Resolution of Singularities and Analysis

Speaker: 

Professor Michael Greenblatt

Institution: 

SUNY Buffalo

Time: 

Tuesday, January 15, 2008 - 2:00pm

Location: 

MSTB 254

We will describe some recent applications of resolution of singularities methods to questions of interest in analysis. In particular, we will describe a recent local resolution of singularities algorithm of the speaker for real-analytic functions. This algorithm is elementary and self-contained, and makes extensive use of Newton polyhedra and local coordinate systems. Some applications will be given. These include applications to oscillatory integrals, asymptotic expansions for sublevel set volumes, and the determination of the supremum of the positive e for which |f|^{- e} is locally integrable. Here f denotes a real-analytic function.

On the Toda and Ablowitz-Ladik equations: comparing two discrete completely integrable systems

Speaker: 

Courant Instructor Irina Nenciu

Institution: 

Courant Institute, NYU

Time: 

Monday, January 14, 2008 - 3:00pm

Location: 

MSTB 254

Completely integrable systems are remarkable evolution equations, the best known of which is probably the Korteweg-deVries equation. Their many "symmetries", or conserved quantities, often allow for a detailed and in-depth description of their solutions.

We will present a number of new results concerning the Ablowitz-Ladik equation (AL). This is a classical, completely integrable discretization of the nonlinear Schroedinger equation. We will contrast its properties with those of one of the most celebrated discrete integrable system, the Toda lattice, while also illustrating the varied nature of the tools and ideas involved in the theory of completely integrable systems: geometric, algebraic, and functional analytic, among others.

Switches, oscillations, and the dynamics of monotone dynamical systems

Speaker: 

Postdoctoral Fellow German Enciso

Institution: 

Harvard University

Time: 

Monday, January 7, 2008 - 2:00pm

Location: 

Nat Sci II 1201

Determining the long-term behavior of large biochemical models has proved to be a remarkably difficult problem. Yet these models exhibit several characteristics that might make them amenable to study under the right perspective. One possible approach (first suggested by Sontag and
Angeli) is their decomposition in terms of so-called monotone systems, which can be thought of as systems with exclusively positive feedback.

In this talk I discuss some general properties of monotone dynamical systems, including recent results regarding their generic convergence
towards an equilibrium. Then I will discuss the use of monotone systems to model biochemical behaviors such as switches and oscillations under
time delays.

Rational curves on algebraic varieties

Speaker: 

Profesor Stefan Kebekus

Institution: 

University of Cologne, Germany

Time: 

Friday, December 7, 2007 - 4:00pm

Location: 

MSTB 254

One approach to investigate the structure of an algebraic variety X is to study the geometry of curves, especially the rational curves, that X contains. This approach relies on classical geometric ideas and strives to understand the intrinsic geometry of varieties. It is nowadays understood that if X contains many rational curves, then their geometry determines X to a large degree.

After Shigefumi Mori showed in his landmark works that many interesting varieties contain rational curves, their systematic study became a standard tool in algebraic geometry. The spectrum of application is diverse and covers long-standing problems such as deformation rigidity, stability of the tangent bundle, classification problems, and generalizations of the Shafarevich hyperbolicity conjecture.

The talk concentrates on examples and basic properties of minimal degree rational curves on projective varieties. Some of the more advanced applications will be discussed.

Modular representation theory and cohomology: an elementary approach.

Speaker: 

Assistant Professor Julia Pevtsova

Institution: 

University of Washington

Time: 

Wednesday, December 5, 2007 - 2:00pm

Location: 

MSTB 254

Abstract. Modular representation theory studies actions of finite groups (Lie algebras, algebraic groups, finite group schemes) on vector spaces over a field of positive characteristic. The simplest example is an action of the cyclic group Z/p on a vector space. Such an action is described by a single matrix which, in turn, is classified by its Jordan canonical form.

I shall describe an approach to the study of modular representations via their restrictions to certain elementary subalgebras which are analogs of one-parameter subgroups. As an application, we can recover the algebraic variety associated to the cohomology ring of a finite group scheme $G$ by purely representation-theoretic means, generalizing Quillen's stratification theorem" for group cohomology. As another application, we construct new numerical invariants of representations. These invariants are expressed in terms of Jordan forms.

Most of our results apply to any finite group scheme, but they are non-trivial even in the case of the finite group Z/p x Z/p, which is a baby example that will be used for illustrative purposes throughout the talk.

Spatial problems in mathematical ecology

Speaker: 

Postdoctoral Fellow Andrew Nevai

Institution: 

Mathematical Biosciences Institute, The Ohio State University

Time: 

Tuesday, December 4, 2007 - 10:00am

Location: 

MSTB 254

In this talk, I will introduce two spatial problems in theoretical ecology together with their mathematical solutions.

The first part of the talk concerns competition between plants for sunlight. In it, I use a mechanistic Kolmogorov-type competition model to connect plant population vertical leaf profiles (or VLPs) to the asymptotic behavior of the resulting dynamical system. For different VLPs, conditions can be obtained for either competitive exclusion to occur or stable coexistence at one or more equilibrium points.

The second part of the talk concerns the spatial spread of infectious diseases. Here, I use a family of SI-type models to examine the ability of a disease, such as rabies, to invade or persist in a spatially heterogeneous habitat. I will discuss properties of the disease-free equilibrium and the behavior of the endemic equilibrium as the mobility of healthy individuals becomes very small relative to that of infecteds. The family of disease models consists variously of systems of difference equations (which I will emphasize), ODEs, and reaction-diffusion equations.

Measurable Group Theory: rigidity of lattices.

Speaker: 

Professor Alex Furman

Institution: 

University of Illinois at Chicago

Time: 

Tuesday, December 4, 2007 - 2:00pm

Location: 

MSTB 254

In this talk we shall discuss rigidity aspects of infinite discrete groups, which arise naturally in Geometry (as fundamental groups of manifolds), in Algebraic groups (as lattices) and, more generally, as symmetries of various mathematical objects.

Starting from classical by now rigidity results of Mostow, Margulis, Zimmer, we shall turn to the recently active area of Measurable Group Theory, which is closely related to Ergodic Theory, von Neumann algebras, and has applications to such fields as Descriptive Set Theory.

Curves, abelian varieties, and the moduli of cubic threefolds

Speaker: 

NSF Postdoctoral Fellow Sebastian Casalaina-Martin

Institution: 

Harvard University/NSF

Time: 

Friday, November 30, 2007 - 4:00pm

Location: 

MSTB 254

A result of Clemens and Griffiths says that a smooth cubic threefold can be recovered from its intermediate Jacobian. In this talk I will discuss the possible degenerations of these abelian varieties, and give a description of the compactification of the moduli space of cubic threefolds obtained in this way. The relation between this compactification and those constructed in the work of Allcock-Carlson-Toledo and Looijenga-Swierstra will also be considered, and is similar in spirit to the relation between the various compactifications of the moduli spaces of low genus curves. This is joint work with Radu Laza.

Brownian motions interacting through ranks and a phase transition phenomenon.

Speaker: 

Visiting Assistant Professor Soumik Pal

Institution: 

Cornell University

Time: 

Tuesday, November 27, 2007 - 11:00am

Location: 

MSTB 254

Consider a particle in a finite dimensional Euclidean space performing a Brownian motion with an instantaneous drift vector at every time point determined by the order in which the coordinates of its location can be arranged as a decreasing sequence. These processes appear naturally in a variety of areas from queueing theory, statistical physics, and economic modeling. One is generally interested in the spacings between the ordered coordinates under such a motion.

For finite n, the invariant distribution of the vector of spacings can be completely described and is a function of the drift. We show, as n grows to infinity, a curious phenomenon occurs. We look at a transformation of the original process by exponentiating the location coordinates and dividing them by their total sum. Irrespective of the drifts, under the invariant distribution, only one of three things can happen to the transformed values: either they all go to zero, or the maximum grows to one while the rest go to zero, or they stabilize and converge in law to some member of a two parameter family of random point processes. This family known as the Poisson-Dirichlet's appears in genetics and renewal theory and is well studied. The proof borrows ideas from Talagrand's analysis of Derrida's Random Energy Model of spin glasses. We also consider another alternative starting with a countable collection of Brownian motions. This countable model is related to the Harris model of elastic collisions and the discrete Ruzmaikina-Aizenmann model for competing particles.

This is based on separate joint works with Sourav Chatterjee and Jim Pitman.

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