Seminars and Colloquia by Series

On a hypergraph generalization of the Balog-Szemeredi-Gowers Theorem

Series
Combinatorics Seminar
Time
Friday, September 19, 2008 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 255
Speaker
Evan BorensteinSchool of Mathematics, Georgia Tech
The Balog-Szemeredi-Gowers theorem is a widely used tool in additive combinatorics, and it says, roughly, that if one has a set A such that the sumset A+A is "concentrated on few values," in the sense that these values v each get close to n representations as v = a+b, with a,b in A, then there is a large subset A' of A such that the sumset A'+A' is "small" -- i.e. it has size a small multiple of n. Later, Sudakov, Szemeredi and Vu generalized this result to handle multiple sums A_1 + ... + A_k. In the present talk we will present a refinement of this result of Sudakov, Szemeredi and Vu, where we get better control on the growth of sums A'+...+A'. This is joint work with Ernie Croot.

Contact homology of Legendrian knots

Series
Geometry Topology Working Seminar
Time
Friday, September 19, 2008 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 269
Speaker
John EtnyreSchool of Mathematics, Georgia Tech
This will be an introduction to Legendrian knots (these are interesting knots that blend topological and geometric concepts) and a powerful invariant of Legendrian knots in R^3 called contact homology. On the first pass this invariant is combinatorial and has a lot of interesting algebraic structure. In a future talk (probably a few weeks from now), I will explain more about the analytic side of the theory as well as deeper algebraic aspects. This talk should be accessible anyone interested in topology and geometry.

Trouble with a chain of stochastic oscillators

Series
Stochastics Seminar
Time
Thursday, September 18, 2008 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 269
Speaker
Jonathan MattinglyDept of Math, Duke University
I will discuss some recent (but modest) results showing the existence and slow mixing of a stationary chain of Hamiltonian oscillators subject to a heat bath.  Surprisingly, even these simple results require some delicate stochastic averaging. This is joint work with Martin Hairer.

Pebbling graphs of diameter three

Series
Graph Theory Seminar
Time
Thursday, September 18, 2008 - 12:05 for 1.5 hours (actually 80 minutes)
Location
Skiles 255
Speaker
Luke PostleSchool of Mathematics, Georgia Tech
Given a configuration of pebbles on the vertices of a connected graph G, a pebbling move is defined as the removal of two pebbles from some vertex, and the placement of one of these on an adjacent vertex. A graph is called pebbleable if for each vertex v there is a sequence of pebbling moves so that at least one pebble can be placed on vertex v. The pebbling number of a graph G is the smallest integer k such that G is pebbleable given any configuration of k pebbles on G. We improve on the bound of Bukh by showing that the pebbling number of a graph of diameter 3 on n vertices is at most the floor of 3n/2 + 2, and this bound is best possible. We give an alternative proof that the pebbling number of a graph of diameter 2 on n vertices is at most n + 1. This is joint work with Noah Streib and Carl Yerger.

Dynamical networks - interplay of topology, interactions and local dynamics

Series
School of Mathematics Colloquium
Time
Thursday, September 18, 2008 - 11:00 for 1 hour (actually 50 minutes)
Location
Skiles 269
Speaker
Leonid BunimovichSchool of Mathematics, Georgia Tech
It has been found about ten years ago that most of the real networks are not random ones in the Erdos-Renyi sense but have different topology (structure of the graph of interactions between the elements of a network). This finding generated a steady flux of papers analyzing structural aspects of networks.  However, real networks are rather dynamical ones where the elements (cells, genes, agents, etc) are interacting dynamical systems. Recently a general approach to the studies of dynamical networks with arbitrary topology was developed. This approach is based on a symbolic dynamics and is in a sense similar to the one introduced by Sinai and the speaker for Lattice Dynamical Systems, where the graph of interactions is a lattice. The new approach allows to analyse a combined effect of all three features which characterize a dynamical network (topology, dynamics of elements of the network and interactions between these elements) on its evolution. The networks are of the most general type, e.g. the local systems and interactions need not to be homogeneous, nor restrictions are imposed on a structure of the graph of interactions. Sufficient conditions on stability of dynamical networks are obtained. It is demonstrated that some subnetworks can evolve regularly while the others evolve chaotically. This approach is a very natural one and thus gives a hope that in many other problems (some will be discussed) on dynamical networks a progress could be expected.

Challenges in Exact Linear Programming: Exact Precision Linear Algebra

Series
ACO Student Seminar
Time
Wednesday, September 17, 2008 - 13:30 for 1.5 hours (actually 80 minutes)
Location
ISyE Executive Classroom
Speaker
Dan SteffyISyE, Georgia Tech
A successful approach to solving linear programming problems exactly has been to solve the problems with increasing levels of fixed precision, checking the final basis in exact arithmetic and then doing additional simplex pivots if necessary. This work is a computational study comparing different techniques for the core element of our exact computation: solving sparse rational systems of linear equations exactly.

Space-Time Dynamics

Series
Research Horizons Seminar
Time
Wednesday, September 17, 2008 - 12:00 for 1 hour (actually 50 minutes)
Location
Skiles 255
Speaker
Leonid BunimovichSchool of Mathematics, Georgia Tech
Dynamics of spatially extended systems is often described by Lattice Dynamical Systems (LDS). LDS were introduced 25 years ago independently by four physicists from four countries. Sometimes LDS themselves are quite relevant models of real phenomena. Besides, very often discretizations of partial differential equations lead to LDS. LDS consist of local dynamical systems sitting in the nodes of a lattice which interact between themselves. Mathematical studies of LDS started in 1988 and introduced a thermodynamic formalism for these spatially extended dynamical systems. They allowed to give exact definitions of such previously vague phenomena as space-time chaos and coherent structures and prove their existence in LDS. The basic notions and results in this area will be discussed.  It is a preparatory talk for the next day colloquium where Dynamical Networks, i.e.  the systems with arbitrary graphs of interactions, will be discussed.

Filling invariants for groups

Series
Geometry Topology Seminar
Time
Monday, September 15, 2008 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 269
Speaker
Pallavi DaniEmory University and LSU
The Dehn function of a finitely presented group measures the difficulty in filling loops in the presentation complex of the group. Higher dimensional Dehn functions are a natural generalization: the n-dimensional Dehn function of a group captures the difficulty of filling n-spheres with (n+1)-balls in suitably defined complexes associated with the group. A fundamental question in the area is that of determining which functions arise as Dehn functions. I will give an overview of known results and describe recent progress in the 2-dimensional case. This is joint work with Josh Barnard and Noel Brady.

Recent Developments in Variable Transformations for Numerical Integration

Series
Analysis Seminar
Time
Monday, September 15, 2008 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 255
Speaker
Avram SidiTechnion, Israel Institute of Technology, Haifa
Variable transformations are used to enhance the normally poor performance of trapezoidal rule approximations of finite-range integrals I[f]=\int^1_0f(x)dx. Letting x=\psi(t), where \psi(t) is an increasing function for 0 < t < 1 and \psi(0)=0 and \psi(1)=1, the trapezoidal rule is applied to the transformed integral I[f]=\int^1_0f(\psi(t))\psi'(t)dt. By choosing \psi(t) appropriately, approximations of very high accuracy can be obtained for I[f] via this approach. In this talk, we survey the various transformations that exist in the literature. In view of recent generalizations of the classical Euler-Maclaurin expansion, we show how some of these transformations can be tuned to optimize the numerical results. If time permits, we will also discuss some recent asymptotic expansions for Gauss-Legendre integration rules in the presence of endpoint singularities and show how their performance can be optimized by tuning variable transformations. The variable transformation approach presents a very flexible device that enables one to write his/her own high-accuracy numerical integration code in a simple way without the need to look up tables of abscissas and weights for special Gaussian integration formulas.

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