All Seminars

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Title: A Ramsey Class of Steiner Systems
Seminar: Combinatorics
Speaker: Christian Reiher of The University of Hamburg
Contact: Dwight Duffus, dwight@mathcs.emory.edu
Date: 2016-10-24 at 4:00PM
Venue: W301
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Abstract:
We construct a Ramsey class whose objects are Steiner systems. In contrast to the situation with general r-uniform hypergraphs, it turns out that simply putting linear orders on their sets of vertices is not enough for this purpose: we also have to strengthen the notion of subobjects used from induced subsystems to something we call strongly induced subsystems. Moreover we study the Ramsey properties of other classes of Steiner systems obtained from this class by either forgetting the order or by working with the usual notion of subsystems. This leads to a perhaps surprising induced Ramsey theorem in which it designs get coloured. This is joint work with Vindya Bhat, Jaroslav Nevsetvril, and Vojtvech Rodl.
Title: Athens-Atlanta Joint Number Theory Seminar
Seminar: Algebra
Speaker: Florian Pop and Ben Bakker of UPenn and UGA
Contact: David Zureick-Brown, dzb@mathcs.emory.edu
Date: 2016-10-18 at 4:00PM
Venue: AT UGA
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Abstract:
AT UGA
Title: Centrality measures and contagion on temporal networks
Defense: Dissertation
Speaker: Isabel Chen of Emory University
Contact: Isabel Chen, imchen@emory.edu
Date: 2016-10-17 at 4:00PM
Venue: W302
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Abstract:
The objective of this dissertation is to study the relationship between network-based centrality measures and epidemic outcome. Determining the key players in contagion processes can inform disease-prevention strategies. We analyze a time-stamped, person-to-person contact network based on human mobility movements within a busy, urban hospital. Movement patterns identified a small number of locations as hubs of activity. Linear algebraic techniques were used to compute a recently proposed temporal centrality measure applied to the empirical network; comparisons with traditional centrality measures were performed to determine if the inclusion of temporal information provides additional insights. Linear regression techniques were employed to describe the relationships between the quantities of interest. We find that while temporal centrality can at times identify key players not captured by traditional measures, it does not necessarily outperform non-temporal measures with respect to predicting epidemic outcome. Strategic removal of connections between highly central nodes resulted in an exponential decrease in the structural connectivity of the network, but this did not translate to a reduction in epidemic outcome. We conclude that contagion on temporal networks is extremely robust to changes in the network, and while network-based centrality can help to identify key players in an epidemic process, more work needs to be done to build an epidemic-containment strategy based on the information afforded by network-based analyses.
Title: CANCELED! Mathematical models for the correspondence problem
Seminar: General interest for MathCS and Radiology
Speaker: Jan Modersitzki of University of Lubeck
Contact: Lars Ruthotto, lruthotto@emory.edu
Date: 2016-10-06 at 4:00PM
Venue: W201
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Abstract:
THIS SEMINAR HAS BEEN CANCELED. WE APOLOGIZE FOR THE SHORT NOTICE. We introduce the fascinating correspondence problem also known as image registration. Roughly spoken, the goal is to automatically establish correspondences between points in different projections of a scene. In particular, in medical imaging, this problem is very important and used for applications such as motion correction or data fusion. Several examples displaying different facets of the problem are discussed. A mathematical framework for the correspondence problem is outlined. Starting point is a variational formulation, where a joint energy is to be minimized on an appropriate set. Modular building blocks such as distance measures and regularizers are briefly discussed and related to particular applications. Finally, a brief outlook on constrained image registration is presented. Constraints are used to improve the modeling by restricting the admissible set in a smart way.
Title: Bounding torsion in geometric families of abelian varieties
Seminar: Algebra
Speaker: Ben Bakker of UGA
Contact: David Zureick-Brown, dzb@mathcs.emory.edu
Date: 2016-10-04 at 4:00PM
Venue: W306
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Abstract:
A celebrated theorem of Mazur asserts that the order of the torsion part of the group of rational points of an elliptic curve over Q is absolutely bounded; it is conjectured that the same is true for abelian varieties over number fields K, though very little progress has been made in proving it. The natural geometric analog where K is replaced by the function field of a complex curve---known as the geometric torsion conjecture---is equivalent to the nonexistence of low genus curves in congruence towers of Siegel modular varieties. In joint work with J. Tsimerman, we prove the conjecture for abelian varieties with real multiplication. We'll discuss a general method for bounding the genus of curves in locally symmetric varieties using hyperbolic geometry to bound Seshadri constants and apply it to some related problems.
Title: Decomposing the Complete r-Graph
Seminar: Combinatorics
Speaker: Imre Leader of The University of Cambridge
Contact: Dwight Duffus, dwight@mathcs.emory.edu
Date: 2016-09-28 at 4:00PM
Venue: W301
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Abstract:
The Graham-Pollak theorem states that, if we wish to decompose the complete graph Kn into complete bipartite subgraphs, then we need at least n-1 of them. What happens for hypergraphs? In other words, if we wish to decompose the complete r-graph on n vertices into complete r-partite r-graphs, how many do we need? In this talk we report on recent progress on this question. This is joint work with Luka Milicevic and Ta Sheng Tan.
Title: Generalized Orbifolds in Conformal Field Theory
Seminar: Algebra
Speaker: Marcel Bischoff of Vanderbilt University
Contact: David Zureick-Brown, dzb@mathcs.emory.edu
Date: 2016-09-20 at 4:00PM
Venue: W306
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Abstract:
I will introduce the notion of a finite hypergroup. It turns out that certain subfactors (unital inclusions of von Neumann algebras with trivial center) can naturally be seen as such a fixed point. Chiral conformal field theory can be axiomatized as local conformal nets of von Neumann algebras.The orbifold of a conformal net is the fixed point with respect to a finite group of automorphisms. We define a generalized orbifold to be the fixed point of a conformal net under a proper hypergroup action. The fixed point is finite index subnet and it turns out that all finite index subnets are generalized orbifolds. A holomorphic conformal net is a conformal net with trivial representation category. For example, every positive even self-dual lattice gives such a conformal net. The representation category of a generalized orbifold of a holomorphic net is the Drinfeld center of a categorification of the hypergroup. Based on arXiv:1506.02606.
Title: Borcherds and Zagier Revisited: Divisors of Modular Forms
Seminar: Algebra
Speaker: Ken Ono of Emory University
Contact: David Zureick-Brown, dzb@mathcs.emory.edu
Date: 2016-09-13 at 4:00PM
Venue: W306
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Abstract:
TBA
Title: Positive Polynomials and Varieties of Minimal Degree
Seminar: Algebra
Speaker: Daniel Plaumann of Universitat Konstanz
Contact: David Zureick-Brown, dzb@mathcs.emory.edu
Date: 2016-09-06 at 4:00PM
Venue: W306
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Abstract:
A celebrated result by Hilbert says that every real nonnegative ternary quartic is a sum of three squares of quadratic forms. We show more generally that every nonnegative quadratic form on a real projective variety X of minimal degree is a sum of dim(X) + 1 squares of linear forms. This provides a new proof for one direction of a recent result due to Blekherman, Smith, and Velasco. We explain the geometry behind this generalization and discuss what is known about the number of equivalence classes of sum-of-squares representations. (Joint work with G. Blekherman, R. Sinn, and C. Vinzant)
Title: Arithmetic Restrictions on Geometric Monodromy
Seminar: Algebra
Speaker: Daniel Litt of Columbia University
Contact: David Zureick-Brown, dzb@mathcs.emory.edu
Date: 2016-08-30 at 4:00PM
Venue: W306
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Abstract:
Let X be an algebraic variety over a field k. Which representations of pi_1(X) arise from geometry, e.g. as monodromy representations on the cohomology of a family of varieties over X? We study this question by analyzing the action of Gal(k/k) on pi_1(X), where k is a finite or p-adic field. As a sample application of our techniques, we show that if A is a non-constant Abelian variety over C(t), such that A[l] is split for some odd prime l, then A has at least four points of bad reduction.