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T. J. P. Penna

Publications and source records attributed to T. J. P. Penna.

30 records · Page 2Linked to original sources
arXiv1998

Sliding Blocks Revisited: A simulational Study

A computational study of sliding blocks on inclined surfaces is presented. Assuming that the friction coefficient $μ$ is a function of position, the probability $P(λ)$ for the block to slide down over a length $λ$ is numerically calculated. Our results are consistent with recent experimental data suggesting a power-law distribution of events over a wide range of displacements when the chute angle is close to the critical one, and suggest that the variation of $μ$ along the surface is responsible for this.

physics.class-ph↗
arXiv1997

Broad Histogram Monte Carlo

We propose a new Monte Carlo technique in which the degeneracy of energy states is obtained with a Markovian process analogous to that of Metropolis used currently in canonical simulations. The obtained histograms are much broader than those of the canonical histogram technique studied by Ferrenberg and Swendsen. Thus we can reliably reconstruct thermodynamic functions over a much larger temperature scale also away from the critical point. We show for the two-dimensional Ising model how our new method reproduces exact results more accurately and using less computer time than the conventional histogram method. We also show data in three dimensions for the Ising ferromagnet and the Edwards Anderson spin glass.

cond-mat.stat-mech↗
arXiv1997

Theoretical approach to biological aging

We present a model for biological aging that considers the number of individuals whose (inherited) genetic charge determines the maximum age for death: each individual may die before that age due to some external factor, but never after that limit. The genetic charge of the offspring is inherited from the parent with some mutations, described by a transition matrix. The model can describe different strategies of reproduction and it is exactly soluble. We applied our method to the bit-string model for aging and the results are in perfect agreement with numerical simulations.

adap-org↗
arXiv1997

Monte Carlo Simulations of Some Dynamical Aspects of Drop Formation

In this work we present some results from computer simulations of dynamical aspects of drop formation in a leaky faucet. Our results, which agree very well with the experiments, suggest that only a few elements, at the microscopic level, would be necessary to describe the most important features of the system. We were able to set all parameters of the model in terms of real ones. This is an additional advantage with respect to previous theoretical works.

cond-mat.stat-mech↗
arXiv1996

Broad Histogram Method

Ferrenberg and Swendsen histogram method is based on Boltzmann probability distribution which presents exponentially decaying tails. Thus, it gives accurate measures only within a narrow window around the simulated temperature. The larger the system, the narrower this window, and the worst the performance of this method. We present a quite different approach, defining a non-biased random walk along the E axis with long range power-law decaying tails, and measuring directly the degeneracy g(E), without thermodynamic constraints. Our arguments are general (model independent), and the method is shown to be exact for the 1D Ising ferromagnet. Also for the 2D Ising ferromagnet, our numerical results for different thermodynamic quantities agree quite well with exact expressions.

cond-mat.stat-mech↗
arXiv1995

Exact Results of the Bit-String Model for Catastrophic Senescence

We succeeded in obtaining exact results of the bit-string model of biological aging for populations whose individuals breed only once. These results are in excellent agreement with those obtained through computer simulations. In addition, we obtain an expression for the minimum birth needed to avoid mutational meltdown.

adap-org↗
arXiv1995

Long Range Anticorrelations and Non-Gaussian Behavior of a Leaky Faucet

We find that intervals between successive drops from a leaky faucet display scale-invariant, long-range anticorrelations characterized by the same exponents of heart beat-to-beat intervals of healthy subjects. This behavior is also confirmed by numerical simulations on lattice and it is faucet-width- and flow-rate-independent. The histogram for the drop intervals is also well described by a Lévy distribution with the same index for both histograms of healthy and diseased subjects. This additional result corroborates the evidence for similarities between leaky faucets and healthy hearts underlying dynamics.

adap-org↗
arXiv1995

Mutation Accumulation and the Catastrophic Senescence of Pacific Salmon

The bit-string model of biological aging is used to simulate the catastrophic senescence of Pacific Salmon. We have shown that reproduction occuring only once and at a fixed age is the only ingredient needed to explain the catastrophic senescence according the mutation accumulation theory. Several results are presented, some of them with up to $10^8$ fishes, showing how the survival rates in catastrophic senescence are affected by changes in the parameters of the model.

adap-org↗
arXiv1995

Preprocessing in Attractor Neural Networks

Preprocessing the input patterns seems the simplest approach to invariant pattern recognition by neural networks. The Fourier transform has been proposed as an appropriate and elegant preprocessor. Nevertheless, we show in this work that the performance of this kind of preprocessor is strongly affected by the number of stored informations. This is because the phase of the Fourier transform plays a more important role than the amplitude in the recognition process.

cond-mat↗
arXiv1995

Efficient Monte Carlo Simulation of Biological Aging

A bit-string model of biological life-histories is parallelized, with hundreds of millions of individuals. It gives the desired drastic decay of survival probabilities with increasing age for 32 age intervals.

cond-mat↗