Estudio de la dinámica temporal de un modelo de urnas : probabilidades de primer pasaje y criticalidad auto-organizada

Tesis (Lic. en Física)--Universidad Nacional de Córdoba, Facultad de Matemática, Astronomía, Física y Computación, 2020.En este trabajo estudiamos una combinación de los clásicos modelos de urnas de Ehrenfest y del votante. En nuestro modelo, a cada paso temporal se efectúa el modelo de Ehrenfest con probabilidad α, o el del votante con probabilidad (1 − α). Este modelo puede ser visto como una caminata aleatoria unidimensional en una red finita. Analizamos los estados de equilibrio del sistema y determinamos una transición de fase para α = 1/N . Asimismo, definiendo el tamaño de avalancha como la cantidad de pasos necesarios para volver al estado de equilibrio por primera vez, estudiamos la distribuciones de tamaños de avalanchas y de retornos, para determinar si el comportamiento del sistema presenta, además, criticalidad auto- organizada. Desde el punto de vista de la sociofı́sica, este modelo representa un modelo de opinión, donde el cambio de opinión puede darse por interacción, (modelo del votante), o sin interacción, (modelo de Ehrenfest). Para valores de α relativamente chicos (∼ 1/N ), el sistema evoluciona a una sociedad polarizada.In this work we present a combination of two classical urn models: Ehrenfest and voter . At every time step of our model either an Ehrenfest step is performed with probability \alpha or a voter step is performed with probability (1 − α). This model can be seen as an unidimensional random walk in a finite lattice. We analyzed the equilibrium states of the system and found a phase transition at α = 1/N. Furthermore, by defining the size of an avalanche as the amount of steps needed for the system to return to the equilibrium state for the first time, we studied the distribution of avalanches sizes and returns, in order to determine also if the system has Self-Organized Criticality. From the point of view of social physics this model represents an opinion model, where an agent can change its opinion by interacting with other agents (voter model) or without any interaction at all (Ehrenfest model). For relatively small α values (∼ 1 / N), the system evolves into a polarized society.Fil: Perachia, Florencia. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía, Física y Computación; Argentina

Port-site metastases following laparoscopic surgery

BackgroundApplication of laparoscopy to the resection of malignancy has been followed by a literature describing cases of metastatic involvement at laparoscopic port sites. These include patients who underwent surgery for early stage carcinoma and instances following laparoscopic procedures during which tumours were not dissected.MethodsRecently published clinical and experimental studies, and case reports related to this problem are reviewed; their relevance is discussed.ResultsExperimental studies incorporating bench top and large animal models have confirmed that tumour cells may be redistributed to port sites during laparoscopic surgery either directly from contaminated instruments or indirectly via the insufflation gas. Small animal models suggest that the incidence of wound metastasis is increased following conventional laparoscopic surgery, and that it may be decreased by gasless laparoscopy or helium insufflation. This evidence suggests that the development of port-site metastases depends not only on the physical redistribution of tumour cells but also on the specific insufflation gas used, possibly because of influences on local metabolic or immune factors acting at the wound site.ConclusionFurther research in this area is urgent. Until the issue is better understood, patients undergoing laparoscopic surgery for malignancy should be entered into clinical trials