are obtained. In the field of nonlinear autonomous polynomial differential systems the compactification allows us to define critical points at infinity. An exact rate of blow up of solutions is derived under appropriate conditions. In the field of nonlinear nonautonomous polynomial differential systems the compactification allows us to characterize a wide family of quadratic systems where every solution exists (globally) for all t in R. The Lorenz system, for all real ranges of its parameters, is a particular autonomous case of this family and it provides an example of a "limit cycle surface" at infinity. Applications to nonlinear polynomial systems of difference equations y_n+1 = f(y_n), is also given. A certain naive expectation about the dominant terms at infinity, of a polynomial difference system or a polynomial differential system, is shown to be false..

Freitag, 20.11.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Martin Burger (Universität Münster)

''Medical Mathematics - A Future Key Discipline''

Abstract: .In this talk I will discuss some aspects in the interplay of mathematics and medicine with particular focus on two examples from cardiology: (i) The quantitative diagnosis of perfusion with PET techniques,(ii) the characterization and therapy of cardiac arrythmias using ECGI techniques. I shall discuss some mathematical questions arising from those imaging and inversion tasks, related to modelling, analysis, and numerical simulation. Moreover, I will highlight the interactive nature between the formulation of diagnostic targets and (to a lower extent) therapeutic strategies, and the formulation and solution of mathematical problems. This work is based on several collaborations with and data from the University Hospital Muenster and the European Institute for Molecular Imaging, Muenster.

Mittwoch, 18.11.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Ulrich Bunke (Universität Regensburg)

''Orbifolds and Differential K-theory''

Abstract: .

Mittwoch, 11.11.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Rudolf Grübel (Universität Hannover)

''Martin Boundary for Randomly Growing Trees''

Abstract: Binary trees are one of the standard objects in enumerative combinatorics, they also arise in connection with some important search algorithms. We regard random sequences of such trees as transient Markov chains and obtain a description of the associated Martin boundary. We establish a relationship in terms of Doob's h-transform between some well-known families of random search trees and we obtain an asymptotic representation for uniformly distributed binary trees.

Mittwoch, 04.11.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Christopher Heil (Georgia Institute of Technology)

E N T F Ä L L T !

''Music, Time-Frequency Shifts, and Linear Independence ''

Abstract: . Fourier series provide a way of writing almost any signal as a superposition of pure tones, or musical notes.  But this representation is not local, and does not reflect the way that music is actually generated by instruments playing individual notes at different times.  We will discuss time-frequency representations, which are a type of local Fourier representation of signals.  This gives us a mathematical model for representing music.  While the model is crude for music, it is in fact a powerful mathematical representation that has appeared widely throughout mathematics (e.g., partial differential equations), physics (e.g., quantum mechanics), and engineering (e.g., time-varying filtering).  We ask one very basic question: are the notes in this representation linearly independent?  This seemingly trivial question leads to surprising
mathematical difficulties.

E N T F Ä L L T !

 

Prof. Dr. Stefan Schröer (Universität Düsseldorf)

''Tensor Products of Irreducible Vector Bundles on Elliptic Curves''

Abstract: The group of isomorphism classes of homogeneous vector bundles on an elliptic curve was determined by Atiyah, who also unravelled its ring structure over the complex numbers. Using Fourier-Mukai transformations, I prove some results on the ring structure in characteristic p>0, which has completely different features.

Mittwoch, 21.10.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Uri Yechiali (Tel Aviv University and Afeka College of Engineering)

''TCP, Polling, and the M/G/I Bulk-Service Queue''

Abstract: The performance of a Transmission Control Protocol (TCP) in a system with N connections sharing a common active queue management (AQM) is analyzed for both Additive-Increase Multiplicative-Decrease (AIMD), and Multiplicative-Increase Multiplicative-Decrease (MIMD) control mechanisms, where reduction signals follow either a cyclic, or a probabilistic, polling-type procedure. The Laplace-Stieltjes Transforms (LST) of the transmission rate of each connection at a polling instant, as well as at an arbitrary moment, are derived. Explicit results are obtained for the mean rate and (in contrast to most polling models) for its second moment.The analysis of the probabilistic MIMD models uses transformations yielding a system-law-of-motion equivalent to that of an M/G/1 queue with bulk service. We thus incorporate the analytical study of a TCP system with notions of Polling Systems and the M/G/1 bulk-service queue, and combine the study of an actual operating communication protocol with queueing theory.

Mittwoch, 23.09.2009, 15 Uhr c.t., 69/125

 

Prof. Dr. Wojciech Gajda (Adam Mickiewicz University Poznan, Polen)

"Arithmetic of abelian varieties and Galois representations"

Abstract: The lecture is meant for the general math public. First we will survey the state of art of Birch & Swinnerton-Dyer conjecture and computations of ranks of Mordell-Weil groups of some abelian varieties. In the second part of the talk we'll focus our attention at certain local-global principle for checking linear dependences in the MW groups. We will conclude discussing briefly Galois representation associated in a natural manner to torsion points of an abelian variety. No prior knowledge of the subject will be assumed.

Mittwoch, 15.07.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Volkmar Welker (Philipps-Universität Marburg)

''On the Combinatorics and Algebra of Permutations''

Abstract: We describe combinatorial matrices that after scaling generalize the matrix of the random to random shuffle and are related to the Varchenko matrix associated to the reflection arrangement of the symmetric group. Our matrices can be interpreted as elements of the group algebra of the symmetric group and generate a subalgebra of the group algebra. We describe results on the matrices and their algebra. In addition, we provide results for generalizations within the symmetric group and other Coxeter groups.

Mittwoch, 08.07.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Demetrio Labate (University of Houston)

''Inversion of the Radon Transform Using the Shearlet Decomposition''

Abstract: In many practical problems, such as the determination of the locations of sources of radiation from measurements taken at an array of sensors, the features of most interest cannot be observed directly, but must be inferred from other observable quantities. Since these inverse problems are typically ill-posed, some form of regularization must be applied, with the result that important features to be recovered are lost, as evident in imaging applications where the regularized reconstructions are blurred versions of the original. The aim of this talk is to describe how the shearlet representation, a method which is optimally efficient in dealing with edges and other sharp discontinuities, can be applied to provide a very efficient regularization scheme for the inversion of certain important classes of operators. In particular, our shearlet-based method is especially efficient in dealing with the classical problem of noisy Radon inversion and it outperforms similar competing strategies using wavelets and curvelets.

Mittwoch, 01.07.2009, 17 Uhr c.t., 69/125

 

Prof. Rick Jardine PhD (University of Western Ontario)

''Path Categories and Concurrency''

Abstract: The path category P(K) of a simplicial complex K is a category which is built from vertices (objects) and 1-simplices (morphisms), subject to commutativity conditions associated to the 2-simplices of K. This construction extends to a functor from simplicial sets to categories which is left adjoint to the nerve. Here is why one cares: path category morphisms specialize to execution paths in higher dimensional automata. These objects are geometric models for behaviour of parallel processing systems, and techniques are required to distinguish execution paths between states in such a system. This calculational problem is non-trivial, since the path category functor is not a standard homotopy invariant and produces categories with little extra structure. The known viable lines of attack arise from higher category theory and homotopy coherence theory.

Freitag, 26.06.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Wolfgang Dahmen (RWTH Aachen)

''Compressed Sensing''

Abstract: The classical paradigm for signal processing is to model a signal as a bandlimited function and capture it by means of its time samples. The Shannon-Nyquist theory says that the sampling rate needs to be at least twice the bandwidth in order to recover the signal completely. However, for broadbanded signals, such high sampling rates may be impossible to implement in circuitry. Compressed Sensing is a new paradigm for signal processing whose aim is to circumvent this dilemma by sampling signals closer to their information rate instead of their bandwidth. Rather than model the signal as bandlimited, Compressed Sensing assumes that the signal can be represented or approximated by a few suitably chosen terms from a basis expansion of the signal. It also enlarges the concept of sample to include the application of any linear functional applied to the signal. We give a brief introduction to Compressed Sensing that centers on the effective- ness and implementation of random sampling. Specifically, we highlight some relevant mathematical concepts relating to Banach space geometry, dimension reduction and random matrices, formulate the notion of instance optimality as a performance bench-mark that applies also to non-sparse signals and sketch instance optimal decoding techniques

Mittwoch, 24.06.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Tammo tom Dieck (Universität Göttingen)

''Duality in Algebraic Topology''

Abstract: Duality is a classical topic in algebraic topology (Poincare, Alexander, Spanier-Whitehead). More recently  A. Dold and D. Puppe found an elementary approach to duality which uses only simple constructions of homotopy theory. It turned out that their theory is the basis for the other classical duality theorems. It also fits with the duality in tensor categories. In my talk I will report on some aspects of the Dold-Puppe theory and its consequences. Its elementary character suggests that it is the correct theory for introductory courses in algebraic topology.

Mittwoch, 10.06.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Günter Last (Universität Karlsruhe)

''Stationary Random Measures and the Mass-Transport Principle''

Abstract: We consider stationary random measures on a locally compact Abelian group G. In the first part of the talk we discuss invariant transport kernels balancing two given random measures. Such kernels can be used to connect the Palm measures associated with the random measure via a so-called transport formula. The well-known deterministic mass-transport principle for invariant (determinsitic) measures on G times G is a special case of this formula. Recent years have seen many successful applications of this principle, for instance, in the theory of random graphs. In the second part of the talk we discuss some extensions to homogeneous and more general spaces. If G operates proper in a certain sense, then the mass-transport principle can be proved in a non-transitive and non-unimodular setting. Parts of the talk are based on joint work with Hermann Thorisson (Reykjavik) and Daniel Gentner (Karlsruhe).

Mittwoch, 03.06.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Birgit Richter (Universität Hamburg)

''Some Features of Algebraic K-Theory of Brave New Rings ''

Abstract: Classical algebraic K-theory tells us something about arithmetic properties of rings. This theory can be extended to investigate cohomology theories that are multiplicative in a very structured way. Such cohomology theories are representable by structured ring spectra, alias brave new rings. Considering ordinary singular cohomology of spaces with coefficients in a ring bridges the two worlds. We will present some examples and structural properties of algebraic K-theory of ring spectra.

Mittwoch, 27.05.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Michael Joswig (Technische Universität Berlin)

''Tropical Convexity''

Abstract: Tropical polytopes (introduced by Develin and Sturmfels) are objects dual to regular subdivisions of products of two simplices. The lecture will explain the role of this concept in various areas, for example graph algorithms, geometric combinatorics, commutative algebra and algebraic geometry.

Mittwoch, 20.05.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Christine Bessenrodt (Leibniz Universität Hannover)

''Blocks of Characters of Finite Groups''

Abstract: The theory of blocks of characters of finite groups was developed by R. Brauer with the aim of proving results on the structure of these groups and their representations. For a fixed prime number p dividing the order of the finite group G, the irreducible complex characters of G are distributed into the p-blocks of G; their invariants are central topics in the p-modular representation theory of finite groups. In recent years, the relationship between the block distributions of characters at different primes has been investigated more closely. In joint work with J. Zhang, we have found a new nilpotency criterion for finite groups in terms of separation properties of blocks, and we have also shown that the covering of its characters by certain blocks implies strong structural consequences for the group.

Mittwoch, 13.05.2009, 17 Uhr c.t., 69/125

 

Prof. Ngo Viet Trung (Vietnamese Academy of Science and Technology)

''Normal Monomial Ideals and Matrices with Integer Rounding Properties''

Abstract: Normal monomial ideals are subjects in Commutative Algebra. Matrices with integer rounding properties come from combinatorial optimization. We shall see that these notions are strongly related to each other and that this relationship has interesting consequences for both fields.

Mittwoch, 06.05.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Christopher Deninger (Universität Münster)

''Number Theory, and Analysis on One-Codimensional Foliated Spaces''

Abstract: We discuss a kind of dictionary between number theory and aspects of the theory of partial differential operators on a certain kind of foliated spaces. In particular we discuss the analogues of the Riemann hypothesis and of a recent conjecture of Lichtenbaum on special values of Hasse-Weil zeta functions.

Mittwoch, 29.04.2009, 17 Uhr c.t., 69/125

 

Prof. Dr. Holger Rauhut (Universität Bonn)

''Sparse Recovery''

Abstract: In recent years sparsity has become a key concept in applied mathematics, in particular, signal processing. The basic observation is that many types of signals can be represented well by a small number of non-vanishing coefficients with respect to a suitable basis, that is, by a sparse representation. This is for instance the reason why compression techniques such as JPEG or MPEG work so well. In 2004, it was observed by Candes, Romberg and Tao, and independently, by Donoho, that sparsity can also be exploited for measuring or capturing signals efficiently, that is, by using only a small number of linear non-adaptive measurements. This principle is called compressed sensing, compressive sampling, or sparse recovery. The talk gives an introduction and overview on sparse recovery with an emphasis on results obtained by the speaker. In particular, we discuss recovery of sparse trigonometric polynomials from a small number of random samples, recovery of sparse time-frequency representations, connections of compressed sensing to Gelfand widths as well as reconstruction of jointly sparse signals.

Mittwoch, 22.04.2009, 17 Uhr c.t., 69/125

 

Dr. Wang-Q Lim (Universität Osnabrück)

''Geometric Image Representations with Shearlets''

Abstract: Standard wavelets bases are optimal to represent functions with pointwise singularities. However they fail to capture the geometric regularity along the singularities of surfaces, because of their isotropic support. In fact, sharp image transitions such as edges are expensive to represent and integrating the geometric regularity in the image representation is a key challenge to improve state of the art applications to image processing. In order to go one step further, today image processing algorithms try to exploit the geometrical regularity of the underlying function and several approaches have been recently proposed. In this talk, I will briefly discuss some of those approaches and their main drawbacks. Finally, I will introduce a new image representation scheme which can resolve those drawbacks and provides efficient geometric representations. Some numerical results also will be presented.

Mittwoch, 18.02.2009, 17 Uhr c.t., 69/E18

 

Prof. Pete Casazza (University of Missouri)

''Applications of Hilbert space frames''

Abstract: Hilbert space frames have traditionally been used in signal/image processing.Recently, there have arisen a variety of new applications to wireless communication, internet coding, physics, Biomedical Engineering, speech recognition technology, distributed processing and more. We will look give a brief introduction to Hilbert space frame theory and then look at some of the new applications of frame theory and how frame theory has begun to impact some of the most famous unsolved problems in ``pure'' mathematics.

Mittwoch, 28.01.2009, 17 Uhr c.t., 69/E18

 

Jun.-Prof. Hannah Markwig (Georg-August-Universität Göttingen)

''Tropical Hurwitz numbers''

Abstract: Hurwitz numbers count genus g, degree d covers of the projective line, with specified ramification profile over a fixed set of points. In tropical geometry, algebraic curves are degenerated to certain piece-wise linear graphs called tropical curves. This process ``loses a lot of information'', but many properties of the algebraic curve can be read off the tropical curve, and many theorems that hold for algebraic curves remarkably continue to hold on the tropical side. One example are for this are Hurwitz numbers: In this talk, we define tropical Hurwitz numbers, i.e. tropical genus g, degree d covers of the tropical projective line, and we show that the thus defined tropical Hurwitz numbers are equal to their classical counterparts. This equality leads to new insights in the structure of Hurwitz numbers.

Donnerstag, 22.01.2009, 17 Uhr c.t., 69/127

 

Prof. Dr. Matthias Kreck (Direktor des Hausdorff Research Institute for Mathematics Bonn)

''Codes und 3-Mannigfaltigkeiten''

Abstract: I will start the lecture by defining codes and explaining their basic idea. Then I will explain how manifolds on which an involution with finitely many fixed points acts give a code. Applying this to 3-dimensional manifolds gives particularly interesting codes: Self dual codes. I will explain that these codes are very special since they lead to unimodular lattices. An obvious question is which self dual codes come from 3-manifolds. In joint work with Volker Puppe we give a complete answer. Finally I will define a geometric condition for 3-manifolds with such an involution which leads to so called doubly even codes.

Mittwoch, 14.01.2009, 17 Uhr c.t., 69/E18

 

Felix Krahmer (Courant Institute of Mathematical Sciences, USA)

''New Results on Stability and Accuracy of One-Bit Quantization''

Abstract:  Sigma Delta-modulation is a popular class of analog-to-digital conversion schemes for  audio signals, which are modeled as bandlimited functions in this context. Sigma Delta-schemes are oversampled one-bit quantization schemes, i.e., the function is sampled at a certain frequency lambda., and the ±1-valued sequence of quantized values tracks local averages of the sampled function values. Exploiting the redundancy of the sequence of sampled values, such a procedure can lead to fast decay rates for the reconstruction error as lambda is increased.

In this talk, I will present a family of schemes that gives rise to the best error  decay rate currently known for oversampled one-bit quantization. Furthermore, I will provide a rigorous error analysis for a family of schemes with linear quantization rules, a class of rules commonly used in practice. In this context, I  will also discuss a novel, generalized stability criterion for Sigma Delta-modulation. The results are joint work with Percy Deift and Sinan Güntürk.

Freitag, 09.01.2009, 17 Uhr c.t., 69/E15

---

2008

Prof. Dr. De Witt Sumners (Florida State University, USA)

''Random Knotting and Viral DNA Packing''

Abstract: Bacteriophages are viruses that infect bacteria. They pack their double-stranded DNA genomes to near-crystalline density in viral capsids and achieve one of the highest levels of DNA genome condensation found in nature. Despite numerous studies, some essential properties of the packaging geometry of the DNA inside the phage capsid are still unknown. Although viral DNA is linear double-stranded with sticky ends, the linear viral DNA quickly becomes cyclic when removed from the capsid, and for some viral DNA the observed knot probability is an astounding 95%. This talk will discuss comparison of the observed viral knot spectrum with the simulated knot spectrum, concluding that the packing geometry of the DNA inside the capsid is non-random and writhe-directed. Simulations of DNA knotting in confined volumes with and without volume exclusion will be discussed.

Donnerstag, 11.12.2008, 17 Uhr c.t., 69/127

Prof. Zuowei Shen (National University of Singapore)

''Tight Frame Approach for Missing Data Recovery''

Abstract: In many practical problems in image processing, the observed data sets are often in complete in the sense that features of interest in the image are missing partially or corrupted by noise. The recovery of missing data from incomplete data is an essential part of any image processing procedures whether the final image is utilized for visual interpretation or for automatic analysis. It is interesting to know that incomplete data can be recovered by over completed system, especially frames. The unitary extension principle provides a great flexibility of designing tight frame wavelet filters that makes the construction of tight frame wavelets absolutely painless. In this talk, I will start by introducing the unitary extension principle that is followed by a brief review of some new theoretic development in the field based on or motivated by this principle. In the second part of this talk, I will discuss our new iterative algorithm for image recovery for missing data which is based on tight framelet systems constructed by the unitary extension principle. We consider in particular few main applications in image processing, inpainting, impulse noise removal, super-resolution image reconstruction and compressed senssing.

Donnerstag, 27.11.2008, 17 Uhr c.t., 69/127

Dr. Jacob Lemvig (Technical University of Denmark)

''Affine and quasi-affine frames for rational dilations''

Abstract

Dienstag, 18.11.2008, 17 Uhr c.t., 69/117

Prof. Dr. Ross Staffeldt (New Mexico State University)

''Introduction to the topology of data sets''

Abstract: It is becoming more important to recognize features of high-dimensional, highly non-linear data sets. For example, type I and type II diabetes were recognized and defined after a subtle statistical analysis of a 5-dimensional data set revealed a geometric structure consisting of a central core, to which two thinner lobes were attached. One lobe corresponds to early-onset diabetes and the other to adult-onset diabetes.Although standard methods, such as principal component analysis, can assign a dimension to a data set, direct methods for discovering voids and clusters at different levels of resolution in high dimensional and highly nonlinear data sets are being sought. Persistent homology is a new tool that holds promise for exploring large data sets for qualitative geometric features appearing at different levels of resolution. I will first describe parametrized constructions that associate a family of topological spaces to a set of "point cloud data." Next I will describe persistent homology, which is an algorithm that associates a "bar code" to each family, just as homology assigns Betti numbers to a topological space. Then we will look at a results of a trial run. Exploration of applications of persistent homology has only recently begun and is often driven by heuristics, so many questions remain open for exploration.

Donnerstag, 26.06.2008, 16 Uhr c.t., 69/E15

Raman Sanyal (Technische Hochschule Berlin)

''Equivariant rigidity and the 3^d-conjecture in dimension 4''

Abstract: The 3^d-conjecture states that every centrally-symmetric d-polytope has at least 3^d non-empty faces. While the claims in dimensions one, two, and three are, respectively, vacuous, clear, and easy to prove, the 3^d-conjecture in dimension d=4 was open until recently. A major ingredient for the proof is a tight lower bound on the flag-functional g₂ for the class of centrally-symmetric polytopes. The quantity g₂ can be defined in terms of Betti numbers of the (intersection) cohomology of an associated toric variety but, surprisingly, it also turns up in the context of the rigidity of frameworks. In this talk we focus on the relation to rigidity and show how to derive the lower bound on g₂ by considering frameworks under group actions. This is joint work with Günter Ziegler and Axel Werner.

Dienstag, 17.06.2008, 16 Uhr c.t., 69/118

Dr. Achim Wübker (Universität Göttingen)

''L²-spectral gaps for Markov Operators induced by Markov Chains''

Abstract: We analyze the spectral properties of Markov operators induced by Markov chains with general state space on certain L²-spaces. Especially we are interested in the question, whether the associated Markov-operator has the spectral gap property or not. For this we work out three approaches: The first one is based on a work of Lawler & Sokal. We extend this by defining a family of constants in order to obtain necessary and sufficient conditions for the existence of an L²-spectral gap. These conditions are simplified in the case of reversible and weak reversible chains. The second approach uses entropy, especially some growth of entropy associated to the Markov-operator. Defining two different growth conditions, we get again necessary and sufficient conditions for the existence of L²-spectral gaps. Finally we investigate the connections between a.s. geometric ergodicity and the spectral gap property and obtain some new results for the non-reversible case.

Freitag, 13.06.2008, 17 Uhr c.t., 69/E15

Dr. Ann Lemahieu (Katholieke Universiteit Leuven)

''The monodromy conjecture for nondegenerate surface singularities''

Abstract: The monodromy conjecture predicts a relation between the geometry and the topology of a singularity. In particular, it says that a pole s₀ of the local topological zeta function in 0 induces an eigenvalue of monodromy e^{2i pi s₀} at a point in the neighbourhood of 0. When the singularity is given by a polynomial that is nondegenerate with respect to its Newton polyhedron, then one can express the local topological zeta function and the zeta function of monodromy in terms of the Newton polyhedron. We analyze these formulas for surface singularities: we provide a set of monodromy eigenvalues and a set of false candidate poles. In this way we obtain a proof for the monodromy conjecture for nondegenerate surface singularities.

Dienstag, 10.06.2008, 16 Uhr c.t., 69/118

Prof. Dr. Ulrich Koschorke (Universität Siegen)

''Fixed Points, Coincidences and Kervaire Invariants''

Abstract:  In the 1920s Salomon Lefschetz and Jakob Nielsen presented groundbreaking work on fixed points of continuous maps.This inspired much topological research in the subsequent decades.We will review some of the classical results and then turn to very recent developments concerning fixed points and, more generally, coincidences. Given two maps between manifolds,we study the geometry of their coincidence locus (using nonstabilized normal bordism theory and pathspaces). We extract an invariant which must necessarily be trivial if the two maps can be deformed away from one another. Often this is also sufficient. Surprisingly however, in certain cases the full answer involves also the Kervaire invariant (which was originally introduced and used in an entirely different area of topology, namely: manifolds without smooth structures and exotic spheres). Similarly other central notions of  topology turn out to play a crucial role here, e.g. various versions of Hopf  invariants (a la James, Hilton, Ganea...).

Freitag, 06.06.2008, 17 Uhr c.t., 69/E15

Prof. Dr. Ulrike Tillmann (University of Oxford)

''Mumford's conjecture and beyond''

Abstract: . Mumford's conjecture - proved by Madsen and Weiss a few years ago - concerns the cohomology of Riemann's moduli space. In my lecture I will review some of its history, and explain how it is part of a more general theory.

Freitag, 16.05.2008, 17 Uhr c.t., 69/117

Dr. Sadok Kallel (University of Lille)

''Analogs of Configuration spaces and Applications''

Abstract: We will introduce and discuss two constructions reminiscent of configuration space type constructions. These are filtered constructions associated to a given space X. We can in both cases completely determine their homology in terms of the (co)homology of X. The first such construction is the space of barycenters of X and its study offers applications to non-linear analysis. This is the space of all simplices "generated" by points of X. If we only consider 1-simplices then we obtain the space of chords, a known construct in differential geometry. The second construction consists of spaces of finite subsets of X. These go back to Borsuk but have had some relevance recently in the determination by Galatius of the classifying space of the group of stable automorphisms of free groups.

Freitag, 25.04.2008, 17 Uhr c.t., 69/E15

Prof. Dr. Aldo Conca (Università di Genova)

''Integrally closed and componentwise linear ideals''

Abstract: In a two dimensional regular local ring  integrally closed ideals have a unique factorization property and have a Cohen-Macaulay associated graded ring. In higher dimension these properties do not hold for general integrally closed ideals. Our goal is to identify a subclass of integrally closed ideals for which they do. We restrict our attention to 0-dimensional homogeneous ideals in polynomial rings R of arbitrary dimension and identify a class of integrally closed ideals, the Goto-class G*, that is closed under product and that has a suitable unique factorization property. Ideals in G* have a Cohen-Macaulay associated graded ring if either they are monomial or dim R=3. Our approach is based on the study of the relationship between the notions of integrally closed, contracted, full and componentwise linear ideals.

Donnerstag, 7. Februar 2008, 17 Uhr c.t., 69/E23

Prof. Dr. Bernd Sturmfels (UC Berkeley and TU Berlin)

''Can Biology Lead To New Theorems?''

Abstract: This lecture argues for an affirmative answer to the question in the title. In future interactions between mathematics and biology, both fields will contribute to each other, and, in particular, research in the life sciences will inspire new theorems in pure mathematics. This opinion is illustrated by four theorems in combinatorics and algebra.

Freitag, 25. Januar 2008, 17 Uhr c.t., 69/E15

2007:

Priv.-Doz. Dr. Gitta Kutyniok (Stanford University)

``Frames, Fusion Frames und optimale Packungen''

Abstract:Eine Vielzahl von modernen Anwendungen, u.a. die Bild- und Signalverarbeitung und die Kommunikationstheorie, benötigen Systeme, die robuste, stabile und normalerweise nicht-eindeutige Darstellungen von Vektoren eines Hilbertraums liefern, da die klassischen Orthonormalbasen jegliche Robustheit vermissen lassen. Diese sogenannten Frames sind das zentrale Studienobjekt des recht jungen Forschungsgebietes der Frame-Theorie, welche z.B. Analysemethoden des Redundanzgrades von Frames und Konstruktionsmethoden liefert und ferner seit kurzem auch verwandt wird, um klassische offene Vermutungen aus einem anderen Blickwinkel zu betrachten. In diesem Vortrag werde ich zunächst eine allgemeine Einführung in die Frame-Theorie geben. Anschließend werde ich eine Erweiterung von Frames - sogenannte Fusion Frames - vorstellen, die ich gemeinsam mit P. Casazza (U. Missouri) zum Zweck der Modellierung von Anwendungen mit verteilter Struktur entwickelt habe. Wir werden dann die Robustheit dieser neuen Objekte untersuchen und feststellen, dass sich eine überraschende Verbindung zu optimalen Packungen herstellen lässt. Dies ist eine gemeinsame Arbeit mit A. Pezeshki, A. R. Calderbank und T. Liu (Princeton U.).

Freitag, 23. November 2007, 17 Uhr c.t., 69/E15

 

Prof. Uwe Nagel (University of Kentucky, USA)

``Hilbert-Funktionen von Gorenstein-Algebren''

Abstract: Gorenstein-Algebren spielen z.B. in der Dualitätstheorie, Kombinatorik und Topologie eine wichtige Rolle. Numerische Informationen über eine solche Algebra sind in der Hilbert-Funktion codiert. An ihr kann man z.B. die Dimension und die Multiplizität der Algebra ablesen. Daher ist man an einer guten Beschreibung der möglichen Hilbert-Funktionen interessiert. Darüber ist aber wenig bekannt. Im Vortrag werden einige neuere Resultate und offene Probleme diskutiert.

Freitag, 9. November 2007, 17 Uhr c.t., 69/E15

 

Prof. Joseph Gubeladze (San Francisco)

``Commutator automorphisms of formal power series rings''

Abstract: A K-theoretical framework for some well known conjectures on automorphisms and idempotent endomorphisms of polynomial rings was initiated at the end of the 1970s. These conjectures proved quite intractable. There are two natural variations of the mentioned nonlinear K-theory that makes things easier and tractable. One variant leads to the s.c. polytopal K-theory}, developed in a joint work with W. Bruns. In this talk we consider the other variation that corresponds to the completion process. It is shown that for a big class of commutative rings R every continuous R-automorphism of R[[X₁,...,X_n]] with the identity linear part is in the commutator subgroup of Aut(R[[X₁,...,X_n]]). An explicit bound for the number of the involved commutators and a K-theoretic interpretation of this result will be provided.

Donnertag, 23. August  2007, 17 Uhr c.t., 69/E15

 

Prof. Adam van Tuyl (Lakehead University, Canada)

``The edge ideals of chordal graphs''

Abstract: Given a simple (no loops or multiple edges) graph G, one can associate to G a quadratic squarefree monomial ideal I(G) in the polynomial ring R = k[x₁,...x,_n]. It is then natural to ask how the properties of G are reflected in I(G) and vice versa. In this talk I will discuss some of my recent projects on this question. In particular, I will talk about the graded Betti numbers of the edge ideal and the sequentially Cohen-Macaulayness of R/I(G). I will highlight the case that G is a chordal (or triangulated) graph; in this situation the edge ideal has nice properties, e.g., the graded Betti numbers can be computed recursively. I will also discuss my recent work on developing a hypergraph analog of chordal graphs to study squarefree monomial ideals.

Freitag, 18. Mai  2007, 17 Uhr c.t., 69/E15

 

Prof. Rekha Thomas (Seattle)

``The Small Chvatal Rank of an Integer Matrix (joint work with Tristram Bogart)''

Abstract: The small Chvatal rank is a new measure of complexity of the integer hulls of rational polyhedra. The Chvatal rank of an integer matrix A is the maximum of the minimum number of rounds of cuttting planes you have to add to any polyhedron of the form Ax <= b to obtain its integer hull. In this talk I will define a related notion called the small Chvatal rank of A which is the minimum number of rounds of an iterated Hilbert basis procedure on the rows of A to obtain all facet directions of the integer hulls of all polyhedra of the form Ax <= b. The small Chvatal rank is bounded above by Chvatal rank and is hence finite. We give families of matrices where the small Chvatal rank is small (even constant) while the Chvatal rank can be arbitrarily high. We give various other examples of the two ranks for families of well-studied polytopes in combinatorial optimization. On the negative side, we show that small Chvatal rank is not a function of dimension (i.e., the number of columns of A) and can be arbitrarily high even when n=3. Finally we relate this notion to that of supernormality, a concept introduced in the study of toric varieties and state some open problems.

Freitag, 11. Mai  2007, 17 Uhr c.t., 69/E15

2006:

Prof. Eugene Levner (Holon Institute of Technology, Holon, Israel)

``Cyclic Scheduling of Robotic Cells with Fixed Robot Routes and Time Windows''

Abstract: Consider single-robot cyclic scheduling problems with a fixed robot operation sequence and time window constraints on processing times. The objective is to minimize the cycle time. We prove that this problem is equivalent to the parametric critical path problem, and propose strongly polynomial-time solution algorithms based on a new node labeling scheme. In the multiple-product case, the algorithm complexity, even though not as good as that for a single-product case, still remains strongly polynomial. These results are obtained together with Vladimir Kats (Institute for Applied Mathematics, Beer Sheva, Israel) and Lei Lei (Rutgers Univesity, Newark, USA).

Montag, 19. Juni  2006, 11 Uhr c.t., 69/E18

 

Prof.  Dr. Hubert Kalf  (Universität München)

``Zur Spektraltheorie des Diracoperators für Teilchen mit einem anomalen magnetischen Moment''

Abstract: Der Diracoperator beschreibt ein geladenes Teilchen mit positiver Ruhmasse, Spin 1/2 und magnetischem Moment 1 (bei Verwendung geeigneter Einheiten) in einem elektromagnetischen Feld. In den 30er Jahren schlug Pauli eine Modification dieses Operators vor, um Teilchen mit magnetischem Moment 1+a zu beschreiben (für das Elektron ist a ungefähr 10(-3)). Für das Coulomb-Potential wird gezeigt, dass die Eigenwerte dieses modifizierten Operators gegen die des üblichen Diracoperators streben, wenn a gegen Null geht, obwohl die Lösungen der Gleichung für a=0 bzw. a=/0 völlig verschieden sind. Es handelt sich also um ein singuläres Störungsproblem.

Freitag, 9. Juni  2006, 17 Uhr c.t., 69/E15

 

Prof. Srikanth Iyengar (University of Lincoln, Nebraska, USA)

``Class and rank of differential modules''

Abstract: A differential module over a ring is a module equipped with a square-zero endomorphism. Complexes of modules are a good source of differential modules, but there are many more of the latter. I will discuss aspects of recent joint work with Avramov and Buchweitz, where we prove a "Class inequality" for differential modules. This is related to the New Intersection Theorem in commutative algebra, and to work of Gunnarson Carlsson on group actions on finite CW complexes.

Dienstag, 30. Mai  2006, 16.15 Uhr , 69/E15

 

Dr. Henrik Holm (University Aarhus)

``Compactly generated homotopy categories''

Abstract: Let Mod R be the category of left modules over a ring R. To any additive subcategory X of Mod R one can associate its homotopy category K(X) [the objects of K(X) are chain complexes consisting of modules from X, and the morphisms of K(X) are chain maps modulo homotopy equivalence]. K(X) carries the structure of a triangulated category, and if X has set-indexed coproducts, then so has K(X).
Under weak assumptions on R, Peter Jørgensen (2005) has proved that K(Proj R) is compactly generated. From this result he inferred that the category of Gorenstein projective R-modules is precovering if R is commutative, noetherian, and admits a dualizing complex.
When R is left noetherian, Henning Krause (2004) has proved that K(Inj R) is compactly generated. He and Srikanth Iyengar (2005) used this result to get a new characterization of commutative Gorenstein rings in terms of (totally) acyclic complexes of injective modules.
On the other hand, Amnon Neeman (2001) has proved that K(Mod Z) [Z being the ring of integers] is not compactly generated.
In the talk I will discuss some sufficient conditions on X and R which ensure that K(X) is compactly generated. It turns out that rings of finite pure global dimension play a central role. This is joint work (and work in progress) with Peter Jørgensen, University of Leeds.

Freitag, 31. März  2006, 17 Uhr c.t., 69/E23

 

Prof. Peter Stollmann (TU Chemnitz)

``Typische Maße sind singulär stetig''

Abstract: Der Satz von Baire hat einige zunächst sehr erstaunliche Konsequenzen: So sind typische stetige Funktionen nirgends differenzierbar. In diesem Vortrag geht es um ein analoges Resultat für Maße, das aus einer gemeinsamen Arbeit mit D. Lenz stammt.

Freitag, 27. Januar 2006, 17 Uhr c.t., 31/E05

 

Dr. Johannes Brasche (TU Clausthal)

``Über eine Dynkinformel und die Approximation von Schrödingeroperatoren mit unendlich hohen Potentialbarrieren''

Abstract

Freitag, 20. Januar 2006, 17 Uhr c.t., 31/E05

 

Dr. Xinxian Zheng  (Universität Essen))

``Cohen-Macaulay graphs''

Abstract

Freitag, 20. Januar 2006, 16 Uhr c.t., 69/125

2005:

Prof. Kohji Yanagawa (Osaka University)

``Castelnuovo-Mumford regularity for complexes and componentwise linear modules''

Abstract: The "Castelnuovo-Mumford regularity" of a finitely generated graded module over a polynomial ring S=k[x₁,...,x__n] is an important invariant in modern commutative algebra. Recently, P. Jorgensen shows that this concept can be naturally generalized to bounded complexes.
The Castelnuovo-Mumford regularity of a complex has a natural meaning under the "Bernstein-Gel'fand-Gel'fand correspondence" (the derived equivalence between S and the exterior algebra E=k<y₁,...,y_n>). As an application, I will refine a result of Herzog and Römer concerning monomial ideals of E.

Freitag, 4. November 2005, 17 Uhr c.t., 31/E05

 

Dr. Leila Khatami (IPM Teheran)

``Gorenstein dimensions''

Abstract: Gorenstein dimensions are relative homological dimensions which can be considered as "refinements" of classical homological dimensions. In this talk, we will review the fundamental definitions and results of the theory of the Gorenstein dimensions over commutative noetherian rings and will also state some new results in the field.

Mittwoch, 19. Oktober 2005, 17 Uhr c.t., 31/E06

 

Dr. Johan van Leeuwaarden (EURANDOM, Eindhoven)

``Tail asymptotics for a tandem queue with coupled processors''

Abstract: We consider the two-stage tandem queue with coupled processors. It is assumed that jobs arrive at the first station according to a Poisson process and require service at both stations before leaving the system. The amount of work that a job requires at each of the stations is an independent, exponentially distributed random variable. When both stations are nonempty, the total service capacity is shared among the stations according to fixed proportions. When one of the stations becomes empty, the total service capacity is given to the nonempty station. We derive asymptotic expressions for the stationary queue length distribution. The biggest challenge in obtaining these expressions is the analytic continuation of P(x,y) defined as the joint pgf of the stationary queue length at both stations. We manage to obtain the analytic continuation by exploiting the specific properties of a functional equation that implicitly defines P(x,y). The analytic continuation of P(x,y) results in knowledge on the dominant singularity of P(x,y). By investigating the behavior of P(x,y) in the vicinity of the dominant singularity, we are able to derive asymptotic expressions. There are some competing methods for deriving asymptotic expressions. We discuss these methods and point out the similarities and differences with our analytic approach.

Montag, 29. August 2005, 15 Uhr c.t., 31/449a

 

Prof. Andrew Baker (University of Glasgow)

``Galois theory for commutative S-algebras''

Abstract: I will describe an extension (due to John Rognes) of the Galois theory of commutative rings to commutative S-algebras in the sense of Elmendorf, Kriz, Mandell and May. I will focus especially on the case of finite Galois groups although far more general versions are possible.

Freitag, 8. Juli 2005, 17 Uhr c.t., 31/E05

 

Dr. Hendrik Vogt (Technische Universität Dresden)

``Stabilität Gaußscher Abschätzungen für Propagatoren unter Potentialstörungen''

Abstract

Freitag, 1. Juli 2005, 17 Uhr c.t., 31/E05

 

Prof. J. K. Verma (IIT Mumbai, Indien)

``Hoskin-Deligne formula and Blowup algebras of complete ideals in 2 dimensional regular local rings''

Abstract: The Hoskin-Deligne formula describes the colength of a complete m-primary ideal I in
a two dimensional regular local ring (R,m) in terms n-adic orders of the transforms of I in all the two
dimensional regular local rings (S,n) birationally dominating (R,m). We will sketch a simple proof of this and derive several consequences due to Zariski, Rees, Lipman, Teissier, Huneke-Sally and Verma concerning complete ideals and Cohen-Macaulay property of blow-up algebras of such ideals.

Freitag, 24. Juni 2005, 17 Uhr c.t., 31/E05

 

Dr. Marius Vladoiu, (University of Bucharest)

``Polymatroidal ideals''

Abstract: Discrete polymatroids were introduced by Herzog and Hibi in connection with the concepts of matroids and polymatroids. I will present some well-known conjectures in the field and some properties of the two algebraic objects associated to a discrete polymatroid: polymatroidal ideals and the so called base rings.

Freitag, 24. Juni 2005, 16 Uhr c.t., 31/E05

 

Prof. Dr. Uwe Nagel (University of Kentucky)

``Die Anzahl der fehlenden Simplizes eines simplizialen Polyeders''

Abstract: Ein Polyeder ist die konvexe Hülle von endlich vielen Punkten. Es ist simplizial, wenn alle seine echten Seiten Simplizes sind. Ein fehlender Simplex des Polyeders ist eine kleinste Teilmenge seiner Eckenmenge, die keine Seite ist. Fehlende Simplizes spielen eine wichtige Rolle in der Klassifikation simplizialer Polyeder. Anfang der achtziger Jahre stellten Kalai, Kleinschmidt und Lee eine Vermutung über die Polyeder auf, welche die Anzahl der fehlenden Seiten fester Dimension maximieren. Im Vortrag werden wir den Beweis dieser Vermutung und die daraus resultierenden expliziten Schranken diskutieren.

Dienstag, 14. Juni 2005, 18 Uhr c.t., 31/E05

 

Prof. Ngo Viet Trung (Hanoi, Vietnam)

``Mixed volumes of polytopes versus mixed multiplicities of ideals''

Abstract: In this talk I will describe the relationship between mixed volumes of lattice polytopes and mixed multiplicities of ideals. This relationship gives new nsights into both the theory of mixed volumes and that of mixed multiplicities. In particular, we can give a purely algebraic proof of Bernstein's theorem which asserts that the number of common zeros of a system of Laurent polynomial equations on the torus is bounded above by the mixed volume of their Newton polytopes.

Dienstag, 3. Mai 2005, 18 Uhr c.t., 31/E05

 

Dipl.-Math. Julia Weber (Universität Münster)

``Die universelle funktorielle äquivariante Lefschetz-Invariante''

Abstract (ps-file / pdf-file)

Freitag, 28. Januar 2005, 17 Uhr c.t., 31/452

 

Prof. Jesus de Loera (UC at Davis, z. Zt. Magdeburg)

``Algorithms & Complexity results on the Computation of Grobner bases of Toric Ideals''

Donnerstag, 27. Januar 2005, 10 Uhr c.t., 31/322

Abstract: A large variety of problems in Computer algebra, Combinatorics, Statistics, Optimization can be formulated in the language of toric ideals. Examples of these are: Hilbert functions of monomial rings, computing Ehrhart polynomials and volumes of polytopes, Counting contingency tables, Solving hard knapsack problems. For this reason Grobner bases of toric ideals have particular importance and fast algorithms are desirable. In this talk we present new algorithmic tools:
Our first result states that, using Barvinok's rational functions and for fixed number of variables and term order, the reduced Grobner basis of a toric ideal can be computed in polynomial time. For computing the normal form of a monomial this can also be done in polynomial time. Our initial experiments suggest that in practice this approach, which is entirely avoids S-pairs and Buchberger's algorithm, has potential for practical use.

For our second result we have also investigated the structure and complexity of toric ideals for 0/1 matrices. We have proved that any Grobner bases of toric ideal is isomorphic to the toric ideal of some special family of 0/1 matrices.

This reports on joint work with subsets of the following people Raymond Hemmecke, Peter Huggins, Dave Haws, Shmuel Onn, Bernd Sturmfels, and Ruriko Yoshida.

 

Prof. Jesus de Loera (UC at Davis, z. Zt. Magdeburg)

``The Many Aspects of Counting Lattice Points in Polytopes''

Dienstag, 25. Januar 2005, 18 Uhr c.t., 31/E05

Abstract: A wide variety of topics in pure and applied mathematics involve the problem of counting the number of lattice points inside a region in Euclidean space. Applications range from the very pure (Number theory, toric Hilbert functions, Kostant's partition function in representation theory) to the applied (cryptography, integer programming, contingency tables).
Perhaps the most basic case is when the region is a convex bounded polyhedron, for short called polytopes. This talk is a survey of this exciting and useful problem. Our goal will be to explain to an audience of non-experts the basic structure theorems about these counting problem. Perhaps the most famous special case are the so called, Ehrhart quasipolynomials, introduced in the 1960s by Eugene Ehrhart. Ehrhart quasipolynomials count the number of lattice points in the different integral dilations of a rational convex polytope. Toward the end of the talk, we present a new very general version of Ehrhart's theorem recently found by the speaker. We conclude with a look at what happens when counting lattice points in more complicated regions of space.

2004:

Prof. Dr. Dan Burghelea (Ohio-State-University, USA)

``Manifold Category, a new potentially interesting structure''

Freitag, 3. Dezember 2004, 17 Uhr c.t., 31/452

Abstract: A manifold category is a topological category whose space of objects and space of morphisms are smooth manifolds (the second with corners) which satisfies one additional (natural) axiom. This structure appears in the geometric study of the dynamics (rest points and trajectories) of nice vector fields on smooth manifolds.
Despite its simplicity the stucture is rich. I will provide examples, and discuss

1. basic homotopy theory (geometric realization)

2. calculus and algebraic topology (De Rham theory)

3. Riemannian geometry of a manifold category.

They are interesting and challenging.

 

Prof. Dr. David Damanik (Caltech Pasadena, USA)

``Unerwartete ausgedehnte Zustände in zufälligen Medien ''

Freitag, 19. November 2004, 17 Uhr c.t., 31/452

Abstract: Eines der wichtigsten Probleme in der Mathematischen Physik ist es, die Existenz von ausgedehnten Zuständen in zufälligen Quantensystemen in Dimension 3 (oder höher) nachzuweisen. In Dimensionen 1 und 2 sollten solche ausgedehnten Zustände nicht existieren. Überraschenderweise wurden für einige zufälligen Modelle dennoch ausgedehnte Zustände nachgewiesen, was in der Physikliteratur Anfang der 90'er Jahre Anlass für angeregte Diskussionen und einige Missinterpretationen gab. Aus mathematischer Sicht wurde dieses Phänomen in den letzten 5 Jahren eingehend auf seine spektralen und quantendynamischen Konsequenzen hin untersucht. In diesem Vortrag werden die wichtigsten Erkenntnisse und Ergebnisse in diesem Zusammenhang vorgestellt.

Prof. Dr. Jürgen Voigt (Universität Dresden)

``Über Randbedingungen für die Wärmeleitungsgleichung ''

Freitag, 5 .November 2004, 17 Uhr c.t., 31/452

Abstract

Prof. Dr. Aldo Conca (Universität Genova)

``Algebras associated with linear spaces''

Freitag, 29. Oktober 2004, 17 Uhr c.t., 31/452

Abstract: Let R be a polynomial ring over a field and V_1,...,V_m vector spaces of linear forms of dimension, say, d₁ ,d₂,...,d_m. We consider the subalgebra A(V) of R generated by the product V₁,...V_m. From a result of myself and Herzog one can deduce that A(V) is always normal (integrally closed in its field of fractions). I conjecture that A(V) is always a Cohen-Macaulay and Koszul algebra. I am able to prove the conjecture in two (extreme) cases: the monomial case (each V_i is generated by a subset d_i variables) and the generic case (each V_i is generated by d_i generic linear forms).

Prof. Dr. Zbigniew Fiedorowicz (Ohio State University, Columbus, USA)

``Iterated monoidal categories''

Freitag, 16. Juli 2004, 17 Uhr c.t, 31/452

Abstract: Recent developments in mathematical physics have given rise to new varieties of algebra and category theory. These, in turn, have intriguing connections to several branches of topology. This talk will discuss one aspect of the relationship between iterated loop spaces and a type of higher category called an iterated monoid.

 

Paolo Vinai (University of Bologna, Italy)

``Arithmetic degree and associated graded ring''

Freitag, 2 .Juli 2004, 15.30 Uhr, 31/452

Abstract

 

Dilip Pantil (IISC Bangalore, Indien)

``Type sequences of some semigroup rings''

Freitag, 2. Juli 2004, 17 Uhr c.t, 31/452

Srikanth Iyengar, (Columbia, Missouri, USA)

``Extremal modules''

Freitag, 25. Juni 2004, 17 Uhr c.t, 31/452

Abstract: Over a local ring R, it is well known that the growth of Betti numbers of any finitely generated module is bounded above by that of the residue field of R. "Extremal modules" are those that achieve this upper bound. In my talk I will discuss some new results that reveal that extremal modules are plentiful. It will be based on recent work with L. Avramov and C. Miller, and reported in our reprint: Homology over local homomorphisms.

 

Taras Panov (Moscow State University)

``Rational Aspects of Topic Topology''

Freitag, 16. April 2004, 17 Uhr c.t, 31/452

Abstract: Since the pioneering work of Davis and Januszkiewicz, algebraic topologists have been drawn increasingly towards the study of spaces which arise from well-behaved actions of the torus Tⁿ . Investigations are no longer confined to the properties of Davis and Januszkiewicz's toric manifolds, but have extended to related geometrical structures, such as moment-angle complexes, subspace arrangements or torus manifolds of Hattori and Masuda, as well as the homotopy types of associated spaces and their rationalisations and localisations. We refer to this enlarged field of activitys as toric topology

 

Björn Jahren ( University of Oslo)

``Homotopy fixpoints and Nielsen-Lefschetz theory''

Freitag, 26. März 2004, 17 Uhr c.t, 31/452

Abstract: A central role in Nielsen fixpoint theory is played by the so-called Reidemeister trace. I will discuss a homological interpretation of this invariant.

 

Jürgen Stückrad ( Universität Leipzig)

``Multiplizitäten''

Freitag, 23. Januar 2004, 17 Uhr c.t, 31/452

Abstract

2003:

Holger Kösters ( Universität Münster)

``Prophetenungleichungen im i.i.d.-Fall bei Beobachtungskosten''

Freitag, 5. Dezember 2003, 17 Uhr c.t, 31/452

Abstract: In der Prophetentheorie wird die folgende Problemstellung betrachtet: Zwei Spieler mit unterschiedlichem Informationsstand, der "Statistiker" und der "Prophet", versuchen im Zeitablauf aus einer Folge von zufallsabhängigen Beobachtungen einen Wert auszuwählen, so dass ihre erwartete Auszahlung maximal wird. Während der Statistiker dabei auf die Verwendung einer Stoppregel angewiesen ist, kann der allwissende Prophet in jedem Fall die größte Beobachtung auswählen. In der Form von "Prophetenungleichungen" sollen nun obere Schranken für den Vorsprung des Propheten gegenüber dem Statistiker angegeben werden.

Im Vortrag soll die Situation betrachtet werden, dass die Beobachtungen durch eine Folge von i.i.d. Zufallsgrößen gegeben sind und dass zudem Beobachtungskosten berücksichtigt werden müssen. Insbesondere wird eine neue Prophetenungleichung bei festem Beobachtungshorizont bewiesen.

 

Erik Pedersen, Binghampton, USA (z. Zt. Universität Münster)

``Topologische Gruppen und Mannigfaltigkeiten''

Freitag, 13. Juni 2003, 17 Uhr c.t, 31/452

 

Hajo Leschke (Universität Erlangen)

"Schrödingeroperatoren für amorphe Festkörper"

(Abstract)

Freitag, 16. Mai 2003, 17 Uhr c.t, 31/452

 

Karin Frank (Umweltforschungszentrum Leipzig, Sektion Ökosystemanalyse)

"Modelle in der angewandten ökologischen Forschung - aktuelle Forschungsfragen und Ansätze"

(Abstract)

Freitag, 31. Januar 2003, 16 Uhr c.t, 31/452

 

Johannes Brasche, Chalmers Technische Hochschule, Göteborg

"Wechselwirkungen entlang Brownscher Pfade"

(Abstract)

Freitag, 24. Januar 2003, 17 Uhr c.t, 31/452

2002:

Wojciech Gajda, Universität Poznan, Polen

"On K(Z) and the classical conjectures in the arithmetic of cyclotomic fields"

Abstract: I am going to discuss some results relating the problem of computing Quillen's K-groups of the integers to classical conjectures in number theory such as the Vandiver conjecture. The main point which will be stressed throughout the talk is that the topological methods of K-theory can be used to prove new theorems in the direction of the number theory conjectures.

Freitag, 13. Dezember 2002, 17 Uhr c.t, 31/452

Jürgen Hurrelbrink (Louisiana State University)

"The u-invariant of quadratic forms: Old and New"

Abstract: The u-invariant  u(F) is a classical invariant in the theory of quadratic forms of over fields F . It has to do with the solvability of polynomial equations q(X₁,...,X_n)=0 over F.
A conjecture, dating back to 1953, stated that in all interesting cases the values of u(F) were given by powers of 2. The conjecture was discovered by A. Merkurjev in 1989. We will report on what is known about u(F) and what is not.

Freitag, 1. November 2002, 17 Uhr c.t, 31/452

Jürgen Hurrelbrink  (Louisiana State University)

"Idealklassengruppen und Einheiten von quadratischen Zahlkörpern"

Freitag, 5. Juli 2002, 17 Uhr c.t, 31/452

Peter Brinkmann (University of Illinois)

"Automorphismen freier Gruppen"

Abstract: Automorphismen und äußere Automorphismen endlich erzeugter freier Gruppen sind in den letzten Jahren intensiv untersucht worden. Ein Zugang, der tiefe Einblicke in die Dynamik solcher Automorphismen ermöglicht, ist das Studium von train tracks, das zu einer Reihe von interessanten Ergebnissen geführt hat. Zu den Anwendungen dieser Methode gehört ein Beweis der Scott-Vermutung und der Tits-Alternative füur Out(F_n), sowie einige Ergebnisse über das Wachstum und die Geometrie von Automorphismen. Der Vortrag besteht aus einer kurzen Einführung in die Theorie der train tracks, gefolgt von einigen Anwendungen auf dynamische, geometrische und algorithmische Fragen.

Freitag, 28. Juni 2002, 17 Uhr c.t, 31/452

Dirk Hundertmark (California Institute of Technology)

"Neue Ergebnisse zu den Lieb-Thirring Ungleichungen"

Abstract:  Lieb-Thirring Ungleichungen sind ein grundlegendes Werkzeug in der mathematischen Theorie der Quantenmechanik. Ihre Stärke liegt insbesondere daran, dass quantenmechanische Objekte wie Momente der (negativen) Eigenwerte von Schrödingeroperatoren über klassische Phasenraummomente abgeschätzt werden. Diese klassische Ausdrücke sind oft leichter zu analysieren als das ursprüngliche quantenmechanische Problem.

Freitag, 14. Juni 2002, 17 Uhr c.t, 31/452

Wolfram Decker (Universität Saarbrücken)

"Algorithmen in der Invariantentheorie"

Abstract:  Die klassische Invariantentheorie des 19. Jahrhunderts beschäfigt sich mit der Berechnung von polynomialen Invarianten, die man benötigt, um zwischen geometrischen Objekten zu unterscheiden, die nicht durch eine Koordinatentransformation ineinander übergeführt werden können. Zwischen 1841 und 1893 interessierten sich zahlreiche berühmte Mathematiker für die Invariantentheorie. Dazu zählen Boole, Cayley, Sylvester, Salmon, Aronhold, Hermite, Eisenstein, Clebsch, Gordan, Lie, Klein, Capelli und Hilbert. Der Vortrag führt in die Invariantentheorie ein, gibt einen historischen Überblick und diskutiert moderne Algorithmen zur Berechnung von Invarianten sowie deren Implementierung.

Freitag, 12. April 2002, 17 Uhr c.t, 31/452

Volker Enß (Universität Aachen)

"Ein geometrischer Zugang zur Streutheorie"

Freitag, 1. Februar 2002, 17 Uhr c.t, 31/452

2001:

Horst Leptin   (Universität Bielefeld)

"Harmonische Analyse auf Automorphismengruppen"
Abstract:  Es werden die Eigenschaften von Automorphismengruppen von Bäumen untersucht. Diese Eigenschaften sind Symmetrie; die Wiener-Eigenschaft oder allgemeinere Eigenschaften der Spektralsynthese. 

Freitag, 14. Dezember 2001, 17 Uhr c.t, 31/452

 

Michael Mürmann   (Universität Heidelberg)

"Der hydrodynamische Limes bei deterministischen Teilchensystemen"

Abstract:  Es wird ein deterministisches Teilchensystem mit nächster Nachbar Wechselwirkung und einer zusätzlichen geschwindigkeitsabhängigen Kraft, die lokale Glättung der Geschwindigkeiten bewirkt, untersucht. Dieses System hat zwei Erhaltungsgrößen, Masse und Impuls. Ihre Grenzdynamik unter Euler Skalierung ist durch die kompressiblen Navier-Stokes Gleichungen mit dichteabhängiger Viskosität gegeben. In Ermangelung eines geeigneten Eindeutigkeitssatzes für die Lösung folgt mit Hilfe von Kompaktheitseigenschaften die Konvergenz von Teilfolgen gegen eine Lösung.

Freitag, 7. Dezember 2001, 17 Uhr c.t, 31/452

 

Michael Demuth   (Universität Clausthal)

"Stetige Spektren für Differentialoperatoren höherer Ordnung

(Abstract)

Freitag, 30. November 2001, 17 Uhr c.t, 31/452

 

Werner Kirsch   (Universität Bochum)

"Schrödingeroperatoren mit zufälligen Potentialen - Mathematische Modelle für Festkörper mit Unordnung"

Abstract:  In der Natur kommen häufig Festkörper vor, deren atomare Struktur - im Gegensatz etwa zu idealen Kristallen - keine regelmäßige Ordnung aufweist. Beispiele sind amorphe Materialien wie Glas oder Gummi, dotierte Halbleiter oder viele Legierungen. Quantenmechanisch beschreibt man solche Materialien durch Potentialfunktionen, die zufällige Parameter enthalten, z.B. die zufälligen Orte der Atomkerne des Festkörpers oder die zufällige Ladung eines Teilchens an einem bestimmten Gitterplatz. Die Bewegung eines Elektrons durch ein solches Material wird dann beschrieben durch einen Schrödingeroperator mit diesem zufälligen Potential. Schrödingeroperatoren mit zufälligen Potentialen weisen ungewohnte und (deshalb) interessante Spektraleigenschaften auf. So kommt es etwa häufig vor, dass der Operator eine (abzählbare) Menge von Eigenwerten hat, die in einer Halbachse dicht liegen. Solch ungewöhnliche Spektraleigenschaften kann man mit der Beweglichkeit des Ladungsträgers und daher mit der Leitfähigkeit des Materials in Verbindung bringen. In dem Vortrag werden wir die mathematische Modellbildung erklären und einige wichtige mathematische Resultate der Theorie vorstellen und erläutern.

Freitag, 23. November 2001, 17 Uhr c.t, 31/452

 

Morten Brun   (z. Zt. Universität Bielefeld)

"Algebraische K-Theorie von Endomorphismen"

Abstract: Dieser Vortrag fängt mit einer allgemeinen Einführung in die algebraische K-Theorie von kommutative Ringen an. Beispiele von Verbindungen zwischen algebraische K-Gruppen und der Zahlentheorie werden aufgezeigt. Danach wird die Algebraische K-Theorie von Endomorphismen von projektiven Modulen über dem Grundring eingeführt. Die zugehörigen K-Gruppen haben eine reiche algebraische Struktur. Diese Struktur wird kurz beschrieben, und Beispiele von anderen mathematischen Objekten mit einer solchen Struktur werden gegeben. Am Ende des Vortrages wird erklärt, wie die Existenz dieser Struktur für Berechnungen von K-Gruppen ausgenutzt werden kann.

Freitag, 9. November 2001, 17 Uhr c.t, 31/452

 

Festkolloquium aus Anlaß des 65. Geburtstages von Prof. Dr. Hans-Jörg Reiffen

Prof. Dr. M. Jarnicki, Universität Krakau

"Invariant Metrics"

Prof. Dr. B. Kaup, Universität Fribourg

"Holomorphe Blätterungen"

Freitag, 2. November 2001, 15 Uhr s.t., 31/E06

 

Rainer Hempel (Technische Universität Braunschweig)

"Spektrale Eigenschaften periodischer Medien mit starkem Kontrast"

Abstract: Ausgehend von der Wärmeleitung in einem Metall mit Einschlüssen von Sandkörnern oder Luftblasen, betrachten wir periodische elliptische Differentialoperatoren, die 2-komponentige Medien im R^d, d=>2, beschreiben (neben der Wärmeleitung denke man an die Akustik, elektromagnetische Wellen und die Quantenmechanik).
Im Limes, wo der Kontrast zwischen den beiden Materialien groß wird, diskutieren wir die Existenz von Lücken im Spektrum und die Konvergenz der Zustandsdichte. Einige unserer Ergebnisse lassen sich auf zufällige Medien übertragen.

Montag, 13. Juli 2001, 17 Uhr c.t, 31/452

 

Sabine Betz  (Züricher Versicherungs AG, Frankfurt)

"Reservierungen in der Sachversicherung mit konkreten Beispiele"

Montag, 9. Juli 2001, 12 Uhr c.t, 32/107

 

Markus Spitzweck (Universität Bonn)

"E_\infty-Algebren und Limesmotive"

Abstract: Im ersten Teil des Vortrags beschreiben wir gewisse triangulierte Kategorien von Garben über einer Mannigfaltigkeit als Modulkategorien über der Kohomologie aufgefasst als E_\infty-Algebra. Im zweiten Teil benutzen wir ein analoges Resultat im Rahmen der algebraischen Geometrie, um eine motivische Versison von Limes-Hodge-Strukturen zu definieren.

Freitag, 6. Juli 2001, 17 Uhr c.t, 31/452

 

Takayuki  Hibi  (Osaka  University)

"Triangulations of convex polytopes and root systems"

Abstract

Freitag, 29. Juni 2001, 17 Uhr c.t, 31/452

 

Peter Brinkmann  (University of Illinois)

"Geometrische  Gruppentheorie"

Abstract: Der Vortrag wird einen Überblick über einige Grundbegriffe aus der geometrischen Gruppentheorie geben, z.B. Entscheidungsprobleme und hyperbolische Gruppen, mit Betonung auf Verbindungen zu anderen Gebieten, z.B. Differentialgeometrie und niedrigdimensionale Topologie.

Freitag, 22. Juni 2001, 17 Uhr c.t, 31/452

 

Jürgen Hurrelbrink (Louisiana State University)

"Quadratische Formen und Milnor K-Gruppen"

Abstract:  Wir behandeln äußerst klassische Probleme, welche mit der Struktur von Idealklassengruppen von Zahlkörpern zu tun haben. So ist etwa das auf Gauß zurückgehende Klassenzahl 1 Problem für reell-quadratische Körper immer noch offen.
In Analogie zu Idealklassengruppen beschreiben wir Milnorsche K-Gruppen K2 über Ringen ganzer Zahlen von Zahlkörpern und ihre Bedeutung. Mittels Heranziehung quadratischer Formen liefern wir auch für solche Gruppen über gewissen quadratischen Körpern Strukturresultate.

Freitag, 15. Juni 2001, 17 Uhr c.t, 31/452

 

Abdallah Al Amrani  (Universität Strassbourg)

"Inertia and regularity"

Abstract:  The notion of "Trägheitsformen" goes back to Mertens (1886).They were studied explicitly first by Hurwitz (1913). In his intensive-exhaustive work on elimination theory, Jouanolou (since the 1980's ) pushed on their study as far as possible (any ground ring, any positive degrees for the variables, ... ). For example, he established an 'inertia lemma', a particular case of which is the Peskine-Szpiro 'acyclicity lemma'. Some aspects of Jouanolou's work on inertia will be shown, including links to Castelnuovo-Mumford regularity and to geometry.

Freitag, 1. Juni 2001, 17 Uhr c.t, 31/452

 

Jürgen Herzog (Universität Essen)

"Über das asymptotische Verhalten der Regularitiät"

Abstract:  Mumford definiert die Regularität einer Garbe F auf dem Pⁿ als die kleinste Zahl m, für die H^j(Pⁿ; F(m-j))=0 für alle j>0. Für einen endlich erzeugten graduierten Modul Müber dem Polynomring S=K[x1,...,xn] ist die Regularität entsprechend definiert. Man setzt reg (M)=max{j :Hⁱ_m(M)j-i}\neq 0 für alle i=> 0}. Hierbei bezeichnet Hⁱ_m((M) die i-te lokale Kohomomologie von M bezüglich dem maximalen Ideal m=(x1,...,xn). Die Invariante reg (M) lässt sich auch charakterisieren als max{ j : Tori(M,K)i+j\neq 0} für alle i=> 0. Insbesondere ergibt sich aus dieser Beschreibung, dass reg (M) kleiner oder gleich dem höchsten Grad eines minimalen Erzeugers von M ist. Für ein Ideal gilt sogar, dass reg (I) gleich dem höchsten Grad eines minimalen Erzeugers des generischen Initialideals Gin (I) von I ist. Die Invariante reg (I) ist daher ein gutes Mass für die Komplexität von I. Das Verhalten der Komplexität von Idealpotenzen Iⁿ in Abhängigkeit von n, also deren Regularität, wurde, ausgehend von Resultaten von Bertram, Ein und Lazardsfeld, von zahlreichen Autoren untersucht. Dabei zeigt sich, dass reg (Iⁿ) für grosse n eine lineare Funktion in n ist. Das liegt im Wesentlichen daran, dass der zugehörige Reesring endlich erzeugt ist. In diesem Vortag möchte ich vorstellen, was über die Regularität von symbolischen Potenzen bekannt ist. Hier ist die Situation erheblich komplizierter, da der symbolische Reesring im allgemeinen nicht endlich erzeugt ist. Wie sich zeigt ist die Regularität symbolischer Potenzen eng verwandt mit der Regularität von in (Iⁿ), wobei (J) das Initialideal eines Ideals J bezüglich einer gewissen Termordnung bezeichnet. Alle Beispiele weisen darauf hin, dass es sowohl für reg in (Iⁿ) als auch für reg (I⁽ⁿ⁾) eine lineare obere Schranke gibt. In einer gemeinsamen Arbeit mit Trung und Hoa haben wir gezeigt, dass dies in gewissen Fällen tatsächlich zutrifft. Allgemein ist eine solche Anbschätzung aber nicht bekannt. Es gilt aber das überraschende Resultat, dass in (I^cn)\subseteq in (I)ⁿ für jedes graduierte Ideal der Kodimension c.

Freitag, 9. Februar 2001, 17 Uhr c.t, 31/452

 

D.N. Verma (Mumbai)

"A New Elementary Approach to the Jacobian Conjecture -- via Generalizing the Wronskian Theorem"

Montag, 5. Februar 2001, 17 Uhr c.t, 31/423

 

Ludwig Cromme (Universität Cottbus)

"Optimierung einer Produktionsanlage"

Freitag, 2. Februar 2001, 17 Uhr c.t, 31/452

 

Martin Lübke (Universität Leiden, Niederlande)

"Die universelle Kobayashi-Hitchin-Korrespondenz"

Abstract:  Es sei E ein differenzierbares komplexes Vektorbündel über einer kompakten komplexen Mannigfaltigkeit. Die "klassische" Kobayashi-Hitchin-Korrespondenz besagt, dass die stabilen holomorphenStrukturen in E (ein algebraisch-geometrisches Konzept) eineindeutig korrespondieren mit den Hermite-Einstein-Strukturen in E (ein differentialgeometrisches Konzept). Genauer gesagt, für jede der beiden Arten von Strukturen hat man einen klassifizierenden Modulraum (komplex- bzw. reell-analytisch), und die Korrespondenz sagt, dass diese Räume isomorph sind als reell-analytische Räume. In dieser Form spielte die Korrespondenz eine wichtige Rolle in der Donaldson-Theorie 4-dimensionaler differenzierbarer Mannigfaltigkeiten; mit ihrer Hilfe war es möglich, differenzierbare Invarianten solcher Mannigfaltigkeiten mit Hilfe komplex-algebraischer Geometrie explizit zu berechnen. Inzwischen hat sich herausgestellt, dass man Varianten der Stabilität und Hermite-Einstein-Strukturen definieren kann, z.B. für Vektorbündel mit Schnitten, und dass auch hierfür eine Kobayashi-Hitchin-Korrespondenz gilt. Die Nützlichkeit solcher Varianten hat sich z.B. in der Seiberg-Witten-Theorie gezeigt. Ziel des Vortrages ist es, eine "universelle" Kobayashi-Hitchin-Korrespondenz zu beschreiben die alle "speziellen" impliziert.

Freitag, 26. Januar 2001, 17 Uhr c.t, 31/452

2000:

Joseph Gubeladze (A. Razmadze Math. Institute,Tbilisi, Georgien)

"Stable and unstable K-homotopy properties of toric varieties"

Abstract:

There are two main conjectures on K-theory of toric varieties which contain the known results in a uniform way and, simultaneo usly, provide their final potential generalizations. The first concerns the nilpotency property of the multiplicative action of the natural numbers on the nil K-theory of these varieties, and the second is about stabilizations. In a number of special cases both these conjectures have been verified. Recently, there has been  progres on the first conjecture, which "almost" proves it. What remains is a question about a very special ring extension which might be attacked by the use of Witt vector actions. Surprisingly, the question is about non-commutative graded rings whereas the conjecture is about usual "commutative" varietes. We shall describe this development and surrounding results.

Fr 15. Dezember 2000, 17 Uhr c.t, 31/452

Martin Markl (Akademie der Wissenschaften Prag)

"Secret structures of configuration spaces"

Abstract:

We discuss various algebraic structures on configuration spaces of points in real or complex manifolds and on compactifications of theses spaces.

Fr 8. Dezember 2000, 17 Uhr c.t, 31/452

B.K. Ghosh, Lehigh University, USA, z.Zt. Uni Münster

"Markov, Chebychev, and sharpet inequalities"

Abstract:

Markov's inequality provides an upper bound for the probability P(x \ge t), t > 0, where x is a non-negative random variable with mean mu. Chebyshev's  inequality gives a lower bound for P(-t< x-mu < t), t > 0, where x is an arbitrary random  variable with mean mu and standard deviation sigma. This talk gives the sharpest upper and lower bounds for P(x \ge t) and P(s<x-mu < t) when mu and sigma are specified. Some of the results are well known but their i proofs given here are simpler.

Fr 1. Dezember 2000, 17 Uhr c.t, 31/452
 

Jürgen Voigt, TU Dresden

"Parabolische Differentialgleichungen in Lp-Räumen"

Abstract:

Ausgangspunkt des Vortrages ist die Wärmeleitungsgleichung mit singulärer Absorption.  Zunächst wird vorgestellt, wie das Anfangswertproblem für diese Gleichung mit Hilfe der Störungstheorie von stark stetigen Halbgruppen von Operatoren im Banachraum behandelt wird. Dabei wird auch die Bedeutung der Kato-Klasse von Potentialen erklärt. Weiterhin wird die Beziehung zu Schrödingeroperatoren hergestellt, und die Stetigkeit von Lösungen des Anfangswertproblemes wird diskutiert. Schließlich werden Verallgemeinerungen, unter anderem für nichtautonome Differentialgleichungen, angesprochen.

Fr 24. November 2000, 17 Uhr c.t., 31/452
 

 Paulo Lima-Filho, Texas A&M University

"Symplectic K-theory and Severi-Brauer varieties"

Mi 22. November 2000, 14 Uhr c.t., 31/450a
 

Michiel Hazewinkel, CWI Amsterdam

"Missing and misleading data in information retrieval. Mathematical andcomputer linguistic aspects."

Abstract:

Suppose your library has subscribed to the journal 'Theoretical Computer Science' from its beginning. Then you now have some 220 volumes on your shelves. That looks nice and is absolutely no use unless you know how to find things in there.
That is the subject of metadata, indexes, thesauri.
The business of attaching in a reliable way metadata to scientific documents is a tricky one. Linguistic tools do not suffice. There are many mathematical, computer science (especially combinatoria) problems that arise and this talk will concentrate on those and their raison d'etre.

Fr 17. November 2000 17 Uhr c.t. , 31/452
 

Hubert Flenner, Bochum

"Rationale Kurven und rationale Singularitäten"

Abstract:

Der Vortragende spricht über eine gemeinsame Arbeit mit Zaidenberg. In ihr geht es um die Thematik, inwieweit viele rationale Kurven auf algebraischen Varietäten die möglichen Singularitäten einschränken. Z.B. zeigen sie, dass eine algebraische Varietät mit isolierten Cohen Macaulay Singularitäten, deren regulärer Ort von abgeschlossenen rationalen Kurven überdeckt wird, höchstens rationale Singularitäten haben kann. Dies liefert eine geometrische Erklärung für klassische Resultate von H.A. Schwarz und Halphen über polynomiale Lösungen der verallgemeinerten Fermatschen Gleichung.

Fr 10. November 2000 17 Uhr c.t. , 31/452
 

Heinrich Focke ( Zürich Financial Services)

"Moderne Methoden der Finanzrückversicherung"

We 8. November 2000, 17 Uhr c.t. , 31/E06
 

Marc Nardmann, Leipzig

"Exotische Strukturen auf R⁴"

Abstract:

In keiner anderen Dimension unterscheiden sich die Begriffe topologische Mannigfaltigkeit und glatte Mannigfaltigkeit so stark wie in Dimension 4. Ein Beispiel für die außergewöhnlichen Phänomene, die bei vierdimensionalen Mannigfaltigkeiten auftreten, ist die Existenz exotischer Strukturen auf der topologischen Mannigfaltigkeit R⁴: Es gibt glatte Mannigfaltigkeiten, die homöomorph, aber nicht diffeomorph zu R⁴ sind. In dem Vortrag wird der Beweis für die Existenz solcher Strukturen, der auf sehr tiefliegenden Sätzen beruht, skizziert.

Fr 3. November 2000 16 Uhr c.t. , 31/452
 

Volker Schnecke (AstraZEneca R&D Mölndal, Schweden)

"Herausforderungen für Informatiker in der modernen Pharmaforschung"

Abstract:

Die modernene Pharmaforschung ist hochgradig interdisziplinär, neue Medikamente werden von Teams aus Chemikern, Biologen, Biochemikern, Pharmakologen und Informatikern entwickelt.  Fortschritte in der Molekularbiologie, insbesondere das Human Genom Projekt, haben die Methoden bei der Suche nach neuen Wirkstoffen revolutioniert, und der Einfluss der Informatik wird in den kommenden Jahren stetig zunehmen. Unter dem Begriff "Bioinformatik" wurde in den letzten Jahren ein Fachgebiet kreiert, was viele extrem interessante und anwendungsnahe Forschungsprobleme behandelt, aber leider im akademischen Bereich noch unterrepräsentiert ist.
In diesem Vortrag werden die Grundlagen der Bioinformatik vorgestellt und eine allgemein verständliche Übersicht über den biochemischen Hintergrund des Arzneimittelentwurfs gegeben. Insbesondere werden Beispiele gezeigt, wie mit Informatikmethoden der üblicherweise mehr als zehn Jahre dauernde Prozess von einer Idee bis zum zugelassenen Medikament verkürzt werden kann, und in welche Richtung sich diese Arbeitsweise in den nächsten Jahren - ausgelöst durch die Sequenzierung des menschlichen Genoms - entwickeln wird.

Fr.11.10.00, 14 ct, 31/449a
 

Pedro Santos, Lissabon

``Algebraic Cycles on Real Algebraic Varietes and Z₂ - Homotopy Theory''

Abstract:

In this talk we will study spaces of algebraic cycles on projective spaces under the Z/2-action induced by complex conjugation. We will start by reviewing basic definitions and results connected with spaces of cycles, such as the suspension theorem and the definition of the topological group of cycles on quasi-projective varieties. We will introduce an equivariant version of the classical Dold-Thom theorem and use it to compute the equivariant homotopy type of the space of stabilized cycles. Finally will we show how use algebraic cycles to construct the total equivariant Chern class for real bundles (in the sense of KR-theory)

Mi 13.09.00, 11 Uhr in 31/452

Pedro Santos, Lissabon

``Quarternionic Algebraic Cycles and Reality''

Abstract:

In this talk we will introduce real Lawson homology as a tool to study real algebraic varieties via its groups of cycles. We will start by defining real Lawson homology and discuss its basic features: exact sequences for pairs, localization sequence, homotopy property, and existence of a cycle map. Using these techniques we will identify the equivariant homotopy of the spaces of cycles on real varieties with a real cell decomposition. On the second part of this talk we will study cycles on the real variety P(Hⁿ) with the real structure induced from multiplication with the unit quaternion j. Using the homotopy property we will prove the quaternionic suspension theorem. The Thom isomorphism and for real varieties and Poincare duality for real manifolds will be discussed. These tools, along with the homotopy property, will be used to compute the equivariant homotopy type of cycles in P(Hⁿ).

Mi 13.09.00, 14 Uhr in 31/452
 

Bhaskar DasGupta, Rutgers University,Camden

"Throughput Maximization Problems in Real-time Scheduling"

Abstract:

In the first part of the talk, we consider the problem of off-line throughput maximization for job scheduling on one or more machines, where each job has a release time, a deadline and a profit. Our main contribution is to provide algorithms that do not use linear programming, are simple and faster to implement, and have either the same or better quality of approximationas compared to the previous best algorithms in the literature.
In the second part of the talk, we derive bounds on performance guarantees of online algorithms for real-time preemptive scheduling of jobs with deadlines on multiple machines when jobs are characterized in terms of their minimum stretch factor a (or, equivalently, their maximum execution rate r=1/a). We consider two well known preemptive models that are of interest from practical applications: the  hard real-time scheduling model and the firm real-time scheduling model. In both models, the objective is to maximize the sum of execution times of the jobs that were executed to completion and preemption is allowed.

Fr 08.09.00, 17 Uhr in 31/328
 

Juergen Hurrelbrink, Baton-Rouge

"Idealklassen, Gruppen und Graphen"

Abstract:

Basic classical topics in Number Theory, dating back to Gauss, are concerned with the ideal class group and the fundamental unit of quadratic number fields. We will explain longstanding problems and show how one can contribute to these topics by just inspecting finite graphs associated with the given fields.

Fr 14.07.00, 17 Uhr in 31/452
 

Roosendaal, Twente

"Scientific Communication?"

Abstract:

Scientific communication is in flux, not only as such but also because university strategies and policies are in flux.
Both developments in research and developments in education will influence the development of scientific communication. ICT is an important driver for all these processes.
The influence of these driving forces on the functions to be performed by Scientific Communication will be discussed.
Changes in the balance of the functions will lead to consequences for the organisation and thus for the value chain in Scientific Communication. New roles for the partners in the value chain and the emergence of new business models will be analysed and speculated upon.

Fr 07.07.00, 17 Uhr in 31/452
 

E. G. Coffman, Jr., Columbia University

"Packing Random Intervals in One and Two Dimensions"

Abstract: onsider a collection of n random intervals, each being the interval between two i.i.d. uniform random draws from [0,1]; and consider algorithms that select subsets of mutually disjoint intervals to (a) maximize the number selected, or (b) minimize the `wasted space'  (the fraction of [0,1] not covered by the intervals selected).   We know for problem (a) that the expected number selected is Theta(n^1/2}) and for problem (b) that the expected wasted space is Theta(log^2 n/n). The problems have obvious generalizations to two dimensions, where intervals become rectangles.  We will discuss a proof that Theta(n^1/2) also applies to the expected number of rectangles in a maximal disjoint subset (this work is with George Lueker, Joel Spencer, and Peter Winkler).  We will also mention cognate problems/results, and some intriguing open questions.

Fr 07.07.00, 17 Uhr in 31/450a
 

Tautenhahn, Magdeburg

"Lokale Suchverfahren in LiSA"

Abstract:

Gegenstand dieses Vortrags ist die technische Umsetzung von lokalen Suchverfahren in LiSA. Das an der Universität Magdeburg entwickelte Programmpaket LiSA - Library of Scheduling Algorithms - setzt sich unter anderem zum Ziel, Werkzeug bei der Entwicklung neuer Scheduling-Algorithmen zu sein. Neben einem extrem einfach zu handhabenden Verfahren zur Einbindung nutzereigener Programme in die Oberfläche verfügt LiSA über verschiedene Bausteine zur schnellen Implementierung. Einer dieser Bausteine ist die Klasse Iterator, die lokale Suchverfahren wie Iterative Improvement, Threshold Accepting, Tabusuche und Simulated Anealing problemunabhängig realisiert. Durch Vererbung einer vordefinierten Klasse Neighborhood wird eine Schnittstelle zu den für jeden Problemtyp spezifisch zu implementierenden Nachbarschaften geschaffen. Bei geringstem Programmieraufwand und relativ hoher Übersichtlichkeit des Codes ist es somit möglich, für ein neues Problem mehrere Verfahrengleichzeitig zu implementieren und in eine komfortable Oberfläche zu integrieren.

Fr 9.06.00, 16.00 Uhr in 31/322
 

Okonek, Zürich

"Poincaré Formeln für projektive Flächen"

Abstract:

Wir definieren Poincare Invarianten für projektive Flächen durch virtuelle Segre Klassen der Hilbert Schemata von Kurven auf diesen Flächen, und zeigen, wie man diese Invarianten in einfachen Fällen berechnen kann. Die erhaltenen Formeln verallgemeinern die klassischen Poincare Formeln für die Brill-Noether Orte spezieller Divisoren auf Kurven. Vermutlich stimmen die Poincare Invarianten projektiver Flächen mit den Seiberg-Witten Invarianten der zugrundeliegenden 4-Mannigfaltigkeiten überein.

Fr 02.06.00, 17 Uhr in 31/452
 

Sudhir Ghorpade, Mumbai (Indien)

"Error correcting codes, matroids and linear sections of 'Grassmannians'"

Abstract:

The objective of the lecture is to motivate and to introduce a rather basic problem in Coding Theory on which I have been doing some work with my French friend G. Lachaud. This problem has equivalent formulations in terms of matroids or arcs in projective spaces over finite fields. As a side product, we also get some results about the geometry of linear sections of Grassmannians using the Grothendieck-Deligne machinery.

Fr 26.05.00, 17 Uhr in 31/452
 

Brunella, Dijon

"Entire leaves"

Abstract:

Given a holomorphic foliation F (possibly with singularities) on a compact complex surface X, an "entire leaf" is a holomorphic map from C to X tangent to F. We shall explain how the existence of a transcendental entire leaf leads to rather strong consequencies on the structure of the full foliation F. We shall also discuss some application to some problems of algebraic geometry-complex analysis.

Fr 19.05.00, 17 Uhr in 31/452
 

Vladimir Nezhinskij, MPI Bonn

"Pseudo-homotopy theory of singular links"

Abstract:

The subject of the talk is singular links of several k-spheres and one p-sphere in 2k+l-sphere. For k > l a theory of such singular links would be constructed, which is parallel to Milnor's theory of one-dimensional singular links.

Fr 12.05.00, 17 Uhr in 31/452
 

Jürgen Krause, Bonn

"Easy and Enjoy - Graphische Benutzungsoberflächen, Visualisierung und Mediendesign"

Fr 11.02.00, 17 Uhr in 31/452
 

Norbert Fuhr, Dortmund

"Informationssuche in vernetzten Digitalen Bibliotheken"

Fr 4.02.00, 17 Uhr in 31/452
 

Volkmar Welker, Marburg

"Komplexe von Graphen: Von Komplexitätstheorie zu Knoteninvarianten"

Fr 28.01.00, 17 Uhr in 31/452
 

1999:

Joachim Cuntz, Universität Münster

"Grundlagen der Nichtkommutativen Geometrie"

Fr 17.12.99, 17 Uhr in 31/452
 

Martin Markl, Akademie der Wissenschaften Prag

"Transporting Algebraic Structures"

Fr 10.12.99, 17 Uhr in 31/452
 

B.Suhr-Erné, Hannoversche Lebensversicherung

"Besonderheiten eines Direktversicherers"

Th 9.12.99, 17 Uhr in 31/450a
 

Andrei Prasolor

"Products in K-Theory of Braiding-Commutative Algebra"

Th 9.12.99, 14 Uhr in 31/450a
 

Andreas Defant, Universität Oldenburg

"Über einen bemerkenswerten Satz von P. Levi und E. Steinitz"

Fr 3.12.99, 17 Uhr in 31/452
 

Nadia Brauner, Université de Liége

"Cyclic Scheduling in Robotic Cells with no buffer space"

Fr 3.12.99, 15 Uhr in 31/450a
 

Joseph Gubeladze, z.Zt. MPI, Bonn

"Higher K-groups of semigroup rings"

Fr 26.11.99, 17 Uhr in 31/452
 

Joachim Weidmann, Universität Frankfurt

"Spektraltheorie gewöhnlicher Differentialoperatoren"

Fr 12.11.99, 17 Uhr in 31/449a
 

Detlev Poguntke, Universität Bielefeld

"Laplacesche Operatoren auf Lieschen Gruppen und C^*-Algebren"

Fr 12.11.99, 15 Uhr in 31/449a
 

Stefan Schwede, Universität Bielefeld

"Exakte Folgen, Lie-Klammern und Hochschild-Kohomologie"

Fr 5.11.99, 17 Uhr in 31/452
 

B. Veit, Universität Tor Vergata, Rom

"Ein Kriterium für Irreduzibilität und Glattheit von Matrizenräumen"

Fr 22.10.99, 17 Uhr in 31/452
 

P. Salvatore, z.Zt. Universität Bonn

"Models for spaces of particles"

Fr 15.10.99, 17 Uhr in 31/452
 

B. Kaup, Universität Fribourg, Schweiz

"Was sind holomorphe Blätterungen mit Singularitäten"

Fr 16.7.99, 17 Uhr in 31/452
 

U. Vetter, Universität Oldenburg

"Linearformen und Koszul-Komplexe"

Fr 9.7.99, 17 Uhr in 31/452
 

A. Schrijver, CWI and University of Amsterdam

"Optimization at Dutch Rail"

Fr 9.7.99, 15 Uhr in 31/322
 

S. Bar-Lev, Haifa

"A general property of reproducibility"

Fr 9.7.99, 14 Uhr in 31/412
 

Uwe Saint-Mont, Universität Düsseldorf

"Prophet regions: some systematic comparisons of sup_i E(X_i) and E(max_i X_i)"

Mo 5.7.99, 14 Uhr in 31/412
 

K. Dietz, Universität Tübingen

"Wie hängt die Ausrottung einer Infektionskrankheit von der Bevölkerungsgröße ab?"

Fr 2.7.99, 14 Uhr in 31/412
 

Paulo Lima-Filho

"Summetrized Grassmannian's and the Chern Character"

Fr 18.6.99, 17 Uhr in 31/452
 

E.G. Evans, Urbana-Champaign

"Ideals with Given Hilbert Function"

Th 10.6.99, 17 Uhr in 31/450a
 

D. Patil (Bangalore, z. Zt. Bochum)

"Derivation modules of curves"

Fr 14.5.99, 17 Uhr in 31/452
 

F. Wagner, FU Berlin

"Transport-, Informatik- und Logistik Consulting GmbH (TLC)"

Fr 14.5.99, 15 Uhr in 31/332
 

Aldo Conca, Genua

"Koszul algebras and filtrations"

We 3.3.99, 17 Uhr in 31/452
 

J. Herzog, Essen

"Generische Initialideale und extremale Bettizahlen"

Fr 12.2.99, 17 Uhr in 31/452
 

Onno J. Boxma (TU Eindhoven, Niederlande)

"Queues with heavy tails"

Fr 12.2.99, 16.00 Uhr in 31/450a
 

Peter Pflug, Oldenburg

"Vollständigkeit bezüglich Invarianter Metriken"

Fr 22.1.99, 17 Uhr in 31/452
 

1998:

R.B. Lenin, Universität Twente, Enschede

"Exact transient solution of truncated birth and death process with linear rates using continued fractions"

Th 11.12.98, 16.00 Uhr in 31/322

Gregor Kemper, Universität Heidelberg

"Die Cohen-Macaulay-Eigenschaft in der Invariantentheorie"

Fr 27.11.98, 17 Uhr in 31/452
 

W. Bein, University of Nevada, Las Vegas

"Monge Properties: Contemporary Tools for Efficient Algorithms"

Fr 27.11.98, 16.00 Uhr in 31/412
 

Christian Kassel, Universität Stra&sazlig;bourg

"Parametrizations of Schubert cells using elementary matrices"

Tu 17.11.98, 11 Uhr in 31/450a
 

Craig Huneke, Universität Bonn

"A survey of tight closure"

Fr 13.11.98, 17 Uhr in 31/452
 

M. Grötschel, TU Berlin

"Elektronische Informationssysteme für die Wissenschaften"

We 11.11.98, 16 Uhr in 31/449a
 

M. Markl, Prag

"Kontsevich's graph complexes via operads"

Th 24.09.98, 14 Uhr in 31/452
 

D.N. Verma, Tata Institute of Fundamental Research, Bombay

"A new, elementary, and unified view of Representations of the Symmetric and the General Linear Group"

Th 23.7.98, 16 Uhr in 31/452
 

P. Rejto, Minneapolis

"A limiting absorption principle for Schrödinger Operators"

Th 16.7.98, 17 Uhr in 31/450a
 

J. Brasche, Universität Bonn

"Schrödingeroperatoren mit maßwertigen Potentialen"

Fr 10.07.98, 17 Uhr in 31/450a
 

C. Luthen, Düsseldorf

"Schadenssätze und Tarifkalkulation in der Feuerversicherung"

Th 9.7.98, 14.30 in 31/452
 

P.R. Parthasarathy, Indian Institute of Technology Madras, Indien

"An inverse problem in birth-and-death processes"

Tu 7.07.98, 16 Uhr in 31/450a
 

Erik van Boorn, Universität Twente, Niederlande

"Markov-modulated fluid queues"

Tu 7.07.98, 14 Uhr in 31/423
 

W. Schmidt, Universität Greifswald

"Probleme der Optimalen Steuerung"

Fr 3.07.98, 17 Uhr in 31/452
 

Paul M.N. Feehan, Ohio State University

"Rank 2 monopoles and the relation between Donaldson and Seiberg-Witten invariants"

Th 17 Uhr in 31/450a
 

Ross Staffeldt, New Mexico State University

"Lokalisierung in algebraischer Topologie"

Fr 12.06.98, 17 Uhr in 31/452
 

Yueshi Lai

"Realisierung der Steuerung eines Computers mittels natürlicher Sprache"

Fr 12.06.98, 12.15 Uhr in 31/452
 

M. Jarnicki, Jagiellonen-Universität Krakau

"Effective formulas for invariant metrics"

Tu 9.6.98, 17 Uhr in 31/450a
 

S. Betz, Züricher Versicherung, Frankfurt

"Mathematische Probleme bei Versicherungen"

Mo 8.06.98, 14 Uhr in 31/412
 

Hanusch, Züricher Versicherung, Frankfurt

"Neue Bedingungen für Lebensversicherungen im liberalisierten Markt" und

"Aufgaben und Arbeitsfeld eines Versicherungsmathematikers"

Th 28.05.98, 14.30 in 32/107
 

A. Lindenstrauss, University of Missouri

"Stable K-theory and Topological Hochschild Homology"

Fr 22.05.98, 17 Uhr in 31/452
 

S. Zacks, University of Binghampton, USA

"Distributions of crossing times for compound Poisson processes with linear boundaries."

We 29.04.98, 15 Uhr in 31/452
 

J. Gubeladze

"Polyhedral algebras, toric arrangements, and their groups"

Th 5.03.98, 17 Uhr in 31/452
 

V. Goryunov, Universität Liverpool

"Enumeration of nuromorphic functions on the line"

Fr 13.02.98, 17 Uhr in 31/452
 

S. Böge, Universität Heidelberg

"Steinberg-Gruppen von Orthogonalen Gruppen"

Fr 30.01.98, 17 Uhr in 31/452
 

K. Altmann, Universität Berlin

"Reflexive Polytope"

Fr 23.01.98, 17 Uhr in 31/452
 

W. Lück, Universität Münster

"L²-Betty Zahlen"

Fr 16.01.98, 17 Uhr in 31/452

1997:

Ralf Meyer, Universität Münster

"Holomorphe Interpolationsprobleme mit konvexem Bildbereich"

Fr 19.12.97, 17 Uhr in 31/452
 

Stefanie Locher-Beland, Universität Hamburg

"Zur Theorie der Lotka Volterra Gleichungen"

We 17.12.97 12.30 Uhr in 31/452
 

Ben de Pagter, Universität Delft, Niederlande

"Singular traces and symmetric functionals"

Fr 12.12.97 17 Uhr in 31/452
 

Anna Guerrieri, Univeristät l'Azuila

"On the structure of the associated graded ring"

Fr 5.12.97 17 Uhr in 31/452
 

W. Gajda, Universität Poznan

"On K-Theory of Algebraic Number Fields"

Fr 28.11.97 17 Uhr in 31/452
 

Andrei Duma, Universität Hagen

"Beiträge zur Theorie der geometrischen Wahrscheinlichkeiten"

Fr 21.11.97, 17 Uhr in 31/452
 

H. Dinges, Universität Frankfurt

"Ein Ergodensatz"

Fr 21.11.97, 15.45 Uhr in 31/452
 

Naoki Terai, University Saga, Japan

"Stanley-Reisner rings and Alexsander duality"

Fr 7.11.97, 17 Uhr in 31/452
 

Ravi Rao, Tata Institute, Bombay

"The transpose-inverse automorphism on the stable K-groups"

Tu 30.9.97, 17 Uhr in 31/450a
 

Ngo Viet Trung, Hanoi

"Bounds for Castelnuovo-Mumford regularity and the arithmetic degree fo monomial ideals"

Th 7.8.97, 17 Uhr in 31/452
 

D. Zuckerman, Jerusalem

"Optimal Surveillance Strategies"

We 23.7.97, 17 Uhr in 31/452
 

A. Elashvili, Tbilisi

"Hermite reciprocity for the regular representation of the cyclic group"

Mo 21.07.97, 17 Uhr in 31/452
 

A.L. Rukhin, University of Maryland, Baltimore

"Estimation of the change point"

We 9.07.97, 16 Uhr in 31/452
 

K. Oeljeklaus, Université de Provence, Aix-Marseille I

"Sphärische komplexe Flächen mit Vektorfeldern"

Fr 4.07.97, 17 Uhr in 31/450a
 

R.Y. Sharp, Sheffield

"A survey of finiteness results for local cohomology modules"

Tu 1.07.97, 17 Uhr in 31/450a
 

G. Boffi, Triest

Tu 24.06.97, 18 Uhr in 31/450a
 

P.R. Parthasarathy, Indian Institute of Technology, Madras

"Exact transient solutions of birth and death processes with quadratic rates and their applications in reliability"

Tu 24.06.97, 16 Uhr in 31/450a
 

Offer Kella, Hebrew University of Jerusalem, Israel

"Stochastic storage networks"

Tu 24.06.97, 14 Uhr in 31/450a
 

P.R. Parthasarathy, Indian Institute of Technology, Madras

"Numerical solutions of birth and death processes using continued fractions"

Th 5.06.97, 16 Uhr in 31/452
 

V. Deineko, Technische Universität Graz

"On Kalmanson matrices, PQ-trees, and the Quadratic Assignment Problem"

Mo 26.05.97, 16 Uhr in 31/452
 

N. Manolache, Universität Oldenburg

"Abelsche Flächen mit einer Polarisierung vom Typ (1,7)"

Fr 16.05.97, 17 Uhr in 31/452
 

N. Gamarnik

"Die speziellen Lösungen des Drei-Festkörper-Problems"

Th 15.05.97, 17 Uhr in 31/450a
 

P.R. Parthasarathy, Indian Institute of Technology, Madras

"Biological and Physical Applications of Birth and Death Processes"

Th 15.05.97, 15.15 Uhr in 31/412
 

T. Kawasaki, Universität Köln

"Macaulayfications and arithmetic Macaulayfications of a local ring"

Fr 25.04.97, 17 Uhr in 31/452
 

H. Maurer, Universität Münster

"Einige Anwendungen optimaler Steuerprozesse"

Th. 24.04.97, 16 Uhr in 31/450a
 

Ch. Kassel, Universität Strasbourg

"Action of Gal(\overline{Q}/Q) on Vasiliev invariants fo knots"

Tu 22.04.97, 15 Uhr in 31/450a
 

D.E. Pearson, University of Hull

"Point Transfer Matrices for Second Order Differential Equations"

Tu 25.3.97, 16 Uhr in 31/452
 

A. Conca, Universität Genua

"Standard monomial theory for Hankel matrices"

Th 20.2.97, 17 Uhr in 31/452
 

J. Gubeladze, Tbilisi

"Algebraic K-Theory of affine toric varieties"

Fr 7.2.97, 17 Uhr in 31/452
 

Krebs und Domansky, St. Petersburg

"Analysis of Demography - Evolution in St. Petersburg"

Fr 31.01.97, 17 Uhr in 31/450a
 

H. Esnault, Essen

"Algebraische Chern-Simons- und Cheeger-Simons-Theorie"

Fr 17.01.97, 17 Uhr in 31/450a