Seminars and Colloquia by Series

Recent advances on the structure of metric measure spaces with Ricci curvature bounded from below

Series
Job Candidate Talk
Time
Thursday, January 19, 2012 - 16:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Nicola Gigli – University of Nice
I'll show how on metric measure spaces with Ricci curvature bounded from below in the sense of Lott-Sturm-Villani there is a well defined notion of Heat flow, and how the study of the properties of this flow leads to interesting geometric and analytic properties of the spaces themselves. A particular attention will be given to the class of spaces where the Heat flow is linear. (From a collaboration with Ambrosio and Savare')

Coupling and Upscaling of Particle Models in Multiscale Physics

Series
Job Candidate Talk
Time
Tuesday, January 17, 2012 - 11:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Matthew Dobson – NSF Postdoctoral Fellow, Ecole des Ponts ParisTech
Multiscale numerical methods seek to compute approximate solutions to physical problems at a reduced computational cost compared to direct numerical simulations. This talk will cover two methods which have a fine scale atomistic model that couples to a coarse scale continuum approximation. The quasicontinuum method directly couples a continuum approximation to an atomistic model to create a coherent model for computing deformed configurations of crystalline lattices at zero temperature. The details of the interface between these two models greatly affects the model properties, and we will discuss the interface consistency, material stability, and error for energy-based and force-based quasicontinuum variants along with the implications for algorithm selection. In the case of crystalline lattices at zero temperature, the constitutive law between stress and strain is computed using the Cauchy-Born rule (the lattice deformation is locally linear and equal to the gradient). For the case of complex fluids, computing the stress-strain relation using a molecular model is more challenging since imposing a strain requires forcing the fluid out of equilibrium, the subject of nonequilibrium molecular dynamics. I will describe the derivation of a stochastic model for the simulation of a molecular system at a given strain rate and temperature.

Counting closed loops in a stratum of quadratic differentials

Series
Job Candidate Talk
Time
Thursday, January 12, 2012 - 11:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Kasra Rafi – University of Oklohama –
In his thesis, Margulis computed the asymptotic growth rate for the number of closed geodesics of length less than R on a given closed hyperbolic surface and his argument has been emulated to many other settings. We examine the Teichmüller geodesic flow on the moduli space of a surface, or more generally any stratum of quadratic differentials in the cotangent bundle of moduli space. The flow is known to be mixing, but the spaces are not compact and the flow is not uniformly hyperbolic. We show that the random walk associated to the Teichmüller geodesic flow is biased toward the compact part of the stratum. We then use this to find asymptotic growth rate of for the number of closed loops in the stratum. (This is a joint work with Alex Eskin and Maryam Mirzakhani.)

Identifiability and estimation of multiple transmission pathways in waterborne disease

Series
Job Candidate Talk
Time
Tuesday, January 10, 2012 - 11:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Marisa Eisenberg – MBI, Ohio State
Waterborne diseases cause over 3.5 million deaths annually, with cholera alone responsible for 3-5 million cases/year and over 100,000 deaths/year. Many waterborne diseases exhibit multiple characteristic timescales or pathways of infection, which can be modeled as direct and indirect transmission. A major public health issue for waterborne diseases involves understanding the modes of transmission in order to improve control and prevention strategies. One question of interest is: given data for an outbreak, can we determine the role and relative importance of direct vs. environmental/waterborne routes of transmission? We examine these issues by exploring the identifiability and parameter estimation of a differential equation model of waterborne disease transmission dynamics. We use a novel differential algebra approach together with several numerical approaches to examine the theoretical and practical identifiability of a waterborne disease model and establish if it is possible to determine the transmission rates from outbreak case data (i.e. whether the transmission rates are identifiable). Our results show that both direct and environmental transmission routes are identifiable, though they become practically unidentifiable with fast water dynamics. Adding measurements of pathogen shedding or water concentration can improve identifiability and allow more accurate estimation of waterborne transmission parameters, as well as the basic reproduction number. Parameter estimation for a recent outbreak in Angola suggests that both transmission routes are needed to explain the observed cholera dynamics. I will also discuss some ongoing applications to the current cholera outbreak in Haiti.

Massey products in Galois cohomology via rational points

Series
Job Candidate Talk
Time
Thursday, December 8, 2011 - 11:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Kirsten Wickelgren – AIM/Harvard University
The cohomology ring of the absolute Galois group Gal(kbar/k) of a field k controls interesting arithmetic properties of k. The Milnor conjecture, proven by Voevodsky, identifies the cohomology ring H^*(Gal(kbar/k), Z/2) with the tensor algebra of k* mod the ideal generated by x otimes 1-x for x in k - {0,1} mod 2, and the Bloch-Kato theorem, also proven by Voevodsky, generalizes the coefficient ring Z/2. In particular, the cohomology ring of Gal(kbar/k) can be expressed in terms of addition and multiplication in the field k, despite the fact that it is difficult even to list specific elements of Gal(kbar/k). The cohomology ring is a coarser invariant than the differential graded algebra of cochains, and one can ask for an analogous description of this finer invariant, controlled by and controlling higher order cohomology operations. We show that order n Massey products of n-1 factors of x and one factor of 1-x vanish, generalizing the relation x otimes 1-x. This is done by embedding P^1 - {0,1,infinity} into its Picard variety and constructing Gal(kbar/k) equivariant maps from pi_1^et applied to this embedding to unipotent matrix groups. This also identifies Massey products of the form <1-x, x, … , x , 1-x> with f cup 1-x, where f is a certain cohomology class which arises in the description of the action of Gal(kbar/k) on pi_1^et(P^1 - {0,1,infinity}). The first part of this talk will not assume knowledge of Galois cohomology or Massey products.

Bernstein's theorem, Newton polygons, and tropical intersections

Series
Job Candidate Talk
Time
Tuesday, December 6, 2011 - 11:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Joseph Rabinoff – Harvard University
Bernstein's theorem is a classical result which computes the number of common zeros in (C*)^n of a generic set of n Laurent polynomials in n variables. The theorem of the Newton polygon is a ubiquitous tool in arithmetic geometry which calculates the valuations of the zeros of a polynomial (or convergent power series) over a non-Archimedean field, along with the number of zeros (counted with multiplicity) with each given valuation. We will explain in what sense both theorems are very special cases of a lifting theorem in tropical intersection theory. The proof of this lifting theorem builds on results of Osserman and Payne, and uses Berkovich analytic spaces and extended tropicalizations of toric varieties in a crucial way, as well as Raynaud's theory of formal models of analytic spaces. Most of this talk will be about joint work with Brian Osserman.

Efficient algorithm for electronic structure calculations

Series
Job Candidate Talk
Time
Thursday, December 1, 2011 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Dr. Jianfeng Lu – Courant Institute, NYU
Electronic structure theories, in particular Kohn-Sham density functional theory, are widely used in computational chemistry and material sciences nowadays. The computational cost using conventional algorithms is however expensive which limits the application to relative small systems. This calls for development of efficient algorithms to extend the first principle calculations to larger system. In this talk, we will discuss some recent progress in efficient algorithms for Kohn-Sham density functional theory. We will focus on the choice of accurate and efficient discretization for Kohn-Sham density functional theory.

Analysis of partial differential equations in non-smooth media

Series
Job Candidate Talk
Time
Thursday, February 10, 2011 - 11:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Svitlana Mayboroda – Purdue University
Despite its long history, the theory of ellipticpartial differential equations in non-smooth media is abundant with openproblems. We will discuss the main achievements in the theory, recentdevelopments, surprising paradoxes related to the behavior of solutions nearthe boundary, and some fundamental questions which still remain open.

On eigenvalues of a sum of random matrices

Series
Job Candidate Talk
Time
Wednesday, February 2, 2011 - 13:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Vladislav Kargin – Department of Mathematics, Stanford University
Let H = A+UBU* where A and B are two N-by-N Hermitian matrices and U is a random unitary transformation. When N is large, the point measure of eigenvalues of H fluctuates near a probability measure which depends only on eigenvalues of A and B. In this talk, I will discuss this limiting measure and explain a result about convergence to the limit in a local regime.

State Transitions and Feedback Loops in the Immune Response

Series
Job Candidate Talk
Time
Tuesday, February 1, 2011 - 11:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Peter Kim – University of Utah –
The immune system is a complex, multi-layered biological system, making it difficult to characterize dynamically. Perhaps, we can better understand the system’s construction by isolating critical, functional motifs. From this perspective, we will investigate two simple, yet ubiquitous motifs:state transitions and feedback regulation.Numerous immune cells exhibit transitions from inactive to activated states. We focus on the T cell response and develop a model of activation, expansion, and contraction. Our study suggests that state transitions enable T cells to detect change and respond effectively to changes in antigen levels, rather than simply the presence or absence of antigen. A key component of the system that gives rise to this change detector is initial activation of naive T cells. The activation step creates a barrier that separates the slow dynamics of naive T cells from the fast dynamics of effector T cells, allowing the T cell population to compare short-term changes in antigen levels to long-term levels. As a result, the T cell population responds to sudden shifts in antigen levels, even if the antigen were already present prior to the change. This feature provides a mechanism for T cells to react to rapidly expandingsources of antigen, such as viruses, while maintaining tolerance to constant or slowly fluctuating sources of stimulation, such as healthy tissue during growth.For our second functional motif, we investigate the potential role of negative feedback in regulating a primary T cell response. Several theories exist concerning the regulation of primary T cell responses, the most prevalent being that T cells follow developmental programs. We propose an alternative hypothesis that the response is governed by a feedback loop between conventional and adaptive regulatory T cells. By developing a mathematical model, we show that the regulated response is robust to a variety of parameters and propose that T cell responses may be governed by a simple feedback loop rather than by autonomous cellular programs.

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