Forthcoming Seminars
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Thu, 18/02/2010 14:30 |
Nicole Snashall (University of Leicester) |
Representation Theory Seminar  |
L3 |
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Thu, 18/02/2010 16:00 |
Stephan Baier (Bristol) |
Number Theory Seminar |
L3 |
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Thu, 18/02/2010 16:30 |
Cameron Hall (OCCAM) |
Differential Equations and Applications Seminar |
DH 1st floor SR |
| If an ideal elastic spring is greatly stretched, it will develop large stresses. However, solid biological tissues are able to grow without developing such large stresses. This is because the cells within such tissues are able to lay down new fibres and remove old ones, fundamentally changing the mechanical structure of the tissue. In many ways, this is analogous to classical plasticity, where materials stretched beyond their yield point begin to flow and the unloaded state of the material changes. Unfortunately, biological tissues are not closed systems and so we are not able to use standard plasticity techniques where we require the flow to be mass conserving and energetically passive. In this talk, a general framework will be presented for modelling the changing zero stress state of a biological tissue (or any other material). Working from the multiplicative decomposition of the deformation gradient, we show that the rate of 'desired' growth can represented using a tensor that describes both the total rate of growth and any directional biases. This can be used to give an evolution equation for the effective strain (a measure of the difference between the current state and the zero stress state). We conclude by looking at a perhaps surprising application for this theory as a method for deriving the constitutive laws of a viscoelastic fluid. | |||
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Thu, 18/02/2010 17:00 |
Nicolai Vorobjov (Bath) |
Logic Seminar |
L3 |
| We study upper bounds on topological complexity of sets definable in o-minimal structures over the reals. We suggest a new construction for approximating a large class of definable sets, including the sets defined by arbitrary Boolean combinations of equations and inequalities, by compact sets. Those compact sets bound from above the homotopies and homologies of the approximated sets. The construction is applicable to images under definable maps. Based on this construction we refine the previously known upper bounds on Betti numbers of semialgebraic and semi-Pfaffian sets defined by quantifier-free formulae, and prove similar new upper bounds, individual for different Betti numbers, for their images under arbitrary continuous definable maps. Joint work with A. Gabrielov. | |||
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Fri, 19/02/2010 10:00 |
Elizabeth Jeffers (Oxford Centre for the Environment) |
Industrial and Interdisciplinary Workshops |
DH 1st floor SR |
| I have reconstructed multiple palaeoecological records from sites across the British Isles; this work has resulted in detailed time series that demonstrate changes in vegetation, herbivore density, nitrogen cycling, fire levels and air temperature across an 8,000 year time span covering the end of the last glacial period. The aim of my research is to use statistics to infer the relationships between vegetation changes and changes in the abiotic and biotic environment in which they occurred. This aim is achieved by using a model-fitting and model-selection method whereby sets of ordinary differential equations (ODE) are ‘fitted’ to the time series data via maximum likelihood estimation in order to find the model(s) that provide the closest match to the data. Many of the differential equation models that I have used in this study are well established in the theoretical ecology literature (i.e. plant – resource dynamics and plant – herbivore dynamics); however, there are no existing ODE models of fire or temperature dynamics that were appropriate for my data. For this workshop, I will present the palaeoecological data that I collected along with the models that I have chosen to work with (including my first attempt at models for fire and temperature dynamics) and I hope to get your feedback on these models and suggestions for other useful modelling methods that could be used to represent these dynamics. | |||
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Fri, 19/02/2010 12:00 |
Lionel Mason (Oxford) |
Twistor Workshop |
Gibson 1st Floor SR |
| After hep-th/0909.0483 | |||
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Fri, 19/02/2010 14:00 |
Prof Dan V Nicolou (University of Liverpool) |
Mathematical Biology Seminar |
L3 |
| TBA | |||
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Fri, 19/02/2010 14:15 |
Carole Bernard (Waterloo University) |
Nomura Seminar |
DH 1st floor SR |
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Mon, 22/02/2010 12:00 |
Riccardo Ricci (Imperial College) |
String Theory Seminar |
L3 |
| According to AdS/CFT a remarkable correspondence exists between strings in AdS5 x S5 and operators in N=4 SYM. A particularly important case is that of fast-spinning folded closed strings and the so called twist-operators in the gauge theory. This is a remarkable tool for uncovering and checking the detailed structure of the AdS/CFT correspondence and its integrability properties. In this talk I will show how to match the expression of the anomalous dimension of twist operators as computed from the quantum superstring with the result obtained from the Bethe ansatz of SYM. This agreement resolves a long-standing disagreement between gauge and string sides of the AdS/CFT duality and provides a highly nontrivial strong coupling test of SYM integrability. | |||
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Mon, 22/02/2010 14:15 |
Eduardo Esteves (IMPA) |
Geometry and Analysis Seminar |
L3 |
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Mon, 22/02/2010 14:15 |
Yi Lei Hu (University of Paris VI, France) |
Stochastic Analysis Seminar |
Eagle House |
| We study a generalized version of the signaling processoriginally introduced and studied by Argiento, Pemantle, Skyrms and Volkov(2009), which models how two interacting agents learn to signal each other andthus create a common language. We show that the process asymptotically leads to the emergence of a graph ofconnections between signals and states which has the property that nosignal-state correspondance could be associated both to a synonym and aninformational bottleneck. | |||
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Mon, 22/02/2010 15:45 |
Massimiliano Gubinelli (Paris, Dauphine) |
Stochastic Analysis Seminar |
Eagle House |
| By means of a series of examples (Korteweg-de Vries equation, non- linear stochastic heat equations and Navier-Stokes equation) we will show how it is possible to apply rough path ideas in the study of the Cauchy problem for PDEs with and without stochastic terms. | |||
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Mon, 22/02/2010 15:45 |
Pierre-Emmanuel Caprace, Bruxelles (Bruxelles) |
Topology Seminar |
L3 |
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Mon, 22/02/2010 16:00 |
TBA (Mathematical Institute, Oxford) |
Junior Number Theory Seminar |
SR1 |
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Mon, 22/02/2010 16:00 |
James Maynard (Mathematical Institute, Oxford) |
Junior Number Theory Seminar |
SR1 |
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Mon, 22/02/2010 17:00 |
Alexander Mielke (WIAS Berlin) |
Partial Differential Equations Seminar |
Gibson 1st Floor SR |
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Tue, 23/02/2010 13:15 |
Siddharth Arora (University of Oxford) |
Junior Applied Mathematics Seminar |
DH 1st floor SR |
| Abstract: Nonlinear models have been widely employed to characterize the underlying structure in a time series. It has been shown that the in-sample fit of nonlinear models is better than linear models, however, the superiority of nonlinear models over linear models, from the perspective of out-of-sample forecasting accuracy remains doubtful. We compare forecast accuracy of nonlinear regime switching models against classical linear models using different performance scores, such as root mean square error (RMSE), mean absolute error (MAE), and the continuous ranked probability score (CRPS). We propose and investigate the efficacy of a class of simple nonparametric, nonlinear models that are based on estimation of a few parameters, and can generate more accurate forecasts when compared with the classical models. Also, given the importance of gauging uncertainty in forecasts for proper risk assessment and well informed decision making, we focus on generating and evaluating both point and density forecasts. Keywords: Nonlinear, Forecasting, Performance scores. | |||
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Tue, 23/02/2010 14:00 |
Kentaro Nagao (Oxford and Kyoto) |
Algebraic and Symplectic Geometry Seminar |
SR1 |
Let  be a quiver with a potential given by successive mutations from a quiver with a potential  . Then we have an equivalence of the derived categories of dg-modules over the Ginzburg dg-algebras satisfying the following condition: a simple module over the dg-algebra for is either concentrated on degree 0 or concentrated on degree 1 as a dg-module over the
dg-algebra for . As an application of this equivalence, I will give a description of the space of stability conditions. |
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Tue, 23/02/2010 14:15 |
Frank Riedel (Bielefeld University) |
Nomura Seminar |
DH 1st floor SR |
| We develop a theory of optimal stopping problems under ambiguity in continuous time. Using results from (backward) stochastic calculus, we characterize the value function as the smallest (nonlinear) supermartingale dominating the payoff process. For Markovian models, we derive a Hamilton–Jacobi–Bellman equation involving a nonlinear drift term that describes the agent’s ambiguity aversion. We show how to use these general results for search problems and American Options. | |||
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Tue, 23/02/2010 14:30 |
Lowell Beineke (Purdue) |
Combinatorial Theory Seminar |
L3 |
| The line graph operation, in which the edges of one graph are taken as the vertices of a new graph with adjacency preserved, is arguably the most interesting of graph transformations. In this survey, we will begin looking at characterisations of line graphs, focusing first on results related to our set of nine forbidden subgraphs. This will be followed by a discussion of some generalisations of line graphs, including our investigations into the Krausz dimension of a graph G, defined as the minimum, over all partitions of the edge-set of G into complete subgraphs, of the maximum number of subgraphs containing any vertex (the maximum in Krausz's characterisation of line graphs being 2). | |||
