Seminars and Colloquia by Series

Regularity of the solutions of the Euler-Cucker-Smale system

Series
PDE Seminar
Time
Tuesday, April 25, 2017 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Lenya RyzhikStanford University
The Cucker-Smale system is a popular model of collective behavior of interacting agents, used, in particular, to model bird flocking and fish swarming. The underlying premise is the tendency for a local alignment of the bird (or fish, or ...) velocities. The Euler-Cucker-Smale system is an effective macroscopic PDE limit of such particle systems. It has the form of the pressureless Euler equations with a non-linear density-dependent alignment term. The alignment term is a non-linear version of the fractional Laplacian to a power alpha in (0,1). It is known that the corresponding Burgers' equation with a linear dissipation of this type develops shocks in a finite time. We show that nonlinearity enhances the dissipation, and the solutions stay globally regular for all alpha in (0,1): the dynamics is regularized due to the nonlinear nature of the alignment. This is a joint work with T. Do, A.Kiselev and C. Tan.

Curves, Graphs, and Tangent Lines

Series
Algebra Seminar
Time
Monday, April 24, 2017 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Yoav LenUniversity of Waterloo
I will discuss the interplay between tangent lines of algebraic and tropical curves. By tropicalizing all the tangent lines of a plane curve, we obtain the tropical dual curve, and a recipe for computing the Newton polygon of the dual projective curve. In the case of canonical curves, tangent lines are closely related with various phenomena in algebraic geometry such as double covers, theta characteristics and Prym varieties. When degenerating them in families, we discover analogous constructions in tropical geometry, and links between quadratic forms, covers of graphs and tropical bitangents.

Joint GT-UGA Seminar at UGA

Series
Geometry Topology Seminar
Time
Monday, April 24, 2017 - 14:30 for 2.5 hours
Location
UGA Room 303
Speaker
Alexandru Oancea and Basak GurelJussieu and University of Central Florida
Alexandru Oancea: Title: Symplectic homology for cobordisms Abstract: Symplectic homology for a Liouville cobordism - possibly filled at the negative end - generalizes simultaneously the symplectic homology of Liouville domains and the Rabinowitz-Floer homology of their boundaries. I will explain its definition, some of its properties, and give a sample application which shows how it can be used in order to obstruct cobordisms between contact manifolds. Based on joint work with Kai Cieliebak and Peter Albers. Basak Gürel: Title: From Lusternik-Schnirelmann theory to Conley conjecture Abstract: In this talk I will discuss a recent result showing that whenever a closed symplectic manifold admits a Hamiltonian diffeomorphism with finitely many simple periodic orbits, the manifold has a spherical homology class of degree two with positive symplectic area and positive integral of the first Chern class. This theorem encompasses all known cases of the Conley conjecture (symplectic CY and negative monotone manifolds) and also some new ones (e.g., weakly exact symplectic manifolds with non-vanishing first Chern class). The proof hinges on a general Lusternik–Schnirelmann type result that, under some natural additional conditions, the sequence of mean spectral invariants for the iterations of a Hamiltonian diffeomorphism never stabilizes. Based on joint work with Viktor Ginzburg.

End-to-end learning for time series classification

Series
Applied and Computational Mathematics Seminar
Time
Monday, April 24, 2017 - 14:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Prof. George MohlerIUPUI Computer Science
In this talk we focus on classification problems where noisy sensor measurements collected over a time window must be classified into one or more categories. For example, mobile phone health and insurance apps take as input time series from the accelerometer, gyroscope and GPS radio of the phone and output predictions as to whether the user is still, walking, running, biking, driving etc. Standard approaches to this problem consist of first engineering features from statistics of the data (or a transform) over a window and then training a discriminative classifier. For two applications we show how these features can instead be learned in an end-to-end modeling framework with the advantages of increased accuracy and decreased modeling and training time. The first application is reconstructing unobserved neural connections from Calcium fluorescence time series and we introduce a novel convolutional neural network architecture with an inverse covariance layer to solve the problem. The second application is driving detection on mobile phones with applications to car telematics and insurance.

Lyapunov exponents and spectral analysis of ergodic Schrödinger operators

Series
CDSNS Colloquium
Time
Monday, April 24, 2017 - 11:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Zehnghe ZhangRice University
One dimensional discrete Schrödinger operators arise naturally in modeling the motion of quantum particles in a disordered medium. The medium is described by potentials which may naturally be generated by certain ergodic dynamics. We will begin with two classic models where the potentials are periodic sequences and i.i.d. random variables (Anderson Model). Then we will move on to quasi-periodic potentials, of which the randomness is between periodic and i.i.d models and the phenomena may become more subtle, e.g. a metal-insulator type of transition may occur. We will show how the dynamical object, the Lyapunov exponent, plays a key role in the spectral analysis of these types of operators.

Counting vertices in labeled rooted trees

Series
Combinatorics Seminar
Time
Friday, April 21, 2017 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Miklós BónaUniversity of Florida
Various parameters of many models of random rooted trees are fairly well understood if they relate to a near-root part of the tree or to global tree structure. In recent years there has been a growing interest in the analysis of the random tree fringe, that is, the part of the tree that is close to the leaves. Distance from the closest leaf can be viewed as the protection level of a vertex, or the seniority of a vertex within a network. In this talk we will review a few recent results of this kind for a number of tree varieties, as well as indicate the challenges one encounters when trying to generalize the existing results. One tree variety, that of decreasing binary trees, will be related to permutations, another one, phylogenetic trees, is frequent in applications in molecular biology.

Conjugacy of circle maps to rotations II (numerical implementation).

Series
Dynamical Systems Working Seminar
Time
Friday, April 21, 2017 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 254
Speaker
Adrian P. BustamanteGeorgia Tech
A classical theorem of Arnold, Moser shows that in analytic families of maps close to a rotation we can find maps which are smoothly conjugate to rotations. This is one of the first examples of the KAM theory. We aim to present an efficient numerical algorithm, and its implementation, which approximate the conjugations given by the Theorem

A random graph model for approximating sparse graphs

Series
ACO Student Seminar
Time
Friday, April 21, 2017 - 13:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Samantha PettiSchool of Mathematics, Georgia Tech
Beginning with Szemerédi’s regularity lemma, the theory of graph decomposition and graph limits has greatly increased our understanding of large dense graphs and provided a framework for graph approximation. Unfortunately, much of this work does not meaningfully extend to non-dense graphs. We present preliminary work towards our goal of creating tools for approximating graphs of intermediate degree (average degree o(n) and not bounded). We give a new random graph model that produces a graph of desired size and density that approximates the number of small closed walks of a given sparse graph (i.e., small moments of its eigenspectrum). We show how our model can be applied to approximate the hypercube graph. This is joint work with Santosh Vempala.

Why industry? Why not!

Series
Professional Development Seminar
Time
Thursday, April 20, 2017 - 16:00 for 1 hour (actually 50 minutes)
Location
Skiles 114
Speaker
Adam FoxMassMutual Financial Group
A conversation with Adam Fox, former GT postdoc who secured his "dream job" as a tenure-track assistant professor at Western New England University, but who recently moved into industry as a Data Scientist.

Counting extensions in random graphs

Series
Stochastics Seminar
Time
Thursday, April 20, 2017 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Lutz WarnkeSchool of Mathematics, GaTech
We consider rooted subgraph extension counts, such as (a) the number of triangles containinga given vertex, or (b) the number of paths of length three connecting two given vertices. In 1989 Spencer gave sufficient conditions for the event that whp all roots of the binomial random graph G(n,p) have the same asymptotic number of extensions, i.e., (1 \pm \epsilon) times their expected number. Perhaps surprisingly, the question whether these conditions are necessary has remained open. In this talk we briefly discuss our qualitative solution of this problem for the `strictly balanced' case, and mention several intriguing questions that remain open (which lie at the intersection of probability theory + discrete mathematics, and are of concentration inequality type). Based on joint work in progress with Matas Sileikis

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