Simón Bolívar University

About: Simón Bolívar University is a education organization based out in Caracas, Venezuela. It is known for research contribution in the topics: Population & Crystallization. The organization has 5912 authors who have published 8294 publications receiving 126152 citations.

Automatic derivation of memoryless policies and finite-state controllers using classical planners

Abstract: Finite-state and memoryless controllers are simple action selection mechanisms widely used in domains such as videogames and mobile robotics. Memoryless controllers stand for functions that map observations into actions, while finite-state controllers generalize memoryless ones with a finite amount of memory. In contrast to the policies obtained from MDPs and POMDPs, finite-state controllers have two advantages: they are often extremely compact, involving a small number of controller states or none at all, and they are general, applying to many problems and not just one. A limitation of finite-state controllers is that they must be written by hand. In this work, we address this limitation, and develop a method for deriving finite-state controllers automatically from models. These models represent a class of contingent problems where actions are deterministic and some fluents are observable. The problem of deriving a controller from such models is converted into a conformant planning problem that is solved using classical planners, taking advantage of a complete translation introduced recently. The controllers derived in this way are 'general' in the sense that they do not solve the original problem only, but many variations as well, including changes in the size of the problem or in the uncertainty of the initial situation and action effects. Experiments illustrating the derivation of such controllers are presented.

An investigation on the capabilities of the PENELOPE MC code in nanodosimetry.

Abstract: The Monte Carlo (MC) method has been widely implemented in studies of radiation effects on human genetic material. Most of these works have used specific-purpose MC codes to simulate radiation transport in condensed media. PENELOPE is one of the general-purpose MC codes that has been used in many applications related to radiation dosimetry. Based on the fact that PENELOPE can carry out event-by-event coupled electron-photon transport simulations following these particles down to energies of the order of few tens of eV, we have decided to investigate the capacities of this code in the field of nanodosimetry. Single and double strand break probabilities due to the direct impact of {gamma} rays originated from Co{sup 60} and Cs{sup 137} isotopes and characteristic x-rays, from Al and C K-shells, have been determined by use of PENELOPE. Indirect damage has not been accounted for in this study. A human genetic material geometrical model has been developed, taking into account five organizational levels. In an article by Friedland et al. [Radiat. Environ. Biophys. 38, 39-47 (1999)], a specific-purpose MC code and a very sophisticated DNA geometrical model were used. We have chosen that work as a reference to compare our results. Single and double strand-breakmore » probabilities obtained here underestimate those reported by Friedland and co-workers by 20%-76% and 50%-60%, respectively. However, we obtain RBE values for Cs{sup 137}, Al{sub K} and C{sub K} radiations in agreement with those reported in previous works [Radiat. Environ. Biophys. 38, 39-47 (1999)] and [Phys. Med. Biol. 53, 233-244 (2008)]. Some enhancements can be incorporated into the PENELOPE code to improve its results in the nanodosimetry field.« less

Evaluation and Comparison of EEG Traces: Latent Structure in Nonstationary Time Series

Abstract: We explore and illustrate the use of time series decomposition methods for evaluating and comparing latent structure in nonstationary electroencephalographic (EEG) traces obtained from depressed patients during brain seizures induced as part of electroconvulsive therapy (ECT). Analysis of the patterns of change over time in the frequency structure of such EEG data provides insight into the neurophysiological mechanisms of action of this effective but poorly understood antidepressant treatment, and allows clinicians to modify ECT treatments to optimize therapeutic benefits while minimizing associated side effects. Our work has introduced new methods of time-frequency analysis of EEG series that identify the complete pattern of time evolution of frequency structure over the course of a seizure, and usefully assist in these scientific and clinical studies. New methods of decomposition of flexible dynamic models provide time domain decompositions of individual EEG series into collections of latent co...

A new fossil genus of South American cricetid rodents allied to Wiedomys, with an assessment of the Sigmodontinae

Abstract: A new genus and species of sigmodontine cricetids, Cholomys pearsonl, from the Lower Pleistocene Vorohue Formation of south-eastern Buenos Aires Province. Argentina, is described. Its closest relative among the extant and extinct cricetids proved to be the living Wiedomys pyrrhorhinus of Brazil. Wiedomys and Cholomys are included in a new tribe of the Sigmodontinae, Wiedomyini. The distinction between the North American and the South American cricetids is stressed, the two groups being postulated to pertain to different subfamilies. The names Neotominae and Sigmodontinae are chosen for these two taxa, respectively. A tribal classification of the Sigmodontinae is advanced. Oryzomyini, Akodontini, Scapteromyini, Phyllotini, Sigmodontini, lehthyomyini and Wiedomyini are recognized as natural tribes of the Sigmodontinae. This broad array of suprageneric groupings reflecting different adaptive types strongly supports the hypothesis that the differentiation of the Sigmodontinae took place in South America from a primitive cricetid immigrant stock which entered South America, probably from North America, not later than during Miocene times.