Forthcoming Seminars

Mon, 09/11/2009
16:00 		James Maynard (Mathematical Institute, Oxford) Junior Number Theory Seminar Add to calendar SR1
Digits of primes
Mon, 09/11/2009
17:00 		John Greenlees (Sheffield) K-Theory Day Add to calendar L3
Homotopy invariant characterizations of complete intersections
Mon, 09/11/2009
17:00 		Reza Pakzad (University of Pittsburgh) Partial Differential Equations Seminar Add to calendar Gibson 1st Floor SR
Elastic models for growing tissues: scaling laws and derivation by Gamma convergence
Certain elastic structures and growing tissues (leaves, flowers or marine invertebrates) exhibit residual strain at free equilibria. We intend to study this phenomena through an elastic growth variational model. We will first discuss this model from a differential geometric point of view: the growth seems to change the intrinsic metric of the tissue to a new target non-flat metric. The non-vanishing curvature is the cause of the non-zero stress at equilibria. We further discuss the scaling laws and $ \Gamma $-limits of the introduced 3d functional on thin plates in the limit of vanishing thickness. Among others, given special forms of growth tensors, we rigorously derive the non-Euclidean versions of Kirchhoff and von Karman models for elastic non-Euclidean plates. Sobolev spaces of isometries and infinitesimal isometries of 2d Riemannian manifolds appear as the natural space of admissible mappings in this context. In particular, as a side result, we obtain an equivalent condition for existence of a $ W^{2,2} $ isometric immersion of a given $ 2 $d metric on a bounded domain into $ \mathbb R3 $.
Tue, 10/11/2009
14:00 		Harald Raecke (Warwick) Combinatorial Theory Seminar Add to calendar L3
Oblivious Routing in the $ L_p $ norm
HTML clipboard Gupta et al. introduced a very general multi-commodity flow problem in which the cost of a given flow solution on a graph $ G=(V,E) $ is calculated by first computing the link loads via a load-function l, that describes the load of a link as a function of the flow traversing the link, and then aggregating the individual link loads into a single number via an aggregation function.   We show the existence of an oblivious routing scheme with competitive ratio $ O(\log n) $ and a lower bound of $ \Omega(\log n/\logl\og n) $ for this model when the aggregation function agg is an $ L_p $-norm.   Our results can also be viewed as a generalization of the work on approximating metrics by a distribution over dominating tree metrics and the work on minimum congestion oblivious. We provide a convex combination of trees such that routing according to the tree distribution approximately minimizes the $ L_p $-norm of the link loads. The embedding techniques of Bartal and Fakcharoenphol et al. [FRT03] can be viewed as solving this problem in the $ L_1 $-norm while the result on congestion minmizing oblivious routing solves it for $ L_\infty $. We give a single proof that shows the existence of a good tree-based oblivious routing for any $ L_p $-norm.
Tue, 10/11/2009
14:15 		Dr Aymeric Spiga (The Open University and LMD/UPMC) Geophysical and Nonlinear Fluid Dynamics Seminar Add to calendar Dobson Room, AOPP
Dynamics of the Martian atmosphere at the mesoscale and the microscale
Tue, 10/11/2009
14:50 		Colin McDiarmid (Oxford) Combinatorial Theory Seminar Add to calendar L3
Random graphs with few disjoint cycles
HTML clipboard Fix a positive integer $ k $, and consider the class of all graphs which do not have $ k+1 $  vertex-disjoint cycles.  A classical result of Erdos and Pósa says that each such graph $ G $ contains a blocker of size at most $ f(k) $.  Here a blocker is a set $ B $ of vertices such that $ G-B $ has no cycles.   We give a minor extension of this result, and deduce that almost all such labelled graphs on vertex set $ 1,\ldots,n $ have a blocker of size $ k $.  This yields an asymptotic counting formula for such graphs; and allows us to deduce further properties of a graph $ R_n $ taken uniformly at random from the class: we see for example that the probability that $ R_n $ is connected tends to a specified limit as $ n \to \infty $.   There are corresponding results when we consider unlabelled graphs with few disjoint cycles. We consider also variants of the problem involving for example disjoint long cycles.   This is joint work with Valentas Kurauskas and Mihyun Kang.
Tue, 10/11/2009
15:45 		Christian Pauly (Montpellier) Algebraic and Symplectic Geometry Seminar Add to calendar L3
Opers, Quot-schemes and Frobenius-destabilised vector bundles over curves
In this talk I will introduce and study opers over a smooth projective curve X defined over a field of positive characteristic. I will describe a bijective correspondence between the set of stable vector bundles E over X such that the pull-back F^*(E) under the Frobenius map F of X has maximal Harder-Narasimhan polygon and the set of opers having zero p-curvature. These sets turn out to be finite, which allows us to derive dimensions of certain Quot-schemes and certain loci of stable Frobenius-destabilized vector bundles over X.
Tue, 10/11/2009
16:30 		Gregory Sorkin (IBM Research NY) Combinatorial Theory Seminar Add to calendar SR2
The Power of Choice in a Generalized Polya Urn Model
HTML clipboard We introduce a "Polya choice" urn model combining elements of the well known "power of two choices" model and the "rich get richer" model. From a set of $ k $ urns, randomly choose $ c $ distinct urns with probability proportional to the product of a power $ \gamma>0 $ of their occupancies, and increment one with the smallest occupancy. The model has an interesting phase transition. If $ \gamma \leq 1 $, the urn occupancies are asymptotically equal with probability 1. For $ \gamma>1 $, this still occurs with positive probability, but there is also positive probability that some urns get only finitely many balls while others get infinitely many.
Tue, 10/11/2009
17:00 		Colva Roney-Dougal (St Andrews) Algebra Seminar Add to calendar L2
Maximal subgroups of finite groups
Wed, 11/11/2009
10:00 		OxMOS Workshop/Meeting/Lecture Add to calendar Gibson 1st Floor SR
Stress Corrosion Cracking Workshop
Wed, 11/11/2009
10:10 		Guido Klingbeil OCCAM Literature Seminar Add to calendar OCCAM Common Room (RI2.28)
NVIDIA Tesla: A Unified Graphics and Computing Architecture
Wed, 11/11/2009
11:30 		Colva Roney-Dougal (St Andrews) Algebra Kinderseminar Add to calendar ChCh, Tom Gate, Room 2
Matrix groups: From geometry to homomorphisms
Thu, 12/11/2009
11:00 		Richard Norton; Siobhan Burke OxMOS Workshop/Meeting/Lecture Add to calendar Gibson 1st Floor SR
OxMOS Team Meeting
Thu, 12/11/2009
11:00 		Franziska Jahnke (Oxford) Advanced Logic Class Add to calendar SR2
Rank and Lascar Rank in differentially closed fields
Thu, 12/11/2009
12:00 		Tom Baird (Oxford) Junior Geometry and Topology Seminar Add to calendar SR1
Group valued moment maps, Loop groups and Dirac structures
I will survey the theory of quasiHamiltonian spaces, a.k.a. group valued moment maps. In rough correspondence with historical development, I will first show how they emerge from the study of loop group representations, and then how they arise as a special case of "presymplectic realizations" in Dirac geometry.
Thu, 12/11/2009
14:00 		Dr. Leigh Lapworth (t.b.c.) (Rolls Royce) Computational Mathematics and Applications Add to calendar Rutherford Appleton Laboratory, nr Didcot
CFD in the Gas Turbine Industry
CFD is an indispensible part of the design process for all major gas turbine components. The growth in the use of CFD from single-block structured mesh steady state solvers to highly resolved unstructured mesh unsteady solvers will be described, with examples of the design improvements that have been achieved. The European Commission has set stringent targets for the reduction of noise, emissions and fuel consumption to be achieved by 2020. The application of CFD to produce innovative designs to meet these targets will be described. The future direction of CFD towards whole engine simulations will also be discussed.

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

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