Seminars and Colloquia by Series

Monday, November 21, 2011 - 16:05 , Location: Skiles 006 , Ye Luo , Georgia Tech , luoye@math.gatech.edu , Organizer: Josephine Yu
Metric graphs arise naturally in tropical tropical geometry and Berkovich geometry. Recent efforts have extend conventional notion of divisors and linear systems on algebraic curves to finite graphs and metric graphs (tropical curves). Reduced divisors are introduced as an essential tool in proving graph-theoretic Riemann-Roch. In short, a q-reduced divisor is the unique divisor in a linear system with respect to a point q in the graph. In this talk, I will show how tropical convexity is related to linear systems on metric graphs, and define a canonical metric on the linear systems. In addition, I will introduce a generalized notion of reduced divisors, which are defined with respect to any effective divisor as in comparison a single point (effective divisor of degree one) in the conventional case.
Monday, November 14, 2011 - 16:05 , Location: Skiles 006 , Luis Garcia , Sam Houston State University , Organizer: Josephine Yu
Geometric modeling builds computer models for industrial design and manufacture from basic units, called patches, such as,  Bézier curves and surfaces. The control polygon of a Bézier curve is well-defined and has geometric significance—there is a sequence of weights under which the limiting position of the curve is the control polygon. For a Bezier surface patch, there are many possible polyhedral control structures, and none are canonical.  In this talk, I will present a not necessarily polyhedral control structure for surface patches, regular control surfaces, which are certain C^0 spline surfaces. While not unique, regular control surfaces are exactly the possible limiting positions of a Bezier patch when the weights are allowed to vary. While our primary interest is to explain the meaning of control  nets for the classical rational Bezier patches, we work in the generality of Krasauskas’ toric Bezier patches. Toric Bezier patches are multi-sided parametric patches based on the geometry of toric varieties and depend on a polytope and some weights. Our results rely upon a construction in computational algebraic geometry called a toric degeneration.
Monday, November 7, 2011 - 16:05 , Location: Skiles 006 , Farbod Shokrieh , Georgia Tech , shokrieh@math.gatech.edu , Organizer: Josephine Yu
A matroid is a structure that captures the notion of "independence". For example, given a set of vectors in a vector space, one can define a matroid. Graphs also naturally give rise to matroids. I will talk about various simplicial complexes associated to matroids. These include the "matroid complex", the "broken circuit complex", and the "order complex" of the associated geometric lattice. They carry some of the most important invariants of matroids and graphs. I will also show how the Bergman fan and its refinement (which arise in tropical geometry) relate to the classical theory. If time permits, I will give an outline of a recent breakthrough result of Huh and Katz on log-concavity of characteristic (chromatic) polynomials of matroids. No prior knowledge of the subject will be assumed. Most of the talk should be accessible to advanced undergraduate students.
Monday, October 31, 2011 - 16:05 , Location: Skiles 006 , Florian Enescu , Georgia State University , Organizer: Anton Leykin
The talk will discuss the notion of Hilbert-Kunz multiplicity, presenting its general theory and listing some of the outstanding open problems together with recent progress on them.
Monday, October 10, 2011 - 16:05 , Location: Skiles 006 , Anders Jensen , Universität des Saarlandes , Organizer: Josephine Yu
In celestial mechanics a configuration of n point masses is called central if it collapses by scaling to the center of mass when released with initial velocities equal to zero. We strengthen a generic finiteness result due to Moeckel by showing that the number of spatial central configurations in the Newtonian five-body problem with positive masses is finite, except for some explicitly given special choices of mass values. The proof will be computational using tropical geometry, Gröbner bases and sum-of-squares decompositions.This is joint work with Marshall Hampton. 
Wednesday, October 5, 2011 - 15:00 , Location: Skiles 005 , Carlos Beltrán , University of Cantabria, Spain , Organizer: Anton Leykin

[Note unusual day and time!]

In the last decades, path following methods have become a very popularstrategy to solve systems of polynomial equations. Many of the advances are due tothe correct understanding of the geometrical properties of an algebraic object, the so-called solution variety for polynomial system solving. I summarize here some of the mostrecent advances in the understanding of this object, focusing also on the certification andcomplexity of the numerical procedures involved in path following methods.
Monday, September 26, 2011 - 16:05 , Location: Skiles 006 , Robert Krone , Georgia Tech , Organizer: Anton Leykin
An ideal of a local polynomial ring can be described by calculating astandard basis with respect to a local monomial ordering. However if we areonly allowed approximate numerical computations, this process is notnumerically stable. On the other hand we can describe the ideal numericallyby finding the space of dual functionals that annihilate it. There areseveral known algorithms for finding the truncated dual up to any specifieddegree, which is useful for zero-dimensional ideals. I present a stoppingcriterion for positive-dimensional cases based on homogenization thatguarantees all generators of the initial monomial ideal are found. This hasapplications for calculating Hilbert functions.
Monday, September 19, 2011 - 16:05 , Location: Skiles 006 , Saikat Biswas , Georgia Tech , Organizer: Josephine Yu
The rational solutions to the equation describing an elliptic curve form a finitely generated abelian group, also known as the Mordell-Weil group. Detemining the rank of this group is one of the great unsolved problems in mathematics. The Shafarevich-Tate group of an elliptic curve is an important invariant whose conjectural finiteness can often be used to determine the generators of the Mordell-Weil group. In this talk, we first introduce the definition of the Shafarevich-Tate group. We then discuss the theory of visibility, initiated by Mazur, by means of which non-trivial elements of the Shafarevich-Tate group of an elliptic curve an be 'visualized' as rational points on an ambient curve. Finally, we explain how this theory can be used to give theoretical evidence for the celebrated Birch and Swinnerton-Dyer Conjecture.
Wednesday, May 25, 2011 - 15:05 , Location: Skiles 006 , Don Zagier , MPI Bonn and College de France , Organizer: Stavros Garoufalidis
Come and see!
Wednesday, April 13, 2011 - 13:00 , Location: Skiles 006 , Christelle Vincent , University of Wisconsin Madison , Organizer: Matt Baker
For q a power of a prime, consider the ring \mathbb{F}_q[T]. Due to the many similarities between \mathbb{F}_q[T] and the ring of integers \mathbb{Z}, we can define for \mathbb{F}_q[T] objects that are analogous to elliptic curves, modular forms, and modular curves. In particular, for \mathfrak{p} a prime ideal in \mathbb{F}_q[T], we can define the Drinfeld modular curve X_0(\mathfrak{p}), and study the reduction modulo \mathfrak{p} of its Weierstrass points, as is done in the classical case by Rohrlich, and Ahlgren and Ono. In this talk we will present some partial results in this direction, defining all necessary objects as we go. The first 20 minutes should be accessible to graduate students interested in number theory.

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