Forthcoming Seminars

Mon, 24/11/2008
12:00 		Jock McOrist (Chicago) String Theory Seminar Add to calendar L3
Summing the Instantons in the Heterotic String
Abstract: I will discuss some recent developments in understanding compactifications of the Heterotic string on Calabi-Yau manifolds. These compactifications are well-described by linear sigma models with (0,2) supersymmetry. I will show how to use these models to compute physical observables, such as genus zero Yukawa couplings, their singularity structure, and dependence on bundle moduli.
Mon, 24/11/2008
13:30 		Yoshihito Oshita (Okayama University, Japan) OxPDE Lunchtime Seminar Add to calendar Gibson 1st Floor SR
Fine structures arising in diblock copolymers and reaction-diffusion systems
We consider a class of energy functionals containing a small parameter ε and a long-range interaction. Such functionals arise from models for phase separation in diblock copolymers and from stationary solutions of FitzHugh–Nagumo type systems. On an interval of arbitrary length, we show that every global minimizer is periodic, and provide asymptotic expansions for the periods. In 2D, periodic hexagonal structures are observed in experiments in certain di-block copolymer melts. Using the modular function and an heuristic reduction of a mathematical model, we present a mathematical account of a hexagonal pattern selection observed in di-block copolymer melts. We also consider the sharp interface problem arising in the singular limit, and prove the existence and the nondegeneracy of solutions whose interface is a distorted circle in a two-dimensional bounded domain without any assumption on the symmetry of the domain.
Mon, 24/11/2008
14:15 		Dr. Anke Wiese (Heriot-Watt University) Stochastic Analysis Seminar Add to calendar Oxford-Man Institute
Numerical Solution of Stochastic Differential Equations Evolving on Manifolds
We present numerical schemes for nonlinear stochastic differential equations whose solution evolves on a smooth finite dimensional manifold. Given a Lie group action that generates transport along the manifold, we pull back the stochastic flow on the manifold to the Lie group via the action and subsequently to the corresponding Lie algebra. We construct an approximation to the stochastic flow in the Lie algebra via closed operations and then push back to the manifold, thus ensuring our approximation lies in the manifold. We call such schemes stochastic Munthe-Kaas methods after their deterministic counterparts. We also present stochastic Lie group integration schemes based on Castell–Gaines methods. They become stochastic Lie group integrator schemes if we use Munthe-Kaas methods as the underlying ordinary differential integrator. Lastly, we demonstrate our methods by presenting some numerical examples
Mon, 24/11/2008
15:45 		Paolo Salvatore (Rome) Topology Seminar Add to calendar L3
The topological cyclic Deligne conjecture
Mon, 24/11/2008
15:45 		Prof. Nathanael Enriquez (Paris X) Stochastic Analysis Seminar Add to calendar Oxford-Man Institute
Random walks in random environment on "Z"
We consider transient random walks in random environment on Z with zero asymptotic speed. In a seminal paper, Kesten, Kozlov and Spitzer proved that the hitting time of the level "n" converges in law, after a proper normalization, towards a positive stable law, but the question of the description of its parameter was left open since that time. A new approach to this problem, based on a precise description of Sinai's potential, leads to a complete characterization of this stable law, making a tight link with Kesten's renewal series. The case of Dirichlet environment turns out to be remarkably explicit. Quenched results on this model will be presented if time permits.
Mon, 24/11/2008
17:00 		Professor Alfio Quarteroni (l'École Polytechnique Fédérale de Lausanne) Alan Tayler Lecture Add to calendar Gulbenkian Lecture Theatre, St Cross Building, Manor Road
Mathematical Modeling In Medicine, Sports and Technology
Tea will be available in the Arumugam Building, St. Catherine's College, from 4.15pm.
Tue, 25/11/2008
12:00 		Christopher Llewellyn-Smith (Theoretical Physics Oxford and ITER Council) Quantum Field Theory Seminar Add to calendar L3
The LHC (Science, Technology and Politics)
Tue, 25/11/2008
14:30 		Artur Czumaj (Warwick) Combinatorial Theory Seminar Add to calendar L3
Testing expansion in bounded degree graphs really fast
In the first part of the talk we will introduce the notion of property testing and briefly discuss some results in testing graph properties in the framework of property testing. Then, we will discuss a recent result about testing expansion in bounded degree graphs. We focus on the notion of vertex-expansion:  
an $ a $-expander is a graph $ G = (V,E) $ in which every subset $ U $ of $ V $ of at most $ |V|/2 $ vertices has a neighborhood of size at least $ a|U| $. Our main result is that one can distinguish good expanders from graphs that are far from being weak expanders in time approximately $ O(n^{1/2}) $. We design a property testing algorithm that accepts every $ a $-expander with probability at least 2/3 and rejects every graph that is $ \epsilon $-far from an $ a^* $-expander with probability at least 2/3, where $ a^* = O(a^2/(d^2 log(n/\epsilon))) $, $ d $ is the maximum degree of the graphs, and a graph is called $ \epsilon $-far from an $ a^* $-expander if one has to modify (add or delete) at least $ \epsilon d n $ of its edges to obtain an $ a^* $-expander. The algorithm assumes the bounded-degree graphs model with adjacency list graph representation and its running time is $ O(d^2 n^{1/2} log(n/\epsilon)/(a^2 \epsilon^3)) $. This is a joint work with Christian Sohler.
Tue, 25/11/2008
15:00 		Georg Dolzmann (University of Regensburg) Partial Differential Equations Seminar Add to calendar Gibson 1st Floor SR
Relaxation and Gamma convergence results in models for crystal plasticity
Tue, 25/11/2008
17:00 		Nikolay Nikolov (Imperial College) Algebra Seminar Add to calendar L2
On the abstract images of profinite groups
I will discuss the following Conjecture B: Finitely generated abstract images of profinite groups are finite. I will explain how it relates to the width of words and conjugacy classes in finite groups. I will indicate a proof in the special case of 'non-universal' profinite groups and propose several directions for future work. This conjecture arose in my discussions with various participants of a workshop in Blaubeuren in May 2007 for which I am grateful. (You know who you are!)
Tue, 25/11/2008
17:00 		Wilhelm Winter (Nottingham) Functional Analysis Seminar Add to calendar L3
Topological and algebraic regularity properties of nuclear C*-algebras
We study a number of regularity properties of C*-algebras which are intimately related in the case of nuclear C*-algebras. These properties can be expressed topologically (as dimension type conditions), C*-algebraically (as stability under tensoring with suitable strongly self-absorbing C*-algebras), and at the level of homological invariants (in terms of comparison properties of projections, or positive elements, respectively). We explain these concepts and some known relations between them, and outline their relevance for the classification program. (As a particularly satisfying application, one obtains a classification result for C*-algebras associated to compact, finite-dimensional, minimal, uniquely ergodic, dynamical systems.) Furthermore, we investigate potential applications of these technologies to other areas, such as coarse geometry.
Wed, 26/11/2008
11:30 		Teru Thomas (University of Oxford) Algebra Kinderseminar Add to calendar
The Langlands Programme (ChCh Tom Gate, Room 2)
Wed, 26/11/2008
12:00 		Kiyoshi Igusa (Brandeis) Topology Seminar Add to calendar L3
Graph cohomology I: Introduction
Wed, 26/11/2008
13:30 		Eduard Kirr (University of Illinois at Urbana Champaign, USA) OxPDE Lunchtime Seminar Add to calendar Gibson 1st Floor SR
Variational Methods in Nonlinear Schroedinger Equations
The talk will survey old and recent applications of variational techniques in studying the existence, stability and bifurcations of time harmonic, localized in space solutions of the nonlinear Schroedinger equation (NLS). Such solutions are called solitons, when the equation is space invariant, and bound-states, when it is not. Due to the Hamiltonian structure of NLS, solitons/bound-states can be characterized as critical points of the energy functional restricted to sets of functions with fixed $ L^2 $ norm. In general, the energy functional is not convex, nor is the set of functions with fixed $ L^2 $ norm closed under weak convergence. Hence the standard variational arguments fail to imply existence of global minimizers. In addition for “critical" and “supercritical" nonlinearities the restricted energy functional is not bounded from below. I will first review the techniques used to overcome these drawbacks. Then I will discuss recent results in which the characterizations of bound-states as critical points (not necessarily global minima) of the restricted energy functional is used to show their orbital stability/instability with respect to the nonlinear dynamics and symmetry breaking phenomena as the $ L^2 $ norm of the bound-state is varied.
Thu, 27/11/2008
10:00 		Kiyoshi Igusa (Brandeis) Topology Seminar Add to calendar L3
Graph cohomology II: Kozul duality of operads
Thu, 27/11/2008
11:00 		Prof. Divakar Viswanath (University of Michigan, USA) Applied Dynamical Systems and Inverse Problems Seminar Add to calendar DH 3rd floor SR
Numerical methods for finding periodic solutions of dynamical systems
Recurrences have been central to the study of dynamical systems ever since the inception of the subject. Periodic solutions make the notion of recurrences exact. The Lorenz attractor is the best known example of a strange attractor and we will describe a method to find periodic solutions that lie on it. Additionally, we will consider a turbulent channel flow and describe the computation of time periodic solutions using nearly $ 300,000 $ degrees of freedom to represent the velocity field.
Thu, 27/11/2008
12:00 		Martijn Kool (Oxford) Junior Geometry and Topology Seminar Add to calendar SR1
Introduction to Deformation Theory
In this talk I will discuss some elementary notions of deformation theory in algebraic geometry like Schlessinger's Criterion. I will describe obstructions and deformations of sheaves in detail and will point out relations to moduli spaces of sheaves.

For any corrections/updates to these listings, please contact seminars [-at-] maths [dot] ox [dot] ac [dot] uk.

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