Competition, efficiency and collective behavior in the "El Farol" bar model

The El Farol bar model, proposed to study the dynamics of competition of agents in a variety of contexts (W. B. Arthur, Amer. Econ. Assoc. Pap. and Proc. 84, 406 (1994)) is studied. We characterize in detail the three regions of the phase diagram (efficient, inefficient and better than random) of the simplest version of the model (D. Challet and Y.-C. Zhang, Physica A, 246, 407 (1997)). The efficient region is shown to have a rich structure, which is investigated in some detail. Changes in the payoff function enhance further the tendency of the model towards a wasteful distribution of resources.Comment: 7 pages Latex, 7 Postscript figures; changed reference, acknowledgments included. Accepted for publication in Europen Physics Journal

Cáculo del riesgo de ingición a partir de imágenes AVHRR (NOAA)

El riesgo de incendio forestal puede resumirse en dos factores principales: el riesgo de ignición y la probabilidad de que el fuego se expanda y acabe produciendo un incendio forestal. El riesgo de ignición puede ser debido a diferentes causas: factor humano y el estado de la vegetación. La probabilidad de expansión es principalmente debido a: condiciones meteorológicas, situación geográfica, características de la vegetación y facilidad de extinción. El principal objetivo del presente trabajo es la obtención de un índice de riesgo de ignición debido al estado de la vegetación. Para cumplir dicho objetivo se ha elaborado una serie temporal de 8 años de imágenes AVHRR (NOAA). A partir de las imágenes diarias se ha calculado el índice de vegetación NDVI, promedios mensuales y también promedios del mismo mes para los distintos años. A partir de la comparación del NDVI mensual del año en curso con el promedio de la serie de temporal para el mes correspondiente se detectan las zonas con diferencias importantes de estado de la vegetación. Las zonas con NDVI promedio más bajos para el año en curso respecto a la serie temporal, son zonas con riesgo de ignición más elevado que el resto de zonas.The risk of forest fires can be summarized in two main factors: the risk of ignition and the probability of fire spreading and causing a forest fires. The risk of ignition may be due to different causes: human factor and the state of vegetation. The probability of expansion is mainly due to: weather, geography, vegetation characteristics and ease of extinction. The main aim of this study is to obtain an index of risk of ignition due to the state of vegetation. To meet this objective a series of 8 years of images AVHRR (NOAA) has been developed. From daily images we have calculated vegetation index NDVI, monthly averages and averages of the same month for different years. From the comparison of monthly NDVI for current month with the average time series for the corresponding month, in order to detect areas with significant differences in the state of the vegetation. Areas with lower average NDVI for the current year with respect to the time series, are areas with higher fire risks than other areas

Energy radiation of moving cracks

The energy radiated by moving cracks in a discrete background is analyzed. The energy flow through a given surface is expressed in terms of a generalized Poynting vector. The velocity of the crack is determined by the radiation by the crack tip. The radiation becomes more isotropic as the crack velocity approaches the instability threshold.Comment: 7 pages, embedded figure

Mean Field Theory of Sandpile Avalanches: from the Intermittent to the Continuous Flow Regime

We model the dynamics of avalanches in granular assemblies in partly filled rotating cylinders using a mean-field approach. We show that, upon varying the cylinder angular velocity $\omega$, the system undergoes a hysteresis cycle between an intermittent and a continuous flow regimes. In the intermittent flow regime, and approaching the transition, the avalanche duration exhibits critical slowing down with a temporal power-law divergence. Upon adding a white noise term, and close to the transition, the distribution of avalanche durations is also a power-law. The hysteresis, as well as the statistics of avalanche durations, are in good qualitative agreement with recent experiments in partly filled rotating cylinders.Comment: 4 pages, RevTeX 3.0, postscript figures 1, 3 and 4 appended

On the driven Frenkel-Kontorova model: II. Chaotic sliding and nonequilibrium melting and freezing

The dynamical behavior of a weakly damped harmonic chain in a spatially periodic potential (Frenkel-Kontorova model) under the subject of an external force is investigated. We show that the chain can be in a spatio-temporally chaotic state called fluid-sliding state. This is proven by calculating correlation functions and Lyapunov spectra. An effective temperature is attributed to the fluid-sliding state. Even though the velocity fluctuations are Gaussian distributed, the fluid-sliding state is clearly not in equilibrium because the equipartition theorem is violated. We also study the transition between frozen states (stationary solutions) and=7F molten states (fluid-sliding states). The transition is similar to a first-order phase transition, and it shows hysteresis. The depinning-pinning transition (freezing) is a nucleation process. The frozen state contains usually two domains of different particle densities. The pinning-depinning transition (melting) is caused by saddle-node bifurcations of the stationary states. It depends on the history. Melting is accompanied by precursors, called micro-slips, which reconfigurate the chain locally. Even though we investigate the dynamics at zero temperature, the behavior of the Frenkel-Kontorova model is qualitatively similar to the behavior of similar models at nonzero temperature.Comment: Written in RevTeX, 13 figures in PostScript, appears in PR

Steady-State Cracks in Viscoelastic Lattice Models II

We present the analytic solution of the Mode III steady-state crack in a square lattice with piecewise linear springs and Kelvin viscosity. We show how the results simplify in the limit of large width. We relate our results to a model where the continuum limit is taken only along the crack direction. We present results for small velocity, and for large viscosity, and discuss the structure of the critical bifurcation for small velocity. We compute the size of the process zone wherein standard continuum elasticity theory breaks down.Comment: 17 pages, 3 figure

Discrete models of dislocations and their motion in cubic crystals

A discrete model describing defects in crystal lattices and having the standard linear anisotropic elasticity as its continuum limit is proposed. The main ingredients entering the model are the elastic stiffness constants of the material and a dimensionless periodic function that restores the translation invariance of the crystal and influences the Peierls stress. Explicit expressions are given for crystals with cubic symmetry: sc, fcc and bcc. Numerical simulations of this model with conservative or damped dynamics illustrate static and moving edge and screw dislocations and describe their cores and profiles. Dislocation loops and dipoles are also numerically observed. Cracks can be created and propagated by applying a sufficient load to a dipole formed by two edge dislocations.Comment: 23 pages, 15 figures, to appear in Phys. Rev.