Forthcoming Seminars
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Thu, 28/10/2010 16:00 |
Rosemary Dyson (University of Nottingham) |
Differential Equations and Applications Seminar  |
DH 1st floor SR |
| Many growing plant cells undergo rapid axial elongation with negligible radial expansion. Growth is driven by high internal turgor pressure causing viscous stretching of the cell wall, with embedded cellulose microfibrils providing the wall with strongly anisotropic properties. We present a theoretical model of a growing cell, representing the primary cell wall as a thin axisymmetric fibre-reinforced viscous sheet supported between rigid end plates. Asymptotic reduction of the governing equations, under simple sets of assumptions about the fibre and wall properties, yields variants of the traditional Lockhart equation, which relates the axial cell growth rate to the internal pressure. The model provides insights into the geometric and biomechanical parameters underlying bulk quantities such as wall extensibility and shows how either dynamical changes in wall material properties or passive fibre reorientation may suppress cell elongation. We then investigate how the action of enzymes on the cell wall microstructure can lead to the required dynamic changes in macroscale wall material properties, and thus demonstrate a mechanism by which hormones may regulate plant growth. | |||
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Thu, 28/10/2010 17:00 |
Raf Cluckers (Leuven) |
Logic Seminar |
L3 |
| Motivic exponential integrals are an abstract version of p-adic exponential integrals for big p. The latter in itself is a flexible tool to describe (families of) finite expontial sums. In this talk we will only need the more concrete view of "uniform in p p-adic integrals" instead of the abstract view on motivic integrals. With F. Loeser, we obtained a first transfer principle for these integrals, which allows one to change the characteristic of the local field when one studies equalities of integrals, which appeared in Ann. of Math (2010). This transfer principle in particular applies to the Fundamental Lemma of the Langlands program (see arxiv). In work in progress with Halupczok and Gordon, we obtain a second transfer principle which allows one to change the characteristic of the local field when one studies integrability conditions of motivic exponential functions. This in particular solves an open problem about the local integrability of Harish-Chandra characters in (large enough) positive characteristic. | |||
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Fri, 29/10/2010 10:00 |
Steven Turnbull (Nuffield Department of Surgery) |
Industrial and Interdisciplinary Workshops |
DH 1st floor SR |
| We will try to cover the following problems in the workshop: (1) Modelling of aortic aneurisms showing the changes in blood flow / wall loads before and after placements of aortic stents; (2) Modelling of blood flows / wall loads in interracial aneurisms when flow diverters are used; (3) Metal artefact reduction in computer tomography (CT). If we run out of time the third topic may be postponed. | |||
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Fri, 29/10/2010 11:45 |
John Allen and Angela Mihai (Oxford) |
Industrial and Interdisciplinary Workshops |
DH 1st floor SR |
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John Allen: The Bennett Pinch revisited Abstract: The original derivation of the well-known Bennett relation is presented. Willard H. Bennett developed a theory, considering both electric and magnetic fields within a pinched column, which is completely different from that found in the textbooks. The latter theory is based on simple magnetohydrodynamics which ignores the electric field. The discussion leads to the interesting question as to whether the possibility of purely electrostatic confinement should be seriously considered. Angela Mihai: A mathematical model of coupled chemical and electrochemical processes arising in stress corrosion cracking Abstract: A general mathematical model for the electrochemistry of corrosion in a long and narrow metal crack is constructed by extending classical kinetic models to also incorporate physically realistic kinematic conditions of metal erosion and surface film growth. In this model, the electrochemical processes are described by a system of transport equations coupled through an electric field, and the movement of the metal surface is caused, on the one hand, by the corrosion process, and on the other hand, by the undermining action of a hydroxide film, which forms by consuming the metal substrate. For the model problem, approximate solutions obtained via a combination of analytical and numerical methods indicate that, if the diffusivity of the metal ions across the film increases, a thick unprotective film forms, while if the rate at which the hydroxide produces is increased, a thin passivating film develops. |
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Fri, 29/10/2010 14:00 |
Dr John MacKenzie (University of Strathclyde)) |
Mathematical Biology and Ecology Seminar |
L1 |
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Fri, 29/10/2010 14:15 |
Matheus Grasselli (McMaster University Canada) |
Nomura Seminar |
DH 1st floor SR |
| A stock loan is a contract between two parties: the lender, usually a bank or other financial institution providing a loan, and the borrower, represented by a client who owns one share of a stock used as collateral for the loan. Several reasons might motivate the client to get into such a deal. For example he might not want to sell his stock or even face selling restrictions, while at the same time being in need of available funds to attend to another financial operation. In Xia and Zhou (2007), a stock loan is modeled as a perpetual American option with a time varying strike and analyzed in detail within the Black-Scholes framework. In this paper, we extend the valuation of such loans to an incomplete market setting, which takes into account the natural trading restrictions faced by the client. When the maturity of the loan is infinite we obtain an exact formula for the value of the loan fee to be charged by the bank based on a result in Henderson (2007). For loans of finite maturity, we characterize its value using a variational inequality first presented in Oberman and Zariphopoulou (2003). In both cases we show analytically how the fee varies with the model parameters and illustrate the results numerically. This is joint work with Cesar G. Velez (Universidad Nacional de Colombia). | |||
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Mon, 01/11/2010 12:00 |
Rhys Davies (Oxford) |
String Theory Seminar |
L3 |
| Recently, two new Calabi-Yau threefolds have been discovered which have small Hodge numbers, and give rise to three chiral generations of fermions via the so-called 'standard embedding' compactification of the heterotic string.In this talk I will describe how to deform the standard embedding on these manifolds in order to achieve the correct gauge group. I will also describe how to calculate the resulting spectrum and interactions, which is still work in progress. | |||
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Mon, 01/11/2010 14:15 |
Martin Huesmann |
Stochastic Analysis Seminar |
Eagle House |
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Mon, 01/11/2010 14:15 |
Andrew Dancer (Oxford) |
Geometry and Analysis Seminar |
L3 |
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Mon, 01/11/2010 15:45 |
Alison Etheridge (University of Oxford) |
Stochastic Analysis Seminar |
Eagle House |
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Mon, 01/11/2010 15:45 |
Tom Leinster (Glasgow) |
Topology Seminar |
L3 |
| There is a close but underexploited analogy between the Euler characteristic of a topological space and the cardinality of a set. I will give a quite general definition of the "magnitude" of a mathematical structure, framed categorically. From this single definition can be derived many cardinality-like invariants (some old, some new): the Euler characteristic of a manifold or orbifold, the Euler characteristic of a category, the magnitude of a metric space, the Euler characteristic of a Koszul algebra, and others. A conjecture states that this purely categorical definition also produces the classical invariants of integral geometry: volume, surface area, perimeter, .... No specialist knowledge will be assumed. | |||
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Mon, 01/11/2010 16:00 |
James Maynard (Oxford) |
Junior Number Theory Seminar |
SR1 |
| The Siegel-Walfisz theorem gives an asymptotic estimate for the number of primes in an arithmetic progression, provided the modulus of the progression is small in comparison with the length of the progression. Counting primes is harder when the modulus is not so small compared to the length, but estimates such as Linnik's constant and the Brun-Titchmarsh theorem give us some information. We aim to look in particular at upper bounds for the number of primes in such a progression, and improving the Brun-Titchmarsh bound. | |||
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Mon, 01/11/2010 17:00 |
Petru Mironescu (Universite Lyon 1) |
Partial Differential Equations Seminar |
Gibson 1st Floor SR |
The maps  which are continuous in  and circle-valued are precisely the maps of the form  , where the phase  is continuous and real-valued.
In the context of Sobolev spaces, this is not true anymore: a map in some Sobolev space  need not have a phase in the same space. However, it is still possible to describe all the circle-valued Sobolev maps. The characterization relies on a factorization formula for Sobolev maps, involving three objects: good phases, bad phases, and topological singularities. This formula is the analog, in the circle-valued context, of Weierstrass' factorization theorem for holomorphic maps.
The purpose of the talk is to describe the factorization and to present a puzzling byproduct concerning sums of Dirac masses. |
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Tue, 02/11/2010 10:00 |
Lionel Mason (Oxford) |
Twistor Workshop |
Gibson 1st Floor SR |
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Tue, 02/11/2010 12:00 |
Francis Bursa (Cambridge) |
Quantum Field Theory Seminar |
L3 |
| String field theory is a candidate for a full non-perturbative definition of string theory. We aim to define string field theory on a space-time lattice to investigate its behaviour at the quantum level. Specifically, we look at string field theory in a one dimensional linear dilaton background, using level truncation to restrict the theory to a finite number of fields. I will report on our preliminary results at level-0 and level-1. | |||
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Tue, 02/11/2010 13:15 |
Athanasios Tsanas (OCIAM and SAMP) |
Junior Applied Mathematics Seminar |
Gibson Grd floor SR |
| This work demonstrates how we can extract clinically useful patternsextracted from time series data (speech signals) using nonlinear signal processing and how to exploit those patterns using robust statistical machine learning tools, in order to estimate remotely and accurately average Parkinson's disease symptom severity. | |||
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Tue, 02/11/2010 15:45 |
Ben Davison (Oxford) |
Algebraic and Symplectic Geometry Seminar |
L3 |
| I will describe recent work on motivic DT invariants for 3-manifolds, which are expected to be a refinement of Chern-Simons theory. The conclusion will be that these should be possible to define and work with, but there will be some interesting problems along the way. There will be a discussion of the problem of upgrading the description of the moduli space of flat connections as a critical locus to the problem of describing the fundamental group algebra of a 3-fold as a "noncommutative critical locus," including a recent topological result on obstructions for this problem. I will also address the question of how a motivic DT invariant may be expected to pick up a finer invariant of 3-manifolds than just the fundamental group. | |||
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Tue, 02/11/2010 16:00 |
Benno Kuckuck (Oxford University) |
Junior Geometric Group Theory Seminar |
DH 3rd floor SR |
| Geoghegan's stack construction is a tool for analysing groups that act on simply connected CW complexes, by providing a topological description in terms of cell stabilisers and the quotient complex, similar to what Bass-Serre theory does for group actions on trees. We will introduce this construction and see how it can be used to give results on finiteness properties of groups. | |||
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Tue, 02/11/2010 17:00 |
Nikolay Nikolov (Imperial College) |
Algebra Seminar |
L2 |
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Tue, 02/11/2010 17:00 |
Mark Opmeer (Bath) |
Functional Analysis Seminar |
L3 |
