Past Seminars

Tue, 14/05
14:30 		Maria Chudnovsky (Columbia) Combinatorial Theory Seminar Add to calendar L3
3-coloring graphs with no induced 6-edge paths
Since graph-coloring is an NP-complete problem in general, it is natural to ask how the complexity changes if the input graph is known not to contain a certain induced subgraph H. Due to results of Kaminski and Lozin, and Hoyler, the problem remains NP-complete, unless H is the disjoint union of paths. Recently the question of coloring graphs with a fixed-length induced path forbidden has received considerable attention, and only a few cases of that problem remain open for k-coloring when k>=4. However, little is known for 3-coloring. Recently we have settled the first open case for 3-coloring; namely we showed that 3-coloring graphs with no induced 6-edge paths can be done in polynomial time. In this talk we will discuss some of the ideas of the algorithm. This is joint work with Peter Maceli and Mingxian Zhong.
Tue, 14/05
14:30 		Mat Bullimore (Oxford) Twistor Workshop Add to calendar Gibson 1st Floor SR
Integrability and amplitudes
Mon, 13/05
17:00 		Gustav Holzegel (Imperial College London) Partial Differential Equations Seminar Add to calendar Gibson 1st Floor SR
The Wave Equation on Asymptotically Anti de Sitter Black Hole Spacetimes
The study of wave equations on black hole backgrounds provides important insights for the non-linear stability problem for black holes. I will illustrate this in the context of asymptotically anti de Sitter black holes and present both stability and instability results. In particular, I will outline the main ideas of recent work with J. Smulevici (Paris) establishing a logarithmic decay in time for solutions of the massive wave equation on Kerr-AdS black holes and proving that this slow decay rate is in fact sharp.
Mon, 13/05
15:45 		KALLE KYTOLA (Helsinki University) Stochastic Analysis Seminar Add to calendar Oxford-Man Institute
Random conformally invariant curves and quantum group techniques
In this talk we consider two questions about conformally invariant random curves known as Schramm-Loewner evolutions (SLE). The first question is about the "boundary zig-zags", i.e. the probabilities for a chordal SLE to pass through small neighborhoods of given boundary points in a given order. The second question is that of obtaining explicit descriptions of "multiple SLE pure geometries", i.e. those extremal multiple SLE probability measures which can not be expressed as non-trivial convex combinations of other multiple SLEs. For both problems one needs to find solutions of a system of partial differential equations with asymptotics conditions written recursively in terms of solution of the same problem with a smaller number of variables. We present a general correspondence, which translates these problems to linear systems of equations in finite dimensional representations of the quantum group U_q(sl_2), and we then explicitly solve these systems. The talk is based on joint works with Eveliina Peltola (Helsinki), and with Niko Jokela (Santiago de Compostela) and Matti Järvinen (Crete).
Mon, 13/05
15:45 		George Raptis (Osnabrueck) Topology Seminar Add to calendar L3
The moduli space of topological realisations of an unstable coalgebra
The mod p homology of a space is an unstable coalgebra over the Steenrod algebra at the prime p. This talk will be about the classical problem of realising an unstable coalgebra as the homology of a space. More generally, one can consider the moduli space of all such topological realisations and ask for a description of its homotopy type. I will discuss an obstruction theory which describes this moduli space in terms of the André-Quillen cohomology of the unstable coalgebra. This is joint work with G. Biedermann and M. Stelzer.
Mon, 13/05
14:15 		THIERRY BODINEAU (Ecole Normale Superieure) Stochastic Analysis Seminar Add to calendar Oxford-Man Institute
Metastability and interface motion in disordered media
We will first review the return to equilibrium of the Ising model when a small external field is applied. The relaxation time is extremely long and can be estimated as the time needed to create critical droplets of the stable phase which will invade the whole system. We will then discuss the impact of disorder on this metastable behavior and show that for Ising model with random interactions (dilution of the couplings) the relaxation time is much faster as the disorder acts as a catalyst. In the last part of the talk, we will focus on the droplet growth and study a toy model describing interface motion in disordered media.
Mon, 13/05
12:00 		Pau Figueras (DAMTP) String Theory Seminar Add to calendar L3
Stationary holographic plasma quenches and numerical methods for non-Killing horizons
In this talk I will explain a new method to numerically construct stationary black holes with non-Killing horizons. As an example, I will use AdS/CFT to describe a time-independent CFT plasma flowing through a static spacetime which asymptotes to Minkowski in the flow's past and future, with a varying spatial geometry in-between. When the boundary geometry varies slowly, the holographic stress tensor is well-described by viscous hydrodynamics. For fast variations it is not, and the solutions are stationary analogs of dynamical quenches, with the plasma being suddenly driven out of equilibrium. We find evidence that these flows become unstable for sufficiently strong quenches and speculate that the instability may be turbulent. The gravitational dual of these flows are the first examples of stationary black holes with non-Killing horizons.
Fri, 10/05
16:00 		David Hobson (Warwick) Nomura Seminar Add to calendar DH 1st floor SR
Option pricing, fake Brownian motion, and minimal variation
Suppose we are given a double continuum (in time and strike) of discounted option prices, or equivalently a set of measures which is increasing in convex order. Given sufficient regularity, Dupire showed how to construct a time-inhomogeneous martingale diffusion which is consistent with those prices. But are there other martingales with the same 1-marginals? (In the case of Gaussian marginals this is the fake Brownian motion problem.) In this talk we show that the answer to the question above is yes. Amongst the class of martingales with a given set of marginals we construct the process with smallest possible expected total variation.
Fri, 10/05
14:00 		Dr Rachele Allena (ENSAM) Mathematical Biology and Ecology Seminar Add to calendar L1
Mechanical models to explore biological phenomena
Mechanics plays an important role during several biological phenomena such as morphogenesis, wound healing, bone remodeling and tumorogenesis. Each one of these events is triggered by specific elementary cell deformations or movements that may involve single cells or populations of cells. In order to better understand how cell behave and interact, especially during degenerative processes (i.e. tumorogenesis and metastasis), it has become necessary to combine both numerical and experimental approaches. Particularly, numerical models allow determining those parameters that are still very difficult to experimentally measure such as strains and stresses. During the last few years, I have developed new finite element models to simulate morphogenetic movements in Drosophila embryo, limb morphogenesis, bone remodeling as well as single and collective cell migration. The common feature of these models is the multiplicative decomposition of the deformation gradient which has been used to take into account both the active and the passive deformations undergone by the cells. I will show how this mechanical approach, firstly used in the seventies by Lee and Mandel to describe large viscoelastic deformations, can actually be very powerful in modeling the biological phenomena mentioned above.
Fri, 10/05
11:30 		Various (University of Oxford) OCCAM Special Seminar Add to calendar OCCAM Common Room (RI2.28)
OCCAM Group Meeting
  • Sean Lim - Full waveform inversion: a first look
  • Alex Raisch - Bistable liquid crystal displays: modelling, simulation and applications
  • Vladimir Zubkov - Mathematical model of kidney morphogenesis
Fri, 10/05
10:00 		Michel Chipot (University of Zurich) OxPDE Special Seminar Add to calendar Gibson Grd floor SR
Asymptotic Behavior of Problems in Cylindrical Domains - Lecture 1 of 4
A mini-lecture series consisting of four 1 hour lectures. We would like to consider asymptotic behaviour of various problems set in cylinders. Let $ \Omega_\ell = (-\ell,\ell)\times (-1,1) $ be the simplest cylinder possible. A good model problem is the following. Consider $ u_\ell $ the weak solution to
$$ \cases{ -\partial_{x_1}^2 u_\ell - \partial_{x_2}^2 u_\ell = f(x_2) \quad \hbox{in } \Omega_\ell, \quad \cr \cr u_\ell = 0 \quad \hbox{ on } \quad \partial \Omega_\ell. \cr} $$
When $ \ell \to \infty $ is it trues that the solution converges toward $ u_\infty $ the solution of the lower dimensional problem below ?
$$ \cases{ - \partial_{x_2}^2 u_\infty = f(x_2) \quad \hbox{in }(-1,1), \quad \cr \cr u_\infty = 0 \quad \hbox{ on } \quad \partial (-1,1). \cr} $$
If so in what sense ? With what speed of convergence with respect to $ \ell $ ? What happens when $ f $ is also allowed to depend on $ x_1 $ ? What happens if $ f $ is periodic in $ x_1 $, is the solution forced to be periodic at the limit ? What happens for general elliptic operators ? For more general cylinders ? For nonlinear problems ? For variational inequalities ? For systems like the Stokes problem or the system of elasticity ? For general problems ? ... We will try to give an update on all these issues and bridge these questions with anisotropic singular perturbations problems. Prerequisites : Elementary knowledge on Sobolev Spaces and weak formulation of elliptic problems.
Thu, 09/05
17:00 		James Kennedy (Ulm) Functional Analysis Seminar Add to calendar L1
Analytical aspects of isospectral drums
Almost 50 years ago, Kac posed the now-famous question `Can one hear the shape of a drum?', that is, if two planar domains are isospectral with respect to the Dirichlet (or Neumann) Laplacian, must they necessarily be congruent? This question was answered in the negative about 20 years ago with the construction of pairs of polygonal domains with special group-theoretically motivated symmetries, which are simultaneously Dirichlet and Neumann isospectral. We wish to revisit these examples from an analytical perspective, recasting the arguments in terms of elliptic forms and intertwining operators. This allows us to prove in particular that the isospectrality property holds for a far more general class of elliptic operators than the Laplacian, as it depends purely on what the intertwining operator does to the form domains. We can also show that the same type of intertwining operator cannot intertwine the Robin Laplacian on such domains. This is joint work with Wolfgang Arendt (Ulm) and Tom ter Elst (Auckland).
Thu, 09/05
17:00 		Dan Isaacson (Oxford) Logic Seminar Add to calendar L3
POSTPONED
Thu, 09/05
16:00 		Chiara Daraio (ETH, Zurich) Industrial and Applied Mathematics Seminar Add to calendar DH 1st floor SR
Discrete nonlinear dynamics and the design of new materials
We develop a physical understanding of how stress waves propagate in uniform, heterogeneous, ordered and disordered media composed of discrete granular particles. We exploit this understanding to create experimentally novel materials and devices at different scales, (for example, for application in energy absorption, acoustic imaging and energy harvesting). We control the constitutive behavior of the new materials selecting the particles’ geometry, their arrangement and materials properties. One-dimensional chains of particles exhibit a highly nonlinear dynamic response, allowing a completely new type of wave propagation that has opened the door to exciting fundamental physical observations (i.e., compact solitary waves, energy trapping phenomena, and acoustic rectification). This talk will focus on energy localization and redirection in one-, two- and three-dimensional systems. (For an extended abstract please contact Ruth preston@maths.ox.ac.uk).
Thu, 09/05
16:00 		Kevin Hughes (Edinburgh) Number Theory Seminar Add to calendar L3
Arithmetic restriction theory and Waring's problem
We will discuss arithmetic restriction phenomena and its relation to Waring's problem, focusing on how recent work of Wooley implies certain restriction bounds.

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

Syndicate content