Descripción: Fil:Braberman, V. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

Descripción: Fil:Eguia, M.C. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

Descripción: In this paper we extend the Paging Problem to the case in which each request specifies a set of pages that must be present in fast memory to serve it. The interest on this extension is motivated by many applications in which the execution of each task may require the presence of more than one page in fast memory. We introduce three different cost models that can be applied in this framework, namely the Full, Uniform and Constant cost models, and study lower and upper bounds for each one of them, using competitive analysis techniques.

Descripción: Fil:Aliaga, J. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

Descripción: Fil:Méndez-Díaz, I. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

Descripción: Aims. We study the modes and stability of non-isothermal coronal loop models with different intensity values of the equilibrium magnetic field. Methods. We use an energy principle obtained via non-equilibrium thermodynamic arguments. The principle is expressed in terms of Hermitian operators and allow to consider together the coupled system of equations: the balance of energy equation and the equation of motion. Results. We determine modes characterized as long - wavelength disturbances that are present in inhomogeneous media. This character of the system introduces additional difficulties for the stability analysis because the inhomogeneous nature of the medium determines the structure of the disturbance, which is no longer sinusoidal. Moreover, another complication is that we obtain a continuous spectrum of stable modes in addition to the discrete one. Conclusions. We obtain a unique unstable mode with a characteristic time that is comparable with the characteristic life-time observed for loops. The feasibility of wave-based and flow-based models is examined. © ESO 2006.

Descripción: Background: The vast computational resources that became available during the past decade enabled the development and simulation of increasingly complex mathematical models of cancer growth. These models typically involve many free parameters whose determination is a substantial obstacle to model development. Direct measurement of biochemical parameters in vivo is often difficult and sometimes impracticable, while fitting them under data-poor onditions may result in biologically implausible values. Results: We discuss different methodological approaches to estimate parameters in complex biological models. We make use of the high computational power of the Blue Gene technology to perform an extensive study of the parameter space in a model of avascular tumor growth. We explicitly show that the landscape of the cost function used to optimize the model to the data has a very rugged surface in parameter space. This cost function has many local minima with unrealistic solutions, including the global minimum corresponding to the best fit. Conclusions: The case studied in this paper shows one example in which model parameters that optimally fit the data are not necessarily the best ones from a biological point of view. To avoid force-fitting a model to a dataset, we propose that the best model parameters should be found by choosing, among suboptimal parameters, those that match criteria other than the ones used to fit the model. We also conclude that the model, data and optimization approach form a new complex system and point to the need of a theory that addresses this problem more generally. © 2010 Fernandez Slezak et al.

Descripción: Fil:Durán, G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

Descripción: An experimental validation of an model for the production of birdsongs was discussed. The derivation of the model with functions whose time dependence came from recordings of muscle activities and air sac pressure was studied. It was found that birds' songs were recorded and compared with the synthetic songs. Some predictions of the model concerning the relative levels of activity in the gating muscles at the beginning and end of a syllable were also tested.

Descripción: The improvement in convergence by means of accurate functional elimination in the context of the monotone Newton theorem is further analyzed and extended to discrete approximations of the Newton method. © 1995.

Descripción: We study the generation of primordial magnetic fields, coherent over cosmologically interesting scales, by gravitational creation of charged scalar particles during the reheating period. We show that magnetic fields consistent with those detected by observation may be obtained if the particle mean life τ(s) is in the range 10-14 s ≤ τ(s) ≤ 10-7 s. We apply this mechanism to minimal gauge-mediated supersymmetry-breaking models, in the case in which the lightest stau τ(l) is the next-to-lightest supersymmetric particle. We show that, for a large range of phenomenologically acceptable values of the supersymmetry-breaking scale √F, the generated primordial magnetic field can be strong enough to seed the galactic dynamo. (C) 2000 Elsevier Science B.V.

Descripción: We determine how the differences in the treatment of the subfilter-scale physics affect the properties of the flow for three closely related regularizations of Navier-Stokes. The consequences on the applicability of the regularizations as subgrid-scale (SGS) models are also shown by examining their effects on superfilter-scale properties. Numerical solutions of the Clark-α model are compared to two previously employed regularizations, the Lagrangian-averaged Navier-Stokes α-model (LANS-α) and Leray-α, albeit at significantly higher Reynolds number than previous studies, namely, Re≈3300, Taylor Reynolds number of Re≈790, and to a direct numerical simulation (DNS) of the Navier-Stokes equations. We derive the de Kármán-Howarth equation for both the Clark-α and Leray-α models. We confirm one of two possible scalings resulting from this equation for Clark-α as well as its associated k-1 energy spectrum. At subfilter scales, Clark-α possesses similar total dissipation and characteristic time to reach a statistical turbulent steady state as Navier-Stokes, but exhibits greater intermittency. As a SGS model, Clark-α reproduces the large-scale energy spectrum and intermittency properties of the DNS. For the Leray-α model, increasing the filter width α decreases the nonlinearity and, hence, the effective Reynolds number is substantially decreased. Therefore, even for the smallest value of α studied Leray-α was inadequate as a SGS model. The LANS-α energy spectrum ∼k1, consistent with its so-called "rigid bodies," precludes a reproduction of the large-scale energy spectrum of the DNS at high Re while achieving a large reduction in numerical resolution. We find, however, that this same feature reduces its intermittency compared to Clark-α (which shares a similar de Kármán-Howarth equation). Clark-α is found to be the best approximation for reproducing the total dissipation rate and the energy spectrum at scales larger than α, whereas high-order intermittency properties for larger values of α are best reproduced by LANS-α. © 2008 American Institute of Physics.

Descripción: We introduce the notion of Timed I/O Components as Timed Automata "á la" Alur & Dill where an "admissible" I/O interface is declared. That notion has, what we consider, a key modeling property: non-zeno preservation under syntactically-checkable "I/O compatibility" among interacting components. Also a reduced parallel composition is posssible based on the ability of statically detect influence of behavior between components [8,10,11]. On the other hand, with some simple extra conditions, modular assume-guarantee style of reasoning like [15,19] is valid in our model. © 2002 Published by Elsevier Science B. V.

Descripción: In this work we investigate experimentally the dynamics of two coupled optical excitable cells, namely, two semiconductor lasers with optical feedback. We analyze the dynamics observed in terms of the statistical properties of the time series and in terms of the phase space reconstruction from the data. We build a model based on a simple set of deterministic equations (on a two torus) plus noise in order to capture the essential features of the dynamics observed. We discuss the validity of our theoretical results in terms of families of excitable systems and coupling terms. © 2002 The American Physical Society.

Descripción: The effects of rapid rotation and bi-stability upon the density contrast between the equatorial and polar directions of a B[e] supergiant are re-investigated. Based on a new slow solution for different high rotational radiation-driven winds and the fact that bi-stability allows a change in the line-force parameters (α, k, and δ), the equatorial densities are about 102-104 times higher than the polar ones. These values are in qualitative agreement with the observations. © ESO 2005.

Descripción: New insights into equilibrium and dynamical aspects of electron photodetachment reactions in small water clusters were given. It focuses on assessing the effects of thermal and polarization fluctuations provided by three cluster environments with different extents of spatial confinement, on the microscopic mechanisms that drive the reaction. These fluctuations, in turn, determine the characteristics of the electron localization and the subsequent detachment following photoexcitation of the probe.

Descripción: In this paper, we introduce a family of robust estimates for the parametric and nonparametric components under a generalized partially linear model, where the data are modeled by y i |(x i , t i ) ∼ F (·, μ i ) with μ i = H(η(t i ) + x i T β), with for some known distribution function F and link function H. It is shown that the estimates of β are root-n consistent and asymptotically normal. Through a Monte Carlo study, the performance of these estimators is compared with that of the classical ones. © Institute of Mathematical Statistics, 2006.