Forthcoming Seminars

Thu, 23/02/2012
16:00 		Chris Wuthrich (Nottingham) Number Theory Seminar Add to calendar L3
On the class group pairing on elliptic curves
Thu, 23/02/2012
16:00 		Brian Sleeman (Leeds University) Industrial and Applied Mathematics Seminar Add to calendar DH 1st floor SR
The Determination of an Obstacle from its Scattering Cross Section
The inverse acoustic obstacle scattering problem, in its most general form, seeks to determine the nature of an unknown scatterer from knowl- edge of its far eld or radiation pattern. The problem which is the main concern here is: If the scattering cross section, i.e the absolute value of the radiation pattern, of an unknown scatterer is known determine its shape. In this talk we explore the problem from a number of points of view. These include questions of uniqueness, methods of solution including it- erative methods, the Minkowski problem and level set methods. We con- clude by looking at the problem of acoustically invisible gateways and its connections with cloaking
Fri, 24/02/2012
11:00 		Eleanor Watson (Poikos) Industrial and Interdisciplinary Workshops Add to calendar DH 1st floor SR
computer imaging (producing accurate measurements of an object in front of a camera)

Problem #1: (marker-less scaling) Poikos ltd. has created algorithms for matching photographs of humans to three-dimensional body scans. Due to variability in camera lenses and body sizes, the resulting three-dimensional data is normalised to have unit height and has no absolute scale. The problem is to assign an absolute scale to normalised three-dimensional data.

Prior Knowledge: A database of similar (but different) reference objects with known scales. An imperfect 1:1 mapping from the input coordinates to the coordinates of each object within the reference database. A projection matrix mapping the three-dimensional data to the two-dimensional space of the photograph (involves a non-linear and non-invertible transform; x=(M*v)_x/(M*v)_z, y=(M*v)_y/(M*v)_z).

Problem #2: (improved silhouette fitting) Poikos ltd. has created algorithms for converting RGB photographs of humans in (approximate) poses into silhouettes. Currently, a multivariate Gaussian mixture model is used as a first pass. This is imperfect, and would benefit from an improved statistical method. The problem is to determine the probability that a given three-component colour at a given two-component location should be considered as "foreground" or "background".

Prior Knowledge: A sparse set of colours which are very likely to be skin (foreground), and their locations. May include some outliers. A (larger) sparse set of colours which are very likely to be clothing (foreground), and their locations. May include several distributions in the case of multi-coloured clothing, and will probably include vast variations in luminosity. A (larger still) sparse set of colours which are very likely to be background. Will probably overlap with skin and/or clothing colours. A very approximate skeleton for the subject.

Limitations: Sample colours are chosen "safely". That is, they are chosen in areas known to be away from edges. This causes two problems; highlights and shadows are not accounted for, and colours from arms and legs are under-represented in the model. All colours may be "saturated"; that is, information is lost about colours which are "brighter than white". All colours are subject to noise; each colour can be considered as a true colour plus a random variable from a gaussian distribution. The weight of this gaussian model is constant across all luminosities, that is, darker colours contain more relative noise than brighter colours.
Fri, 24/02/2012
14:00 		Kobi Kremnizer (Oxford) Quantum Mathematics and Computation Add to calendar Comlab
Homotopy Type Theory

In recent years, surprising connections between type theory and homotopy theory have been discovered. In this talk I will recall the notions of intensional type theories and identity types. I will describe "infinity groupoids", formal algebraic models of topological spaces, and explain how identity types carry the structure of an infinity groupoid. I will finish by discussing categorical semantics of intensional type theories.

The talk will take place in Lecture Theatre B, at the Department of Computer Science.
Fri, 24/02/2012
14:15 		Peter Forsyth (Waterloo) Nomura Seminar Add to calendar DH 1st floor SR
Comparison between the Mean Variance Optimal and the Mean Quadratic Variation Optimal Trading Strategies
Algorithmic trade execution has become a standard technique for institutional market players in recent years, particularly in the equity market where electronic trading is most prevalent. A trade execution algorithm typically seeks to execute a trade decision optimally upon receiving inputs from a human trader. A common form of optimality criterion seeks to strike a balance between minimizing pricing impact and minimizing timing risk. For example, in the case of selling a large number of shares, a fast liquidation will cause the share price to drop, whereas a slow liquidation will expose the seller to timing risk due to the stochastic nature of the share price. We compare optimal liquidation policies in continuous time in the presence of trading impact using numerical solutions of Hamilton Jacobi Bellman (HJB)partial differential equations (PDE). In particular, we compare the time-consistent mean-quadratic-variation strategy (Almgren and Chriss) with the time-inconsistent (pre-commitment) mean-variance strategy. The Almgren and Chriss strategy should be viewed as the industry standard. We show that the two different risk measures lead to very different strategies and liquidation profiles. In terms of the mean variance efficient frontier, the original Almgren/Chriss strategy is signficently sub-optimal compared to the (pre-commitment) mean-variance strategy. This is joint work with Stephen Tse, Heath Windcliff and Shannon Kennedy.
Fri, 24/02/2012
14:30 		Dr. Adrian Jenkins (British Antarctic Survey, Cambridge) Mathematical Geoscience Seminar Add to calendar DH 3rd floor SR
Ocean forcing of ice sheet change in West Antarctica
The part of the West Antarctic Ice Sheet that drains into the Amundsen Sea is currently thinning at such a rate that it contributes nearly 10 percent of the observed rise in global mean sea level. Acceleration of the outlet glaciers means that the sea level contribution has grown over the past decades, while the likely future contribution remains a key unknown. The synchronous response of several independent glaciers, coupled with the observation that thinning is most rapid at their downstream ends, where the ice goes afloat, hints at an oceanic driver. The general assumption is that the changes are a response to an increase in submarine melting of the floating ice shelves that has been driven in turn by an increase in the transport of ocean heat towards the ice sheet. Understanding the causes of these changes and their relationship with climate variability is imperative if we are to make quantitative estimates of sea level into the future. Observations made since the mid‐1990s on the Amundsen Sea continental shelf have revealed that the seabed troughs carved by previous glacial advances guide seawater around 3‐4°C above the freezing point from the deep ocean to the ice sheet margin, fuelling rapid melting of the floating ice. This talk summarises the results of several pieces of work that investigate the chain of processes linking large‐scale atmospheric processes with ocean circulation over the continental shelf and beneath the floating ice shelves and the eventual transfer of heat to the ice. While our understanding of the processes is far from complete, the pieces of the jigsaw that have been put into place give us insight into the potential causes of variability in ice shelf melting, and allow us to at least formulate some key questions that still need to be answered in order to make reliable projections of future ice sheet evolution in West Antarctica.
Mon, 27/02/2012
12:00 		Aristomenis Donos (Imperial College London) String Theory Seminar Add to calendar L3
Holographic stripes and helical superconductors
The AdS/CFT correspondence is a powerful tool to analyse strongly coupled quantum field theories. Over the past few years there has been a surge of activity aimed at finding possible applications to condensed matter systems. One focus has been to holographically realise various kinds of phases via the construction of fascinating new classes of black hole solutions. In this framework, I will discuss the possibility of describing finite temperature phase transitions leading to spontaneous breaking of translational invariance of the dual field theory at strong coupling. Along with the general setup I will also discuss specific string/M theory embeddings of the corresponding symmetry breaking modes leading to the description of such phases.
Mon, 27/02/2012
14:15 		Michael Scheutzow (TU Berlin) Stochastic Analysis Seminar Add to calendar Oxford-Man Institute
Long-time behaviour of stochastic delay equations

Abstract: First we provide a survey on the long-time behaviour of stochastic delay equations with bounded memory, addressing existence and uniqueness of invariant measures, Lyapunov spectra, and exponential growth rates.

Then, we study the very simple one-dimensional equation $dX(t)=X(t-1)dW(t)$ in more detail and establish the existence of a deterministic exponential growth rate of a suitable norm of the solution via a Furstenberg-Hasminskii-type formula.

Parts of the talk are based on joint work with Martin Hairer and Jonathan Mattingly. 

Mon, 27/02/2012
15:45 		Ieke Moerdijk (Utrecht and Sheffield) Topology Seminar Add to calendar L3
Infinity categories and infinity operads
I will discuss some aspects of the simplicial theory of infinity-categories which originates with Boardman and Vogt, and has recently been developed by Joyal, Lurie and others. The main purpose of the talk will be to present an extension of this theory which covers infinity-operads. It is based on a modification of the notion of simplicial set, called 'dendroidal set'. One of the main results is that the category of dendroidal sets carries a monoidal Quillen model structure, in which the fibrant objects are precisely the infinity operads,and which contains the Joyal model structure for infinity-categories as a full subcategory. (The lecture will be mainly based on joint work with Denis-Charles Cisinski.)
Mon, 27/02/2012
15:45 		NATHAEL GOZLAN (mlv France) Stochastic Analysis Seminar Add to calendar Oxford-Man Institute
Optimal transport, concentration of measure and functional inequalities.

This talk is devoted to Talagrand's transport-entropy inequality and its deep connections to the concentration of measure phenomenon, large deviation theory and logarithmic Sobolev inequalities. After an introductive part on the field, I will present recent results obtained with P-M Samson and C. Roberto establishing the equivalence of Talagrand's inequality to a restricted version of the Log-Sobolev inequality. If time enables, I will also present some works in progress about transport inequalities in a discrete setting.
Mon, 27/02/2012
17:00 		Peter M. Topping (University of Warwick) Partial Differential Equations Seminar Add to calendar Gibson 1st Floor SR
Mean Curvature Flow from Cones
This talk will consist of a pure PDE part, and an applied part. The unifying topic is mean curvature flow (MCF), and particularly mean curvature flow starting at cones. This latter subject originates from the abstract consideration of uniqueness questions for flows in the presence of singularities. Recently, this theory has found applications in several quite different areas, and I will explain the connections with Harnack estimates (which I will explain from scratch) and also with the study of the dynamics of charged fluid droplets. There are essentially no prerequisites. It would help to be familiar with basic submanifold geometry (e.g. second fundamental form) and intuition concerning the heat equation, but I will try to explain everything and give the talk at colloquium level. Joint work with Sebastian Helmensdorfer.
Tue, 28/02/2012
12:00 		Mahdi Godazgar (DAMTP, Cambridge) Relativity Seminar Add to calendar L3
Peeling of the Weyl tensor and gravitational radiation in higher dimensions.

Abstract: In this talk, I will discuss the peeling behaviour of the Weyl tensor near null infinity for asymptotically flat higher dimensional spacetimes. The result is qualitatively different from the peeling property in 4d. Also, I will discuss the rewriting of the Bondi energy flux in terms of "Newman-Penrose" Weyl components.
Tue, 28/02/2012
14:30 		Penny Haxell (Waterloo) Combinatorial Theory Seminar Add to calendar L3
On packing and covering in hypergraphs
We discuss some recent developments on the following long-standing problem known as Ryser's conjecture. Let $ H $ be an $ r $-partite $ r $-uniform hypergraph. A matching in $ H $ is a set of disjoint edges, and we denote by $ \nu(H) $ the maximum size of a matching in $ H $. A cover of $ H $ is a set of vertices that intersects every edge of $ H $. It is clear that there exists a cover of $ H $ of size at most $ r\nu(H) $, but it is conjectured that there is always a cover of size at most $ (r-1)\nu(H) $.
Tue, 28/02/2012
15:45 		Oliver Fabert (Freiburg) Algebraic and Symplectic Geometry Seminar Add to calendar L3
Local symplectic field theory and stable hypersurfaces in symplectic blow-ups
Symplectic field theory (SFT) can be viewed as TQFT approach to Gromov-Witten theory. As in Gromov-Witten theory, transversality for the Cauchy-Riemann operator is not satisfied in general, due to the presence of multiply-covered curves. When the underlying simple curve is sufficiently nice, I will outline that the transversality problem for their multiple covers can be elegantly solved using finite-dimensional obstruction bundles of constant rank. By fixing the underlying holomorphic curve, we furthermore define a local version of SFT by counting only multiple covers of this chosen curve. After introducing gravitational descendants, we use this new version of SFT to prove that a stable hypersurface intersecting an exceptional sphere (in a homologically nontrivial way) in a closed four-dimensional symplectic manifold must carry an elliptic orbit. Here we use that the local Gromov-Witten potential of the exceptional sphere factors through the local SFT invariants of the breaking orbits appearing after neck-stretching along the hypersurface.
Tue, 28/02/2012
16:00 		Wladimir Neves (UFRJ-Federal University of Rio de Janeiro) OxPDE Special Seminar Add to calendar Gibson 1st Floor SR
Generalized Buckley-Leverett System

We show the solvability of a proposed Generalized Buckley-LeverettSystem, which is related to multidimensional Muskat Problem. More-over, we discuss some important questions concerning singular limitsof the proposed model.
Tue, 28/02/2012
17:00 		Ashot Minasyan (University of Southampton) Algebra Seminar Add to calendar L2
"Tits alternatives for graph products of groups".

 Graph products of groups naturally generalize direct and free products and have a rich subgroup structure. Basic examples of graph products are right angled Coxeter and Artin groups. I will discuss various forms of Tits Alternative for subgroups and
their stability under graph products. The talk will be based on a joint work with Yago Antolin Pichel.
Tue, 28/02/2012
17:00 		Richard Smith (University College Dublin) Functional Analysis Seminar Add to calendar L3
Strictly convex norms and topology

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

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