bluuebunny/crossref_metadata_2025 huggingface dataset:
Computational Power of Quantum Machines, Quantum Grammars and Feasible Computation
Authors: Krishnamurthy E. V.
Published: October 2003

This paper studies the computational power of quantum computers to explore as to whether they can recognize properties which are in nondeterministic polynomial-time class (NP) and beyond. To study the computational power, we use the Feynman's path integral (FPI) formulation of quantum mechanics. From a computational point of view the Feynman's path integral computes a quantum dynamical analogue of the k-ary relation computed by an Alternating Turing machine (ATM) using AND-OR Parallelism. Hence, if we can find a suitable mapping function between an instance of a mathematical problem and the corresponding interference problem, using suitable potential functions for which FPI can be integrated exactly, the computational power of a quantum computer can be bounded to that of an alternating Turing machine that can solve problems in NP (e.g, factorization problem) and in polynomial space. Unfortunately, FPI is exactly integrable only for a few problems (e.g., the harmonic oscillator) involving quadratic potentials; otherwise, they may be only approximately computable or noncomputable. This means we cannot in general solve all quantum dynamical problems exactly except for those special cases of quadratic potentials, e.g., harmonic oscillator. Since there is a one to one correspondence between the quantum mechanical problems that can be analytically solved and the path integrals that can be exactly evaluated, we can say that the noncomputability of FPI implies quantum unsolvability. This is the analogue of classical unsolvability.

The Feynman's path graph can be considered as a semantic parse graph for the quantum mechanical sentence. It provides a semantic valuation function of the terminal sentence based on probability amplitudes to disambiguate a given quantum description and obtain an interpretation in a linear time. In Feynman's path integral, the kernels are partially ordered over time (different alternate paths acting concurrently at the same time) and multiplied. The semantic valuation is computable only if the FPI is computable. Thus both the expressive power and complexity aspects quantum computing are mirrored by the exact and efficient integrability of FPI.
New Algorithm for Tensor Calculation in Field Theories
Authors: Ichinose Shoichi
Published: October 2003

Tensor calculation of suffix-contraction is carried out by a C-program. Tensors are represented graphically, and the algorithm makes use of the topology of the graphs. Classical and quantum gravity, in the weak-field perturbative approach, is a special interest. Examples of the leading order calculation of some general invariants such as RμνλσRμνλσ are given. Application to Weyl anomaly calculation is also discussed.
Exact Solutions of (2+1)-Dimensional Soliton Equations Applying SymbolicC++
Authors: Steeb W.-H. , Tan Kiat Shi , Stoop R.
Published: October 2003

We show how SymbolicC++, a symbolic and numeric computer algebra system written in C++, can be used to find exact solutions of soliton equations in (2+1)-dimensions.
An Eighth Order Exponentially Fitted Method for the Numerical Solution of the Schrödinger Equation
Authors: Simos T. E.
Published: October 2003

An eighth order exponentially fitted method is developed for the numerical integration of the Schrödinger equation. The formula considered contains certain free parameters which allow it to be fitted automatically to exponential functions. This is the first eighth order exponentially fitted method in the literature. Numerical results also indicate that the new method is much more accurate than other classical and exponentially fitted methods.
Large Lattice Simulation of Random Site Percolation
Authors: Macleod Shane , Jan Naeem
Published: October 2003

We simulate systems up to 4*1012 sites on a square lattice at the percolation threshold, pc=0.592746. We confirm τ=187/91, and report that the correction to scaling exponent, Δ1=0.65±0.05. The mass of the largest and second largest cluster scales with the fractal dimensionality, df=91/48. These exponents, apart from the correction-to-scaling exponent, Δ1, are expected from exact calculations and previous substantial numerical work.
Caribou Overgrazing in the Absence of Predators
Authors: Feingold S. J.
Published: October 2003

Overgrazing in a nonmigrating caribou population, in the absence of predators, is simulated using the Penna life-history model. Despite the lack of natural enemies, the population size may not increase permanently. A balanced ecosystem with moderate predation gives better results.
Split Mode Method for the Elliptic 2D Sine-Gordon Equation: Application to Josephson Junction in Overlap Geometry
Authors: Caputo Jean-Guy , Flytzanis Nikos , Gaididei Yuri , Moulitsa Irene , Vavalis Emmanuel
Published: October 2003

We introduce a new type of splitting method for semilinear partial differential equations. The method is analyzed in detail for the case of the two-dimensional static sine-Gordon equation describing a large area Josephson junction with overlap current feed and external magnetic field. The solution is separated into an explicit term that satisfies the one-dimensional sine-Gordon equation in the y-direction with boundary conditions determined by the bias current and a residual which is expanded using modes in the y-direction, the coefficients of which satisfy ordinary differential equations in x with boundary conditions given by the magnetic field. We show by direct comparison with a two-dimensional solution that this method converges and that it is an efficient way of solving the problem. The convergence of the y expansion for the residual is compared for Fourier cosine modes and the normal modes associated to the static one-dimensional sine-Gordon equation and we find a faster convergence for the latter. Even for such large widths as w=10 two such modes are enough to give accurate results.
Universality of Derrida Coarsening in the Triangular Potts Model
Authors: Hennecke Michael
Published: October 2003

The temperature dependence of the dynamics of the fraction F(t) of persistent spins in the triangular Q-state Potts model is investigated by large scale Monte Carlo simulations. After extending Derrida's approach of measuring the fraction of spins that remain in one phase to allQ low-temperature phases, it is shown that the exponent θ of algebraic decay of F(t) is independent of temperature.
Learning Algorithm for the Uniform Graph Partitioning Problem
Authors: Chua C. B. , Chen Kan
Published: October 2003

We study the uniform graph partitioning problem using the learning algorithm proposed by one of us. We discuss the characteristics of the learning algorithm and compare the performance of the algorithm empirically with the Kernighan–Lin algorithm on a range of instances. Even with a simple implementation, the learning algorithm is capable of producing very good results.
Random Site Percolation in Three Dimensions
Authors: Jan Naeem , Stauffer Dietrich
Published: October 2003

We simulate sytems up to 10 001 × 10 001 × 10 001 in three dimensions at the percolation threshold pc of 0.31160. We find that the fractal dimension is ≃2.53±0.02, the cluster size distribution exponent, τ, is 2.186±0.002 and an exponent of 0.85 describing how the mass of the clusters scale with rank. Corrections-to-scaling exponents of ≃-0.7 are observed for ns and for the mass of the largest cluster. We also check the percolation threshold and report good agreement with recent values.
Drift and Trapping in Biased Diffusion on Disordered Lattices
Authors: Dhar Deepak , Stauffer Dietrich
Published: October 2003

We re-examine the theory of transition from drift to no-drift in biased diffusion on percolation networks. We argue that for the bias field B equal to the critical value Bc, the average velocity at large times t decreases to zero as 1/ log (t). For B<Bc, the time required to reach the steady-state velocity diverges as exp ( const /|Bc-B|). We propose an extrapolation form that describes the behavior of average velocity as a function of time at intermediate time scales. This form is found to have a very good agreement with the results of extensive Monte Carlo simulations on a three-dimensional site-percolation network and moderate bias.
Simulations of Carbon Containing Semiconductor Alloys: Bonding, Strain Compensation, and Surface Structure
Authors: Kelires P. C.
Published: October 2003

This paper reviews recent Monte Carlo simulations within the empirical potential approach, which give insights into fundamental aspects of the bulk and surface structure of group-IV semiconductor alloys containing carbon. We focus on the binary Si 1-x C x and ternary Si 1-x-y Ge x C y alloys strained on silicon substrates. The statistical treatment of these highly strained alloys is made possible by using the semigrand canonical ensemble. We describe here improvements in the algorithm which considerably speed up the method. We show that the identity switches, which are the basic ingredients in this statistical ensemble, must be accompanied by appropriate relaxations of nearest neighbors in order to reach "quasiequilibrium" in metastable systems with large size mismatch between the constituent atoms. This effectively lowers the high formation energies and large barriers for diffusion which make molecular dynamics methods impractical for this problem. The most important findings of our studies are: (a) The prediction of a repulsive Ge–C interaction and of a preferential C–C interaction in the lattice. (b) The prediction for significant deviations of the structural parameters and of the elastic constants from linearly interpolated values (Vegard's law). As a result, for a given amount of carbon, strain compensation is shown to be more drastic than previously thought. (c) Investigation of the surface problem shows that the competition between the reconstruction strain field and the preferential arrangement of carbon atoms leads to new complicated structural patterns.
An Iterative Algorithm to Determine the Dynamic User Equilibrium in a Traffic Simulation Model
Authors: Gawron C.
Published: October 2003

An iterative algorithm to determine the dynamic user equilibrium with respect to link costs defined by a traffic simulation model is presented. Each driver's route choice is modeled by a discrete probability distribution which is used to select a route in the simulation. After each simulation run, the probability distribution is adapted to minimize the travel costs. Although the algorithm does not depend on the simulation model, a queuing model is used for performance reasons. The stability of the algorithm is analyzed for a simple example network. As an application example, a dynamic version of Braess's paradox is studied.
A Monte Carlo Simulation of an In-Plane Switching Liquid Crystal Display
Authors: Chiccoli Cesare , Pasini Paolo , Guzzetti Stefano , Zannoni Claudio
Published: October 2003

The Monte Carlo (MC) method is applied to the modeling of a liquid crystal display based on the in-plane switching effect. This is the first attempt to simulate this device features starting from purely microscopic molecular interactions. The optical textures are obtained from the Monte Carlo generated microscopic equilibrium configurations by means of a Müller matrix approach. Suitable order parameters are also calculated to quantify the ordering and the molecular organization across the display cell.
Why Sex? — Monte Carlo Simulations of Survival after Catastrophes
Authors: Sá Martins J. S. , de Oliveira S. Moss
Published: October 2003

Using the Penna bit-string model for biological ageing we compare two kinds of reproductive regimes: Sexual reproduction (SR) and meiotic parthenogenesis (MP). The last one is a common type of asexual reproduction with recombination, found in diploid organisms. We show that although both regimes present roughly the same survival rates, the diversity generated by SR is much larger, and can prevent the extinction of a population submitted to a natural disaster. The fixation of bad genes inside an MP population, after many generations, explains our results. We also study the consequences of cloning (simple copy) on population diversity.
Evidence of Discrete Scale Invariance in DLA and Time-to-Failure by Canonical Averaging
Authors: Johansen A. , Sornette D.
Published: October 2003

Discrete scale invariance, which corresponds to a partial breaking of the scaling symmetry, is reflected in the existence of a hierarchy of characteristic scales l0,l0λ,l0λ2,…, where λ is a preferred scaling ratio and l0 a microscopic cut-off. Signatures of discrete scale invariance have recently been found in a variety of systems ranging from rupture, earthquakes, Laplacian growth phenomena, "animals" in percolation to financial market crashes. We believe it to be a quite general, albeit subtle phenomenon. Indeed, the practical problem in uncovering an underlying discrete scale invariance is that standard ensemble averaging procedures destroy it as if it was pure noise. This is due to the fact, that while λ only depends on the underlying physics, l0 on the contrary is realization-dependent. Here, we adapt and implement a novel so-called "canonical" averaging scheme which re-sets the l0 of different realizations to approximately the same value. The method is based on the determination of a realization-dependent effective critical point obtained from, e.g., a maximum susceptibility criterion. We demonstrate the method on diffusion limited aggregation and a model of rupture.
Some New Facts of Life
Authors: Malarz K. , Kulakowski K. , Antoniuk M. , Grodecki M. , Stauffer D.
Published: October 2003

We investigate the asymptotic density of living cells as dependent on an initial density, the results of scattering of gliders on small stable configurations and the damage spreading in an open system, subjected to a stream of gliders with the rules of the cellular automaton "Game of Life." We find that a damage increases approximately with the second power of time through about the first ten steps; later its time dependence becomes almost linear.
Simulation of Phosphorus Implantation into Silicon with a Single Parameter Electronic Stopping Power Model
Authors: Cai David , Snell Charles M. , Beardmore Keith M. , Grønbech-Jensen Niels
Published: October 2003

We simulate dopant profiles for phosphorus implantation into silicon using a new model for electronic stopping power. In this model, the electronic stopping power is factorized into a globally averaged effective charge [Formula: see text], and a local charge density dependent electronic stopping power for a proton. There is only a single adjustable parameter in the model, namely the one electron radius [Formula: see text] which controls [Formula: see text]. By fine tuning this parameter, we obtain excellent agreement between simulated dopant profiles and the SIMS data over a wide range of energies for the channeling case. Our work provides a further example of implant species, in addition to boron and arsenic, to verify the validity of the electronic stopping power model and to illustrate its generality for studies of physical processes involving electronic stopping.
Application of Chaotic Automata to Improve the Pattern Recognition Ability of Attractor Neural Networks
Authors: de Menezes M. Argollo , Penna T. J. P.
Published: October 2003

We present a preprocessor for attractor neural networks consisting of a chaotic cellular automaton. We show that this preprocessor improves on the ability to recognize correlated memories. The preprocessor, although chaotic, is reversible because the patterns have a finite number of neurons and appropriate boundary conditions were chosen. The performance of the present model was checked using numerical simulations in both the RS and the Hopfield models of neural networks. However, this is a very general approach that can be used in any other architecture with different learning rules.
Dynamics in Eigen Quasispecies Model
Authors: Malarz K. , Tiggemann D.
Published: October 2003

In this work the Eigen model of quasispecies is reconsidered. Our main aim is to find to what extent mutational meltdown changes previously presented results, when the finite population can decrease or increase during evolution.
Computer Simulation of Sexual Selection on Age-Structured Populations
Authors: Martins S. G. F. , Penna T. J. P.
Published: October 2003

Using computer simulations of a bit-string model for age-structured populations, we found that sexual selection of older males is advantageous, from an evolutionary point of view. These results are in opposition to a recent proposal of females choosing younger males. Our simulations are based on findings from recent studies of polygynous bird species. Since secondary sex characters are found mostly in males, we could make use of asexual populations that can be implemented in a fast and efficient way.
Broad Histogram Simulation: Microcanonical Ising Dynamics
Authors: de Oliveira Paulo Murilo Castro
Published: October 2003

We revisit here a new Monte Carlo approach, namely the Broad Histogram Method. It is based on two quantities, the numbers N up and N dn of potential modifications which could be performed starting from the system's current state, increasing or decreasing its energy E, respectively. Thus, the energy degeneracy g(E) can be directly determined from the microcanonical averages  and  of these two quantities. This method was first tested by sampling states from a random walk along the energy axis, for which the control of possible correlations between successive averaging states is not an easy task. Neverthless, the resulting microcanonical averages could not depend upon the particular dynamics used to sample the Markovian chain of averaging states. Here, we test the same method within an alternative dynamics for which the quoted control becomes trivial.
Gauge Theory of Finance?
Authors: Sornette Didier
Published: October 2003

The recent stimulating proposal of a "Gauge Theory of Finance" by Ilinsky et al. is connected here with traditional approaches. First, the derivation of the log-normal distribution is shown to be equivalent both in information and mathematical content to the simpler and well-known derivation, dating back from Bachelier and Samuelson. Similarly, the re-derivation of Black–Scholes equation is shown equivalent to the standard one because the limit of no uncertainty is equivalent to the standard risk-free replication argument. Both re-derivations of the log-normality and Black–Scholes result do not provide a test of the theory because it is not uniquely specified in the limits where these results apply. Third, the choice of the exponential form a la Boltzmann, of the weight of a given market configuration, is a key postulate that requires justification. In addition, the "Gauge Theory of Finance" seems to lead to "virtual" arbitrage opportunities for a pure Markov random walk market when there should be none. These remarks are offered in the hope to improve the formulation of the "Gauge Theory of Finance" into a coherent and useful framework.
Symbolic Computation of Turbulence and Energy Dissipation in the Taylor Vortex Model
Authors: Fateman Richard J.
Published: October 2003

Using a classic example proposed by G. I. Taylor, we reconsider through the use of computer algebra, the mathematical analysis of a fundamental process in turbulent flow, namely: How do large scale eddies evolve into smaller scale ones to the point where they are effectively absorbed by viscosity? The explicit symbolic series solution of this problem, even for cleverly chosen special cases, requires daunting algebra, and so numerical methods have become quite popular. Yet an algebraic approach can provide substantial insight, especially if it can be pursued with modest human effort.

The specific example we use dates to 1937 when Taylor and Green8 first published a method for explicitly computing successive approximations to formulas describing the three-dimensional evolution over time of what is now called a Taylor–Green vortex.

With the aid of a computer algebra system, we have duplicated Taylor and Green's efforts and obtained more detailed time-series results. We have extended their approximation of the energy dissipation from order 5 in time to order 14, including the variation with viscosity.

Rather than attempting additional interpretation of results for fluid flow (for which, see papers by Brachet et al.,2,3 we examine the promise of computer algebra in pursuing such problems in fluid mechanics.
Calculation of Percolation Thresholds in High Dimensions for FCC, BCC and Diamond Lattices
Authors: van der Marck Steven C.
Published: October 2003

Site and bond percolation thresholds are calculated for the face centered cubic, body centered cubic and diamond lattices in four, five and six dimensions. The results are used to study the behavior of percolation thresholds as a functions of dimension. It is shown that the predictions from a recently proposed invariant for percolation thresholds are not satisfactory for these lattices.
Number of Magic Squares from Parallel Tempering Monte Carlo
Authors: Pinn K. , Wieczerkowski C.
Published: October 2003

There are 880 magic squares of size 4 by 4, and 275 305 224 of size 5 by 5. It seems very difficult if not impossible to count exactly the number of higher order magic squares. We propose a method to estimate these numbers by Monte Carlo simulating magic squares at finite temperature. One is led to perform low temperature simulations of a system with many ground states that are separated by energy barriers. The Parallel Tempering Monte Carlo method turns out to be of great help here. Our estimate for the number of 6 by 6 magic squares is (0.17745± 0.00016)×1020.
Evolution of Hunter–Gatherer Strategies with a Genetically Inspired Algorithm
Authors: Sutton Pat , Georgallas A. , Hunter D. L. , Jan N. , Nash R. J. , Quinn J. P.
Published: October 2003

We address the following: Are there several optimal strategies for a hunter-gatherer society in a given environment, in which the society has an evolving level of technology and/or skills? We use the Genetic Algorithm, with point mutations, to facilitate the search for these strategies. We find that, in a generous environment, several optimal strategies are possible; there is no unique optimal strategy. We are now in a position, with the Genetic Algorithm, to assess different approaches to resource exploitation and the role of contingency.
Species Formation in Simple Ecosystems
Authors: Bagnoli F. , Bezzi M.
Published: October 2003

In this paper we consider a microscopic model of a simple ecosystem. The basic ingredients of this model are individuals, and both the phenotypic and genotypic levels are taken in account. The model is based on a long range cellular automaton (CA); introducing simple interactions between the individuals, we get some of the complex collective behaviors observed in a real ecosystem. Since our fitness function is smooth, the model does not exhibit the error threshold transition; on the other hand the size of total population is not kept constant, and the mutational meltdown transition is present. We study the effects of competition between genetically similar individuals and how it can lead to species formation. This speciation transition does not depend on the mutation rate. We present also an analytical approximation of the model.
A BUU Code for Parallel Computers
Authors: Barbosa V. C. , Donangelo R. , Wedemann R. S.
Published: October 2003

We have developed a distributed algorithm for the simulation of systems that evolve continuously, but which are also subject to discrete events that affect their evolution. As an example we consider the description through the BUU equations of the time evolution of a highly excited nuclear system, and show that this algorithm attains almost optimal speedups.
Is Finite-Size Scaling Universal for Ising Models?
Authors: Müller B.
Published: October 2003

The sensitivity on shape and boundaries at the Curie point of the universal critical ratio LdM2/χ for the Ising model is confirmed by a comparison of square and rectangular lattices. Tests with three different algorithms give the same results for the universal critical ratio on various lattice types and always the same dependence on the system's aspect ratio.
Ground State Symmetry of an Electrorheological Fluid Containing Two Types of Particles
Authors: Lukkarinen A. , Kaski K.
Published: October 2003

The ground state symmetry of an electrorheological fluid is studied numerically using a finite element method. The free energies of body-centered cubic, body-centered tetragonal, simple hexagonal, hexagonal close-packed, simple cubic, simple hexagonal, and honeycomb structures are calculated for systems containing homogeneous particles with the dielectric constant greater or smaller than that of the base fluid. In accordance with other studies, the body-centered tetragonal symmetry was found to be the most favored lattice symmetry. The ground state of an ER system containing two types of particles, i.e., particles with the dielectric constant greater and smaller than that of the base fluid, was found to consist of separated particle columns each of which has only one type of particles. These separated particle columns preferred body-centered tetragonal symmetry. However, simple hexagonal and honeycomb symmetries were found to be energetically quite close to a body-centered tetragonal symmetry. In addition, simple hexagonal and honeycomb symmetries do not need phase separation between different types of particles and are thus more likely to form. Dynamical effects were studied using a point dipole model. Within the time-span studied the system seemed to form a polycrystalline structure.
The RANLUX Generator: Resonances in a Random Walk Test
Authors: Shchur Lev N. , Butera Paolo
Published: October 2003

Using a recently proposed directed random walk test, we systematically investigate the popular random number generator RANLUX developed by Lüscher and implemented by James. We confirm the good quality of this generator with the recommended luxury level. At a smaller luxury level (for instance equal to 1) resonances are observed in the random walk test. We also find that the lagged Fibonacci and Subtract-with-Carry recipes exhibit similar failures in the random walk test. A revised analysis of the corresponding dynamical systems leads to the observation of resonances in the eigenvalues of Jacobi matrix.
Monte Carlo Investigation of Rare Magnetization Fluctuations in Ising Models
Authors: Stauffer D.
Published: October 2003

For square and simple cubic Ising models at T=Tc, we look at the tails of the magnetization distribution for untypically large magnetizations. No indications for a stretched exponential or power law behavior are found.
Dynamics of Two Populations with Different Birth Rates
Authors: Hoffmann Julia , Pekalski Andrzej
Published: October 2003

We propose a simple model describing the dynamics of a system of two populations — more numerous natives and less numerous immigrants. The immigrants' birth rate is higher than that of the natives. Several modifications of this model taking into account changes of the birth rates due to external factors and/or possibility of contacts between the populations, are also introduced. The model is studied within two approaches — by solving a set of differential equations and through a Monte Carlo simulations. We show that the question of which population will eventually dominate depends on such factors as the probability of producing offsprings of mixed origin, assimilation of the immigrants, the ratio of the birth rates, initial numbers of the populations and the average age of an individual. In all, but two extreme cases, both populations will survive.
Effects of Boundary Conditions on the Critical Spanning Probability
Authors: Acharyya Muktish , Stauffer Dietrich
Published: October 2003

The fractions of samples spanning a lattice at its percolation threshold are found by computer simulation of random site-percolation in two- and three-dimensional hypercubic lattices using different boundary conditions. As a byproduct we find pc=0.311605(5) in the cubic lattice.
An Extension of the LCAO Method — Isothermal Dynamics
Authors: Mazzone A. M.
Published: October 2003

This work presents a dynamical version of the LCAO method which includes extension variables, such as temperature and pressure. As an illustrative example, the method is applied to the temperature response of small silicon clusters. A comparison is made with molecular dynamics calculations using standard interatomic potentials.
A New Conjecture Extends the GM Law for Percolation Thresholds to Dynamical Situations
Authors: Galam Serge , Vandewalle Nicolas
Published: October 2003

The universal law for percolation thresholds proposed by Galam and Mauger (GM) is found to apply also to dynamical situations. This law depends solely on two variables, the space dimension d and a coordinance number q. For regular lattices, q reduces to the usual coordination number while for anisotropic lattices it is an effective coordination number. For dynamical percolation we conjecture that the law is still valid if we use the number q2 of second nearest neighbors instead of q. This conjecture is checked for the dynamic epidemic model which considers the percolation phenomenon in a mobile disordered system. The agreement is good.
Ising Cellular Automata for Proton Diffusion on Protein Surfaces
Authors: Careri G. , Stauffer D.
Published: October 2003

Q2R cellular automata are combined with Kawasaki dynamics to give an Ising ferromagnet with conserved magnetization and conserved energy. In its lattice gas interpretation, it may serve to describe the diffusion of hydrogen ions on the surface of globular proteins. Our computer simulations test to what extent the phase transition at the Curie point is still visible in small 10×10 lattices and t~104.
Spin–Spin Correlations in Site Disordered ±J 2D Ising Spin Glasses
Authors: Kawecka-Magiera B. , Maksymowicz A. Z. , Kowal M. , Kulakowski K.
Published: October 2003

Spin–spin correlation functions  as dependent on interatomic distance R are studied in the random-site two-dimensional Ising S=1/2 ±J system. Oscillations of the correlation functions are found, which is not a case in the random-bond system.
On the Phase Transition of Hopfield Networks — Another Monte Carlo Study
Authors: Volk Daniel
Published: October 2003

A Hopfield-type neural network has content addressable memory which emerges from its collective properties. I reinvestigate the controversial question of its critical storage capacity at zero temperature. To locate the discontinuous transition from good retrieval to bad retrieval in infinite systems the decreasing average quality of retrieved information is traced until it falls below a threshold. The cutoff points found for different system sizes are extrapolated towards infinity and yield αc=0.143±0.002.
A Predator–Prey Model with Genetics: Transition to a Self-Organized Critical State
Authors: Ray Tane S. , Moseley Leo , Jan Naeem
Published: October 2003

A two species predator–prey model based on the Lotka–Volterra equations is proposed, where the fitness of an individual animal depends upon the relative strength of its genes. Simulations of the model show that the system passes from the standard oscillatory solution of the Lotka–Volterra equations into a steady-state regime, which exhibits many of the characteristics of self-organized criticality, including a 1/f power spectrum.
Sparseness and Roughness of Foreign Exchange Rates
Authors: Vandewalle N. , Ausloos M.
Published: October 2003

An accurate multiaffine analysis of 23 foreign currency exchange rates has been performed. The roughness exponent H1which characterizes the excursion of the exchange rate has been numerically measured. The degree of intermittency C1has been also estimated. In the (H1,C1) phase diagram, the currency exchange rates are dispersed in a wide region around the Brownian motion value (H1=0.5,C1=0) and have a significantly intermittent component (C1≠0).
Why do Women Live Longer than Men? A Monte Carlo Simulation of Penna-type Models with X and Y Chromosomes
Authors: Schneider J. , Cebrat S. , Stauffer D.
Published: October 2003

This work presents a simple simulation of the differences in male and female mortality and ageing. The simulation is based on Penna's bit-string model of biological ageing.
Monte Carlo Algorithms for the Fully Frustrated XY Model
Authors: Pawig S. Große , Pinn K.
Published: October 2003

We investigate local update algorithms for the fully frustrated XY model on a square lattice. In addition to the standard updating procedures like the Metropolis or heat bath algorithm we include overrelaxation sweeps, implemented through single spin updates that preserve the energy of the configuration. The dynamical critical exponent (of order two) stays more or less unchanged. However, the integrated autocorrelation times of the algorithm can be significantly reduced.
Influence of the Tail Length in the Aggregation of Surfactants
Authors: Merschmann Luiz Henrique C. , Bernardes Américo T.
Published: October 2003

A lattice model is used to simulate the dependence of the condensation temperature of ternary mixtures (water-oil-surfactants) on varying tail length. Nonionic single-tailed amphiphiles are assumed. We have obtained the result that the longer the tail length, the higher is the condensation temperature. We also discuss the influence of the head–head interaction on that temperature.
Inclusion of Retardation Effects in Hydrodynamical Calculations
Authors: Kodama T. , Donangelo R. , Guidry M. W.
Published: October 2003

In numerical large-scale hydrodynamics calculations, such as the description of a supernova explosion, instantaneous thermalization of the fluid matter is assumed independently of the size of the volume element used in the calculation. One expects, however, the appearance of transient processes such as convection currents, vortices, and other collective motion on smaller and smaller scales, which can delay equilibration. To account for these effects in a simple one-dimensional hydrodynamical calculation, we introduce retardation in the hydrodynamic equations and show that, when strong shocks are present, such effects may have considerable influence on the evolution of the system.
Three-Dimensional Massively Parallel Computing of Suspensions
Authors: Wachmann B. , Schwarzer S.
Published: October 2003

Numerical simulations of suspensions often suffer from the fact that the simulated systems are rather small compared to experimental setups. We present a numerical scheme for non-Brownian particle-liquid mixtures in three dimensions at particle Reynolds numbers between 0.01 and 20 and describe its parallel implementation. The fluid equations are solved by a time-explicit pressure-implicit Navier–Stokes algorithm and the particle motion is tracked by molecular-dynamics methods. The two are coupled by imposing no-slip boundary conditions between the particles and the fluid. We integrate the stress distribution on the particle surface numerically to obtain forces and torques. The building blocks of the algorithm are local and scalable and we have reached particle numbers up to 106 (1.41*108 fluid nodes) on a 512 node CRAY-T3E. We compare our simulation results to theoretical predictions and experimental data and find good agreement for particle volume fractions up to 0.30.
Three Species Lotka–Volterra Model
Authors: Pekalski Andrzej , Stauffer Dietrich
Published: October 2003

We present a model of a three-step food chain. Population A grows at a certain rate and at another rate is eaten by the population B which has its own birth rate but is, in turn, eaten at a different rate by the population C. The dynamics of the model is given by a set of differential equations and via Monte Carlo simulations. Our system undergoes sudden cataclysms in the form of partial destruction of one of the populations. We show that there exist threshold values for the possible percentage of destroyed populations, above which the system returns to its previous state, thus showing a self-regulatory character.
Strategies of Reproduction and Longevity
Authors: de Menezes M. Argollo , Racco A. , Penna T. J. P.
Published: October 2003

In this work we try to verify whether the increased lifespan of trees and some lobsters, like Homarus, whose fertility increases with advancing age, can be explained by the mutation accumulation theory of biological ageing. Computer simulations of the Penna model seems to support this hypothesis, showing that it is a robust strategy of reproduction.
Using Cellular Automata to Learn About the Immune System
Authors: Zorzenon Dos Santos Rita Maria
Published: October 2003

Cellular automata are very simple systems that can exhibit complex dynamics on its time evolution. Over the last decade there have been many applications of cellular automata to modeling of biological systems. Those applications have been stimulated by the study of complex systems which has brought many insights into the cooperative and global behavior of the biological systems. Along with this discussion we present two different applications of deterministic and also of probabilistic cellular automata that are used to model the dynamics involved in cooperative and collective behavior of the immune system. In the first example, we use a deterministic cellular automata to model the time evolution of the immune repertoire, as a network, according to the Jerne's theory. Using this model we could reproduce some recent experimental results about immunization and aging of the immune system. In the second example, we use a probabilistic cellular automata model to study the evolution of HIV infection and the onset of AIDS. The results are in excellent agreement with experimental data obtained from infected patients. Besides the examples above, other interesting applications, such as models for cancer and recurrent epidemics, are being considered in the present framework.
Monte Carlo Comparative Study of Model Detergent and Lipid Aggregation on a Lattice
Authors: Shida Claudio S. , Henriques Vera B.
Published: October 2003

We have studied the aggregation of model amphiphilic molecules on a square lattice through Monte Carlo simulations via Metropolis. Amphiphilic molecules are modeled with a hydrophilic head represented by a small set of "water-loving" sites whereas the hydrophobic double-tail is represented by a second set of oil-loving sites. We have compared aggregation properties of single-stail (detergents) and double-tail (phospholipids) amphiphiles with equivalent hydrophobicity ratios. Equilibrium quantities such as average particle energy, specific heat, free amphiphile density show similar behavior for both model systems. The transition region associated with aggregation occurs at high temperatures for phospholipids as compared to detergents.
Studying the Number of Lineages Through Monte Carlo Simulations of Biological Ageing
Authors: de Oliveira S. Moss , de Medeiros G. A. , de Oliveira P. M. C. , Stauffer D.
Published: October 2003

We studied different versions of the Penna bit-string model for biological ageing and found that, after many generations, the number of lineages N (maternal family names) always decays to one as a power-law N∝t-z with an exponent z roughly equal to one. Measuring the mean correlation between the ancestor genome and those of the actual population we obtained the result that it goes to zero much earlier before the number of families goes to one, the population keeping thus its biological diversity. Considering maternal and paternal family names (doubled names) we also finished with only one pair of common ancestors. Computing the number of families of a given size as a function of the size (number of individuals the family has had during its whole existence) again a power-law decay is obtained.
A Crossover Between Two Growth Universality Classes
Authors: da Silva T. J.
Published: October 2003

We investigated two simple SOS growth models where the roughness of the profile is softened by surface relaxation or by refuse. In the first case, a particle can relax until it reaches a local minimum and this process is described by a linear equation. In the second case, a particle can be evaporated and this process is described by the KPZ equation. In this work, we have mixed both the models in the same growth process. Numerical simulations in (1+1)-dimensions show a process that for short times behaves as the surface relaxation model and for intermediate times, behaves as the refuse model. Numerical simulations indicate for this crossover a dynamical exponent z≈1.4, indicating that it is in the KPZ universality class.
Domain Analysis and Magnetic Relaxation in Thin Films
Authors: Rappoport Tatiana G. , de Menezes F. S. , Sampaio L. C. , Albuquerque M. P. , Mello F.
Published: October 2003

We have simulated the magnetic relaxation (M(t)) and the nucleation of magnetic domains in the presence of magnetic field in thin films with anisotropy perpendicular to the film plane. We have used Monte Carlo simulations based on the two-dimensional classical Ising model including the long-range dipole–dipole and Zeeman interactions. Domains nucleated during the magnetic relaxation exhibit very rough interfaces. We analyze the roughness and the M(t) as a function of the relative strength of dipole–dipole and Zeeman terms.
Invasion Percolation Quantities on Correlated Networks
Authors: Vidales A. M. , Miranda E. , Zgrablich G.
Published: October 2003

Invasion percolation is studied on correlated square networks described through a site-bond model which has proven to be useful for the characterization of real heterogeneous media. It is shown how the correlation degree affects the mean front velocity, the number of islands of trapped defender fluid (which are completely surrounded by invaded elements), their size distribution and total number of steps to reach the final state. The correlation degree seems to affect the fractal dimension of the percolating cluster. A characteristic correlation length is found to exist which maximizes the mean invasion velocity.
Immiscible Displacement at Pore Level for Correlated Porous Media
Authors: Vidales A. M. , Riccardo J. L. , Zgrablich G.
Published: October 2003

Immiscible displacement at pore level on a three-dimensional correlated porous network is simulated allowing flow of the wetting phase along crevices of the pore walls (possibility of snap-off in throats) and advance through the centers of the pore space with different pore and throat filling conditions, leading to a cooperative filling. When these two mechanisms compete, different patterns arise. We study the effect of the correlation strength on the onset of each pattern. We do not take buoyancy forces into account.
Brittle-Ductile Transition in Fiber-Reinforced Composites
Authors: Menezes-Sobrinho Ismael L. , Moreira José-Guilherme , Bernardes Américo T.
Published: October 2003

Fiber-reinforced composites are a class of material with increasing industrial applications. Computer simulations have been used in order to understand the microscopic mechanism which can explain their mechanical behavior and several models have been introduced in the last decade. In this paper we introduce a criterion to define the brittle-ductile transition region in unidirectional fiber-reinforced composites. In order to simulate a fiber bundle, a recently introduced stochastic model is used. The results obtained with our criterion are compared with those obtained by using a self-organized criticality (SOC) approach.
Molecular Dynamics Simulation of Zone Melting
Authors: Coura P. Z. , Mesquita O. N. , Costa B. V.
Published: October 2003

We use molecular dynamics technique for simulating the zone melting process. By tuning the parameters in the molecule–molecule potential we were able to reproduce segregation and fingering at the interface.
Critical Dynamics Behavior of the Wolff Algorithm in the Site-Bond-Correlated Ising Model
Authors: Campos P. R. A. , Onody R. N.
Published: October 2003

Here we apply the Wolff single-cluster algorithm to the site-bond-correlated Ising model and study its critical dynamical behavior. We have verified that the autocorrelation time diminishes in the presence of dilution and correlation, showing that the Wolff algorithm performs even better in such situations. The critical dynamical exponents are also estimated.
Lattice Simulation of Nuclear Multifragmentation
Authors: Sá Martins J. S. , de Oliveira P. M. C.
Published: October 2003

Motivated by the decade-long debate over the issue of criticality supposedly observed in nuclear multifragmentation, we propose a dynamic lattice model to simulate the phenomenon. Its Ising Hamiltonian mimics a short range attractive interaction which competes with a thermal-like dissipative process. The results here presented, generated through an event-by-event analysis, are in agreement with both experiment and those produced by a percolative (nondynamic) model.
Sliding Blocks Revisited: A Simulational Study
Authors: de Lima A. R. , Moukarzel C. , Penna T. J. P.
Published: October 2003

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.
Unphysical Frozen States in Monte Carlo Simulation of 2D Ising Model
Authors: Papa Andres R. R.
Published: October 2003

We show that for two-dimensional square Ising systems unphysical frozen states are obtained by just changing the instant of application of periodic boundary conditions during Monte Carlo simulations. The strange behavior is observed up to sample sizes currently used in literature. The anomalous results appear to be associated to the simultaneous use of type writer updating algorithms; they disappear when random access routines are implemented.
A Fast Algorithm for Backbones
Authors: Moukarzel C.
Published: October 2003

A matching algorithm for the identification of backbones in percolation problems is introduced. Using this procedure, percolation backbones are studied in two- to five-dimensional systems containing 1.7×107 sites, two orders of magnitude larger than was previously possible using burning algorithms.
A Simple Cellular Automaton that Solves the Density and Ordering Problems
Authors: Sipper Moshe , Capcarrere Mathieu S. , Ronald Edmund
Published: October 2003

We show that there exists a simple solution to the density problem in cellular automata, under fixed boundary conditions, in contrast to previously used periodic ones.
Distributed Monte Carlo Integration Using Corba and Java
Authors: Solms Fritz , Steeb Willi-Hans
Published: October 2003

The object-oriented middleware standard, CORBA, is a very useful platform for distributed computing, and in particular for sharing a workload among a collection of possibly polymorphic computers. CORBA has, however, received relatively little attention from the scientific computing community. In this article we demonstrate how CORBA and Java can be used to implement a distributed multi-dimensional Monte Carlo integration algorithm which runs on the internet.