## Seminars and Colloquia by Series

Friday, November 1, 2019 - 15:00 , Location: Skiles 006 , , Yale University , Organizer: Lutz Warnke

TBA

Thursday, October 17, 2019 - 15:00 , Location: Skiles 006 , , Georgia Tech , , Organizer: Galyna Livshyts

TBA

Wednesday, October 2, 2019 - 15:00 , Location: Skiles 006 , Josephine Yu , Georgia Tech , , Organizer: Galyna Livshyts

TBA

Wednesday, September 4, 2019 - 13:55 , Location: Skiles 005 , , University of Crete , , Organizer:
Monday, August 26, 2019 - 14:00 , Location: Skiles 006 , , University of Michigan , , Organizer: Justin Lanier
Monday, August 12, 2019 - 14:00 , Location: Skile 006 , Jonathan Beardsley , University of Washington , Organizer:
Wednesday, July 3, 2019 - 11:00 , Location: Skiles 005 , , Georgia Tech , , Organizer:

The first half of this dissertation concerns the following problem: Given a lattice in $\mathbf{R}^d$ which refines the integer lattice $\mathbf{Z}^d$, what can be said about the distribution of the lattice points inside of the half-open unit cube $[0,1)^d$? This question is of interest in discrete geometry, especially integral polytopes and Ehrhart theory. We observe a combinatorial description of the linear span of these points, and give a formula for the dimension of this span. The proofs of these results use methods from classical multiplicative number theory.

In the second half of the dissertation, we investigate oriented matroids from the point of view of tropical geometry. Given an oriented matroid, we describe, in detail, a polyhedral complex which plays the role of the Bergman complex for ordinary matroids. We show how this complex can be used to give a new proof of the celebrated Bohne-Dress theorem on tilings of zonotopes by zonotopes with an approach which relies on a novel interpretation of the chirotope of an oriented matroid.

Wednesday, July 3, 2019 - 11:00 , Location: Skiles 005 , , Georgia Tech , , Organizer: Marcel Celaya

The first half of this dissertation concerns the following problem: Given a lattice in $\mathbf{R}^d$ which refines the integer lattice $\mathbf{Z}^d$, what can be said about the distribution of the lattice points inside of the half-open unit cube $[0,1)^d$? This question is of interest in discrete geometry, especially integral polytopes and Ehrhart theory. We observe a combinatorial description of the linear span of these points, and give a formula for the dimension of this span. The proofs of these results use methods from classical multiplicative number theory.

In the second half of the dissertation, we investigate oriented matroids from the point of view of tropical geometry. Given an oriented matroid, we describe, in detail, a polyhedral complex which plays the role of the Bergman complex for ordinary matroids. We show how this complex can be used to give a new proof of the celebrated Bohne-Dress theorem on tilings of zonotopes by zonotopes with an approach which relies on a novel interpretation of the chirotope of an oriented matroid.

Tuesday, June 25, 2019 - 14:00 , Location: Skiles 006 , Igor Belykh , Georgia State , , Organizer: Rachel Kuske

The pedestrian-induced lateral oscillation of London's Millennium bridge on the day it opened in 2000 has become a much cited paradigm of an instability caused by phase synchronization of coupled oscillators. However, a closer examination of subsequent theoretical studies and experimental observations have brought this interpretation into question.

To elucidate the true cause of instability, we study a model in which each pedestrian is represented by a simplified biomechanically-inspired two-legged inverted pendulum. The key finding is that synchronization between individual pedestrians is not a necessary ingredient of instability onset. Instead, the side-to-side pedestrian motion should on average lag that of the bridge oscillation by a fraction of a cycle. Using a multi-scale asymptotic analysis, we derive a mathematically rigorous general criterion for bridge instability based on the notion of effective negative damping. This criterion suggests that the initiation of wobbling is not accompanied by crowd synchrony and crowd synchrony is a consequence but not the cause of bridge instability.

Friday, June 21, 2019 - 14:00 , Location: Skiles 317 , Joan Gimeno , Universitat de Barcelona (BGSMath) , , Organizer: Yian Yao
Abstract: Many real-life phenomena in science can be modeled by an Initial Value
Problem (IVP) for ODE's. To make the model more consistent with real phenomenon,
it sometimes needs to include the dependence on past values of the state.
Such models are given by retarded functional differential equations.
When the past values depend on the state, the IVP is not always defined.
Several examples illustrating the problems and methods to integrate IVP of
these kind of differential equations are going to be explained in this talk.