<html><head><style type='text/css'>p { margin: 0; }</style></head><body><div style='font-family: times new roman,new york,times,serif; font-size: 12pt; color: #000000'>Prezados colegas e alunos, Esta mensagem é para divulgar a palestra desta semana do ciclo de Seminários de Probabilidade e Sistemas Complexos, ICMC-USP & UFSCar, São Carlos. Uma lista das palestras passadas, e as próximas, pode ser encontrada no seguinte link: <a target="_blank" href="http://www.icmc.usp.br/pessoas/pablor/seminar/pcs_seminar.html">http://www.icmc.usp.br/pessoas/pablor/seminar/pcs_seminar.html</a> Segue a informação da próxima sexta. <h3>Ferromagnetic Ising model with periodical external field</h3> Speaker: Manuel González Navarrete (IME-USP) Date: 22/05/2015 • Time: 16h00 • Room: 4-111 (ICMC) Abstract: We study the low-temperature phase diagram for a ferromagnetic Ising model on Z², with a periodical external magnetic field. The external field takes two values: h and -h, where h>0. The sites associated with positive and negative values of external field form a cell-board configuration with rectangular cells of sides L1 × L2 sites, such that the total value of the external field is zero. As a main result, we show the presence of a first-order phase transition. The phase transition holds if h< (2J ⁄ L1)+ (2J ⁄ L2), where J is an interaction constant. We use the reflection positivity (RP) method. We apply a key inequality which is usually referred to as the chessboard estimate. Furthermore, we prove uniqueness for Gibbs measure in h>4J, using a uniqueness condition obtained in terms of disagreement percolation. Joint work with Eugene Pechersky and Anatoly Yambartsev. Estão todos convidados! Favor divulgar a possíveis interessados. Um abraço, <div>Pablo Martín Rodríguez Professor Doutor (MS-3) Department of Applied Mathematics and Statistics, ICMC-USP C.P. 668 - São Carlos, SP, Brazil - CEP 13560-970 </div></div></body></html>