About the seminars

The aim of the "Probability and Complex Systems Seminar" is to get together people interested in the analysis and applications of stochastic processes, complex systems and networks, and related fields. Each session features one or two talks in which the speaker will show and discuss new methods, results and applications of these areas. Current Organizers: Sandro Gallo and Tiago Pereira.

Support: The activity is, and has been, strongly supported by FAPESP (research grants), PIPGEs UFSCar/USP, SME-ICMC-USP and DEs-UFSCar.

Forthcoming Talks

During 2017 the seminar will be held on Thursdays at 14h00 in the ICMC-USP or DEs-UFSCar. View the list of forthcoming talks below.

Past Talks 2017

Abstracts

Teoria do Transporte Ergódico

Speaker: Artur Lopes (IM-UFRGS)
Date: 08/06/2017 • Time: 14h00 • Room: 43 Des-UFSCar

Abstract: Vamos apresentar alguns resultados recentes em que questões de Teoria Ergódica são analisados dentro do contexto da Teoria de Transporte. Por exemplo, dada uma medida não invariante (não estacionória) como encontrar a medida invariante (estacionória) mais próxima segundo a distância de Wasserstein. Outros tópicos - Dualidade de Kantorovich no contexto de medidas invariantes, planos de Gibbs, a condição twist e o kernel de involução.

Long-range in light scattering by cold atoms

Speaker: Romain Bachelard (DF-UFSCar)
Date: 25/05/2017 • Time: 14h00 • Room: 4-111 Auditório Fávaro ICMC-USP

Abstract: The scattering of light by cold atomic clouds, and many over light scattering problems, is a long-range problem by nature. Indeed, the light creates an effective interaction between the particles, which presents the slow decay of waves over large distances. This allows to reach a strong 'long-range' coupling between the particles, a paradigmatic example being the mean-field coupling of atoms in 1D optical cavities. 3D systems also exhibit strong inter-particle coupling, with a 1/r decay over long distances. As a result from this all-to-all strong coupling, a variety of 'cooperative' effects were observed in light scattering by atomic clouds, that are the signatures of the presence of macroscopic modes. In this presentation, we will discuss such cooperative phenomena in large atomic clouds, and the techniques and models to deal with them. These phenomena include superradiance and subradiance, Mie scattering and cooperative radiation spectrum.

Phase Transitions and the lack of regularity for g-functions on Dyson Models

Speaker: Rodrigo Bissacot (IME-USP)
Date: 18/05/2017 • Time: 14h00 • Room: 4-111 Auditório Fávaro ICMC-USP

Abstract: We discuss Dyson models and the stability of the phase transition phenomenon under a particular type perturbations (using external fields decaying to zero). For Dyson models with slow polynomial decay and in the absence of external fields, we show that its Gibbs measures deep in the phase transition region are not g-measures. In the sense that the natural g-functions are not continuous. Joint work with Aernout van Enter (Groningen), Arnaud Le Ny (Université Paris-Est) and Eric O. Endo (USP).

Laplacian matrices over weighted graphs: Genericity properties and applications to the Synchronization of Networks

Speaker: Camille Poignard (ICMC-USP)
Date: 11/05/2017 • Time: 14h00 • Room: 3-012 ICMC-USP

Abstract: This work deals with the spectrum of Laplacian matrices over weighted graphs, for which Fiedler [70's] showed their topological descriptions rely on two objects of fundamental importance: the second eigenvalue of the spectrum (“spectral gap”) and one of its associated eigenvectors, the so called “Fiedler eigenvector”. Since the seminal work of Fiedler, the use of Spectral graph theory in the study of dynamical networks has been really successful. First, I will show that given a Laplacian matrix, it is possible to perturb slightly the weights of its existing links so that its spectrum be composed of only simple eigenvalues, and its Fiedler eigenvector be composed of only non zero entries. These genericity properties with the constraint of not adding links in the underlying network are stronger than the classical ones, for which any “topological” perturbation is allowed. Then, I will show how this result can be useful for the synchronization of diffusively coupled networks, more precisely for the problem of identifying the links for which the perturbations of the weights modify this dynamics, i.e decreases or enhances the synchronization. The talk does not require any background on graphs theory or on dynamical systems.

Impact of isolation on epidemic diseases

Speaker: Tiago Pereira da Silva (ICMC-USP)
Date: 04/05/2017 • Time: 14h00 • Room: Auditório do Centro de Tecnologia da Informação do ICMC-USP.

Abstract: Infectious diseases are among the most prominent threats to mankind. The final severity of such an initially small infection is significantly decided upon by the infectious period. We study isolation as a means to control epidemic outbreaks, focusing on the consequences of delays in isolating infected individuals. Our analysis uncovers a tipping point: if infected nodes are isolated before a critical day, the disease is effectively controlled, whereas for longer delays the disease is endemic. In fact, if the critical isolation day is missed and the time spent in quarantine is large, the disease outbreak becomes seasonal (periodic). We discuss these predictions in the context of recent Ebola and H1N1 outbreaks. This is a joint work with Lai-Sang Young (NYU), Serhiy Yanchuk and Stefan Ruschel (TU Berlin).