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Monday, September 28, 2009 - 13:00 ,
Location: Skiles 255 ,
Chad Topaz ,
Macalester College ,
Organizer:

Biological aggregations such as insect swarms, bird flocks, and fish schools are arguably some of the most common and least understood patterns in nature. In this talk, I will discuss recent work on swarming models, focusing on the connection between inter-organism social interactions and properties of macroscopic swarm patterns. The first model is a conservation-type partial integrodifferential equation (PIDE). Social interactions of incompressible form lead to vortex-like swarms. The second model is a high-dimensional ODE description of locust groups. The statistical-mechanical properties of the attractive-repulsive social interaction potential control whether or not individuals form a rolling migratory swarm pattern similar to those observed in nature. For the third model, we again return to a conservation-type PIDE and, via long- and short-wave analysis, determine general conditions that social interactions must satisfy for the population to asymptotically spread, contract, or reach steady state.

Series: CDSNS Colloquium

This talk continues from last week's colloquium.

Fourier's Law assert that the heat flow through a point in a solid is proportional to the temperature gradient at that point. Fourier himself thought that this law could not be derived from the mechanical properties of the elementary constituents (atoms and electrons, in modern language) of the solid. On the contrary, we now believe that such a derivation is possible and necessary. At the core of this change of opinion is the introduction of probability in the description. We now see the microscopic state of a system as a probability measure on phase space so that evolution becomes a stochastic process. Macroscopic properties are then obtained through averages. I will introduce some of the models used in this research and discuss their relevance for the physical problem and the mathematical results one can obtain.

Series: Probability Working Seminar

In this talk, we will introduce the classical Cramer's Theorem. The
pattern of proof is one of the two most powerful tools in the theory
of large deviations. Namely, the upper bound comes from optimizing
over a family of Chebychef inequalities; while the lower bound comes
from introducing a Radon-Dikodym factor in order to make what was
originally "deviant" behavior look like typical behavior.
If time permits, we will extend the Cramer's Theorem to a more general
setting and discuss the Sanov Theorem.
This talk is based on Deuschel and Stroock's .

Friday, September 25, 2009 - 15:00 ,
Location: Skiles 269 ,
Anh Tran ,
Georgia Tech ,
Organizer:

(This is a 2 hour lecture.)

In this talk I will give a quick review of classical invariants of
Legendrian knots in a 3-dimensional contact manifold (the topological knot type, the
Thurston-Bennequin invariant and the rotation number). These classical invariants do not
completely determine the Legendrian isotopy type of Legendrian knots, therefore we will
consider Contact homology (aka Chekanov-Eliashberg DGA), a new invariant that has been
defined in recent years. We also discuss the linearization of Contact homology, a method
to extract a more computable invariant out of the DGA associated to a Legendrian knot.

Series: Combinatorics Seminar

Since the seminal work of Erdos and Renyi the phase transition of the largest components in random graphs became one of the central topics in random graph theory and discrete probability theory. Of particular interest in recent years are random graphs with constraints (e.g. degree distribution, forbidden substructures) including random planar graphs. Let G(n,M) be a uniform random graph, a graph picked uniformly at random among all graphs on vertex set [n]={1,...,n} with M edges. Let P(n,M) be a uniform random planar graph, a graph picked uniformly at random among all graphs on vertex set [n] with M edges that are embeddable in the plane. Erodos-Renyi, Bollobas, and Janson-Knuth-Luczak-Pittel amongst others studied the critical behaviour of the largest components in G(n,M) when M= n/2+o(n) with scaling window of size n^{2/3}. For example, when M=n/2+s with s=o(n) and s \gg n^{2/3}, a.a.s. (i.e. with probability tending to 1 as n approaches \infty) G(n,M) contains a unique largest component (the giant component) of size (4+o(1))s. In contract to G(n,M) one can observe two critical behaviour in P(n,M), when M=n/2+o(n) with scaling window of size n^{2/3}, and when M=n+o(n) with scaling window of size n^{3/5}. For example, when M=n/2+s with s = o(n) and s \gg n^{2/3}, a.a.s. the largest component in P(n,M) is of size (2+o(1))s, roughly half the size of the largest component in G(n,M), whereas when M=n+t with t = o(n) and t \gg n^{3/5}, a.a.s. the number of vertices outside the giant component is \Theta(n^{3/2}t^{-3/2}). (Joint work with Tomasz Luczak)

Series: SIAM Student Seminar

In the study of one dimensional dynamical systems one often assumes that the functions involved have a negative Schwarzian derivative. In this talk we consider a generalization of this condition. Specifically, we consider the interval functions of a real variable having some iterate with a negative Schwarzian derivative and show that many known results generalize to this larger class of functions. The introduction of this class was motivated by some maps arising in neuroscience

Series: Stochastics Seminar

I will describe recent work on the behavior of solutions to
reaction diffusion equations (PDEs) when the coefficients in the
equation are random. The solutions behave like traveling waves moving
in a randomly varying environment. I will explain how one can obtain
limit theorems (Law of Large Numbers and CLT) for the motion of the
interface. The talk will be accessible to people without much knowledge
of PDE.

Series: School of Mathematics Colloquium

The asymmetric simple exclusion process (ASEP) is a continuous time Markov process of interacting particles on a lattice \Gamma. ASEP is defined by two rules: (1) A particle at x \in \Gamma waits an exponential time with parameter one, and then chooses y \in \Gamma with probability p(x, y); (2) If y is vacant at that time it moves to y, while if y is occupied it remains at x. The main interest lies in infinite particle systems. In this lecture we consider the ASEP on the integer lattice {\mathbb Z} with nearest neighbor jump rule: p(x, x+1) = p, p(x, x-1) = 1-p and p \ne 1/2. The integrable structure is that of Bethe Ansatz. We discuss various limit theorems which in certain cases establishes KPZ universality.

Series: Analysis Seminar

We consider multipoint Padé approximation to Cauchy transforms of
complex measures. First, we recap that if the support of a measure is
an analytic Jordan arc and if the measure itself is absolutely
continuous with respect to the equilibrium distribution of that arc
with Dini-continuous non-vanishing density, then the diagonal
multipoint Padé approximants associated with appropriate interpolation
schemes converge locally uniformly to the approximated Cauchy
transform in the complement of the arc. Second, we show that this
convergence holds also for measures whose Radon–Nikodym derivative is
a Jacobi weight modified by a Hölder continuous function. The
asymptotics behavior of Padé approximants is deduced from the analysis
of underlying non–Hermitian orthogonal polynomials, for which the
Riemann–Hilbert–∂ method is used.

Series: Other Talks

I will discuss how various geometric categories (e.g. smooth manifolds, complex manifolds) can be be described in terms of locally ringed spaces. (A locally ringed space is a topological spaces endowed with a sheaf of rings whose stalks are local rings.) As an application of the notion of locally ringed space, I'll define what a scheme is.