Forthcoming Seminars

Tue, 26/01/2010
14:30 		Jan Hladky (University of Warwick) Combinatorial Theory Seminar Add to calendar L3
Tree packing conjectures; Graceful tree labelling conjecture
A family of graphs $ H_1,...,H_k $ packs into a graph $ G $ if there exist pairwise edge-disjoint copies of $ H_1,...,H_k $ in $ G $. Gyarfas and Lehel conjectured that any family $ T_1, ..., T_n $ of trees of respective orders $ 1, ..., n $ packs into $ K_n $. A similar conjecture of Ringel asserts that $ 2n $ copies of any trees $ T $ on $ n+1 $ vertices pack into $ K_{2n+1} $. In a joint work with Boettcher, Piguet, Taraz we proved a theorem about packing trees. The theorem implies asymptotic versions of the above conjectures for families of trees of bounded maximum degree. Tree-indexed random walks controlled by the nibbling method are used in the proof. In a joint work with Adamaszek, Adamaszek, Allen and Grosu, we used the nibbling method to prove the approximate version of the related Graceful Tree Labelling conjecture for trees of bounded degree. In the talk we shall give proofs of both results. We shall discuss possible extensions thereof to trees of unbounded degree.
Tue, 26/01/2010
15:45 		Richard Thomas (Imperial College London) Algebraic and Symplectic Geometry Seminar Add to calendar L3
(HoRSe seminar) Symmetric and reduced obstruction theories
I will describe some more of the deformation theory necessary for the first talk. This leads to a number of natural questions and counterexamples. This talk requires a strong stomach, or a fanatical devotion to symmetric obstruction theories.
Tue, 26/01/2010
16:00 		Alexander Coward (Oxford/University of California at Davis) Junior Geometric Group Theory Seminar Add to calendar SR1
Unknotting operations on knots and the mapping class group
Tue, 26/01/2010
17:00 		Nikolai Nikolov (Imperial College London) Algebra Seminar Add to calendar L2
Wreath products and bounded generation
Tue, 26/01/2010
17:00 		Gordon Blower (Lancaster) Functional Analysis Seminar Add to calendar L3
Linear systems associated with Painleve's equation VI
Wed, 27/01/2010
10:10 		Jan Haskovec and Christian Schmeiser (TBA) OCCAM Literature Seminar Add to calendar OCCAM Common Room (RI2.28)
Stochastic Particle Approximation for Measure Valued Solutions of the 2D Keller-Segal System
tba
Wed, 27/01/2010
11:30 		Nikolay Nikolov (Imperial College, London) Algebra Kinderseminar Add to calendar ChCh, Tom Gate, Room 2
Linear algebra and geometric combinatorics
Thu, 28/01/2010
11:00 		Philip Scowcroft (Oxford and Wesleyan) Advanced Logic Class Add to calendar SR2
Some Examples of Theories lacking the Independence Property
Thu, 28/01/2010
12:30 		François Genoud (OxPDE, University of Oxford) OxPDE Lunchtime Seminar Add to calendar Gibson 1st Floor SR
Statistical Theories of Liquid Crystals: Onsager, Maier-Saupe and Beyond
I will present in detail the celebrated theories of Onsager (1949) and Maier-Saupe (1958) explaining the phenomenon of long-range orientational order in nematic liquid crystals. The models are not rigorous from the mathematical viewpoint and my talk will stay at the formal level. If time permits, I will suggest directions towards a rigorous mean-field theory.
Thu, 28/01/2010
13:15 		Steven Rayan (Oxford) Junior Geometry and Topology Seminar Add to calendar SR1
Co-Higgs bundles II: fibrations and moduli spaces
After reviewing the salient details from last week's seminar, I will construct an explicit example of a spectral curve, using co-Higgs bundles of rank 2. The role of the spectral curve in understanding the moduli space will be made clear by appealing to the Hitchin fibration, and from there inferences (some of them very concrete) can be made about the structure of the moduli space. I will make some conjectures about the higher-dimensional picture, and also try to show how spectral varieties might live in that picture.
Thu, 28/01/2010
14:00 		Dr. Catherine Powell (University of Manchester) Computational Mathematics and Applications Add to calendar 3WS SR
Preconditioning Stochastic Finite Element Matrices
In the last few years, there has been renewed interest in stochastic finite element methods (SFEMs), which facilitate the approximation of statistics of solutions to PDEs with random data. SFEMs based on sampling, such as stochastic collocation schemes, lead to decoupled problems requiring only deterministic solvers. SFEMs based on Galerkin approximation satisfy an optimality condition but require the solution of a single linear system of equations that couples deterministic and stochastic degrees of freedom. This is regarded as a serious bottleneck in computations and the difficulty is even more pronounced when we attempt to solve systems of PDEs with random data via stochastic mixed FEMs. In this talk, we give an overview of solution strategies for the saddle-point systems that arise when the mixed form of the Darcy flow problem, with correlated random coefficients, is discretised via stochastic Galerkin and stochastic collocation techniques. For the stochastic Galerkin approach, the systems are orders of magnitude larger than those arising for deterministic problems. We report on fast solvers and preconditioners based on multigrid, which have proved successful for deterministic problems. In particular, we examine their robustness with respect to the random diffusion coefficients, which can be either a linear or non-linear function of a finite set of random parameters with a prescribed probability distribution.
Thu, 28/01/2010
16:00 		Jahan Zahid (Oxford) Number Theory Seminar Add to calendar L3
Minimizing p-adic projective varieties
Thu, 28/01/2010
16:00 		Tamas Hausel (Oxford) Seminar Add to calendar SR1
Mirror Symmetry, Langlands duality and the Hitchin System
tba
Thu, 28/01/2010
16:30 		Phil Hall (Imperial College London) Differential Equations and Applications Seminar Add to calendar DH 1st floor SR
STEADY STREAMING, VORTEX WAVE INTERACTION THEORY, SELF SUSTAINED PROCESSES AND COHERENT STRUCTURES IN TURBULENT SHEAR FLOWS
Some years ago Hall and Smith in a number of papers developed a theory governing the interaction of vortices and waves in shear flows. In recent years immense interest has been focused on so-called self-sustained processes in turbulent shear flows where the importance of waves interacting with streamwise vortex flows has been elucidated in a number of; see for example the work of Waleffe and colleagues, Kerswell, Gibson, etc. These processes have a striking resemblance to coherent structures observed in turbulent shear flow and for that reason they are often referred to as exact coherent structures. It is shown that the structures associated with the so-called 'lower branch' state, which has been shown to play a crucial role in these self-sustained process, is nothing but a Rayleigh wave vortex interaction with a wave system generating streamwise vortices inside a critical layer. The theory enables the reduction of the 3D Navier Stokes equations to a coupled system for a steady streamwise vortex and an inviscid wave system. The reduced system for the streamwise vortices must be solved with jump conditions in the shear across the critical layer and the position of that layer constitutes a nonlinear pde eigenvalue problem. Remarkable agreement between the asymptotic theory and numerical simulations is found thereby demonstrating the importance of vortex-wave interaction theory in the mathematical description of coherent structures in turbulent shear flows. The theory offers the possibility of drag reduction in turbulent shear flows by controlling the flow to the neighborhood of the lower branch state. The relevance of the work to more general shear flows is also discussed.
Thu, 28/01/2010
17:00 		Jeroen Demeyer (Gwent) Logic Seminar Add to calendar L3
To Be Announced
Thu, 28/01/2010
17:00 		Jeroen Demeyer (Ghent) Logic Seminar Add to calendar L3
Diophantine Sets of Polynomials over Number Fields
 Let R be a number field (or a recursive subring of anumber field) and consider the polynomial ring R[T].We show that the set of polynomials with integercoefficients is diophantine (existentially definable) over R[T].Applying a result by Denef, this implies that everyrecursively enumerable subset of R[T]^k is diophantine over R[T].
Fri, 29/01/2010
10:00 		Harvey Burd (Department of Engineering Science, University of Oxford) Industrial and Interdisciplinary Workshops Add to calendar DH 1st floor SR
Mechanics of the accommodation mechanism in the human eye
When the human eye looks at a distant object, the lens is held in a state of tension by a set of fibres (known as zonules) that connect the lens to the ciliary body. To view a nearby object, the ciliary muscle (which is part of the ciliary body) contracts. This reduces the tension in the zonules, the lens assumes a thicker and more rounded shape and the optical power of the eye increases. This process is known as accommodation. With increased age, however, the accommodation mechanism becomes increasingly ineffective so that, from an age of about 50 years onwards, it effectively ceases to function. This condition is known as presbyopia. There is considerable interest in the ophthalmic community on developing a better understanding of the ageing processes that cause presbyopia. As well as being an interesting scientific question in its own right, it is hoped that this improved understanding will lead to improved surgical procedures (e.g. to re-start the accommodation process in elderly cataract patients).
Fri, 29/01/2010
11:45 		Sarah McBurnie and Dave Hewett Industrial and Interdisciplinary Workshops Add to calendar DH 1st floor SR
OCIAM internal seminar
McBurnie: “Sound propagation through bubbly liquids”. Hewett: "Switching on a time-harmonic acoustic source".
Fri, 29/01/2010
12:00 		Lionel Mason (Oxford) Twistor Workshop Add to calendar Gibson 1st Floor SR
Unification of Grassmannian Residues and Dualities after Arkani-Hamed et. al.
This is a review of hep-th/0912.4912
Fri, 29/01/2010
14:15 		Marco Frittelli (Milan) Nomura Seminar Add to calendar DH 1st floor SR
Dual Representation of Conditional Quasiconvex Maps

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

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