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At least 199 records · Page 11Linked to original sources
arXiv1994

Learning and Complexity in Genetic Auto-Adaptive Systems

We describe and investigate the learning capablities displayed by a population of self-replicating segments of computer-code subject to random mutation: the tierra environment. We find that learning is achieved through phase transitions that adapt the population to whichever environment it encounters, with a learning rate characterized by the environmental variables. Our results suggest that most effective learning is achieved close to the edge of chaos.

adap-org↗
arXiv1994

Transport and current reversal in stochastically driven ratchets

We present analytic results for the current in a system moving in an arbitrary periodic potential and driven by weak Gaussian noise with an arbitrary power spectrum which are valid to order (t_c/t_r)^2, where t_c is the largest characteristic time of the noise, and t_r is the characteristic intrawell relaxation time. The dependence of the current on the shape of the potential, and on the shape of the power spectrum of the noise is illustrated. It is demonstrated that the direction of the current is opposite when the power spectrum of the noise has minimum or maximum at zero frequency. A simple physical mechanism for this behavior is suggested. The behavior of the system in the limit of slow noise (t_c >>t_r) is also discussed. .

cond-mat↗
arXiv1996

Resonance Broadening Induced Nonlinear Saturation of Kinetic Alfven Turbulence in the Interplanetary Plasma

The saturation of ion cyclotron Alfven turbulence excited by beam particles is investigated using resonance broadening theory. The stochastic scattering which decorrelates particles, includes both random acceleration by electric fields and a turbulent magnetic mirroring effect. Turbulent mirroring is shown to yield non-Gaussian corrections to the orbits even if the random electric and magnetic fields are Gaussian. The predicted steady-state turbulence level exhibits a peaked anglular distribution, with a maximum near Theta ~ 60 degrees.

physics.plasm-ph↗
arXiv1994

On The Avalanche-finiteness Of Abelian Sandpiles

We prove a necessary and sufficient condition for an Abelian Sandpile Model (ASM) to be avalanche-finite, namely: all unstable states of the system can be brought back to stability in finite number of topplings. The method is also computationally feasible since it involves no greater than $\mbox{O} \left( N^3 \right)$ arithmetic computations where $N$ is the total number of sites of the system. Key words: Abelian sandpile model; avalanche-finiteness; self-organized criticality

adap-org↗
arXiv1995

Statistics Of The Burst Model At Super-critical Phase

We investigate the statistics of a model of type-I X-ray burst [Phys. Rev. E, {\bf 51}, 3045 (1995)] in its super-critical phase. The time evolution of the burnable clusters, places where fire can pass through, is studied using simple statistical arguments. We offer a simple picture for the time evolution of the percentage of space covered by burnable clusters. A relation between the time-average and the peak percentage of space covered by burnable clusters is also derived.

cond-mat↗
arXiv1995

Scaling and universality in binary fragmenting with inhibition

We investigate a new model of binary fragmentation with inhibition, driven by the white noise. In a broad range of fragmentation probabilities, the power-law spatiotemporal correlations are found to arise due to self-organized criticality (SOC). We find in the SOC phase a non-trivial power spectrum of the temporal sequence of the fragmentation events. The $1/f$~ behaviour is recovered in the irreversible, near-equilibrium part of this phase.

adap-org↗
arXiv1996

Self-Organized Critical Directed Percolation

We introduce and study a dynamic transport model exhibiting Self-Organized Criticality. The novel concepts of our model are the probabilistic propagation of activity and unbiased random repartition of energy among the active site and its nearest neighbors. For space dimensionality $d\geq 2$ we argue that the model is related to $d+1$ dimensional directed percolation, with time interpreted as the preferred direction.

adap-org↗
arXiv1995

Critical Properties of the One-Dimensional Forest-Fire Model

The one-dimensional forest-fire model including lightnings is studied numerically and analytically. For the tree correlation function, a new correlation length with critical exponent ν~ 5/6 is found by simulations. A Hamiltonian formulation is introduced which enables one to study the stationary state close to the critical point using quantum-mechanical perturbation theory. With this formulation also the structure of the low-lying relaxation spectrum and the critical behaviour of the smallest complex gap are investigated numerically. Finally, it is shown that critical correlation functions can be obtained from a simplified model involving only the total number of trees although such simplified models are unable to reproduce the correct off-critical behaviour.

cond-mat↗
arXiv1995

Two Lane Traffic Simulations using Cellular Automata

We examine a simple two lane cellular automaton based upon the single lane CA introduced by Nagel and Schreckenberg. We point out important parameters defining the shape of the fundamental diagram. Moreover we investigate the importance of stochastic elements with respect to real life traffic.

cond-mat↗
arXiv1996

Self-organized criticality and the lattice topology

We examine exhaustively the behavior of avalanches in critical height sandpile models based in two- and three-dimensional lattices of various topologies. We get that for two-dimensional lattices the spatial and temporal distributions characterizing bulk avalanches do not depend on the lattice topology. For the three-dimensional case, we detect a small dependence of the topology for the temporal distribution, while the spatial ones are independent. The two-dimensional lattices studied are: the plane ($R^2$), the cylinder ($S^1\times R$), and the Möbius-strip ($M$); and the three-dimensional are: $R^3$, $S^1\times R^2$, $S^1\times S^1\times R$, $M\times R$, $S^2\times R$, $K\times R$, and $RP\times R$, where $K$ and $RP$ are respectively the Klein bottle and the real projective plane.

adap-org↗
arXiv1995

Numerical investigation of surface level instability due to tube in vibrating bed of powder

Surface level instability when tube is injected into vibrating bed of powder, which was originally found in experiments, is investigated numerically. We find that thicker (thiner) tube makes surface level inside tube higher (lower) than surface level outside tube. With fixed acceleration amplitude of vibration, surface level inside tube becomes higher as amplitude of vibration increases, which can be explained by considering the dependence upon strength of convective flow.

adap-org↗
arXiv1996

Fragmentation versus Stability in Bimodal Coalitions

Competing bimodal coalitions among a group of actors are discussed. First, a model from political sciences is revisited. Most of the model statements are found not to be contained in the model. Second, a new coalition model is built. It accounts for local versus global alignment with respect to the joining of a coalition. The existence of two competing world coaltions is found to yield one unique stable distribution of actors. On the opposite a unique world leadership allows the emergence of unstable relationships. In parallel to regular actors which have a clear coalition choice, ``neutral" ``frustrated" and ``risky" actors are produced. The cold war organisation after world war II is shown to be rather stable. The emergence of a fragmentation process from eastern group disappearance is explained as well as continuing western group stability. Some hints are obtained about possible policies to stabilize world nation relationships. European construction is analyzed with respect to european stability. Chinese stability is also discussed.

cond-mat↗
arXiv2000

Nature's Way of Optimizing

We propose a general-purpose method for finding high-quality solutions to hard optimization problems, inspired by self-organizing processes often found in nature. The method, called Extremal Optimization, successively eliminates extremely undesirable components of sub-optimal solutions. Drawing upon models used to simulate far-from-equilibrium dynamics, it complements approximation methods inspired by equilibrium statistical physics, such as Simulated Annealing. With only one adjustable parameter, its performance proves competitive with, and often superior to, more elaborate stochastic optimization procedures. We demonstrate it here on two classic hard optimization problems: graph partitioning and the traveling salesman problem.

cond-mat.stat-mech↗
arXiv1997

The Study of Correlation Structures of DNA Sequences: A Critical Review

The study of correlation structure in the primary sequences of DNA is reviewed. The issues reviewed include: symmetries among 16 base-base correlation functions, accurate estimation of correlation measures, the relationship between $1/f$ and Lorentzian spectra, heterogeneity in DNA sequences, different modeling strategies of the correlation structure of DNA sequences, the difference of correlation structure between coding and non-coding regions (besides the period-3 pattern), and source of broad distribution of domain sizes. Although some of the results remain controversial, a body of work on this topic constitutes a good starting point for future studies.

adap-org↗
arXiv1997

Quasi-Linear Cellular Automata

Simulating a cellular automaton (CA) for t time-steps into the future requires t^2 serial computation steps or t parallel ones. However, certain CAs based on an Abelian group, such as addition mod 2, are termed ``linear'' because they obey a principle of superposition. This allows them to be predicted efficiently, in serial time O(t) or O(log t) in parallel. In this paper, we generalize this by looking at CAs with a variety of algebraic structures, including quasigroups, non-Abelian groups, Steiner systems, and others. We show that in many cases, an efficient algorithm exists even though these CAs are not linear in the previous sense; we term them ``quasilinear.'' We find examples which can be predicted in serial time proportional to t, t log t, t log^2 t, and t^a for a < 2, and parallel time log t, log t log log t and log^2 t. We also discuss what algebraic properties are required or implied by the existence of scaling relations and principles of superposition, and exhibit several novel ``vector-valued'' CAs.

adap-org↗
arXiv1997

Predicting Non-linear Cellular Automata Quickly by Decomposing Them into Linear Ones

We show that a wide variety of non-linear cellular automata (CAs) can be decomposed into a quasidirect product of linear ones. These CAs can be predicted by parallel circuits of depth O(log^2 t) using gates with binary inputs, or O(log t) depth if ``sum mod p'' gates with an unbounded number of inputs are allowed. Thus these CAs can be predicted by (idealized) parallel computers much faster than by explicit simulation, even though they are non-linear. This class includes any CA whose rule, when written as an algebra, is a solvable group. We also show that CAs based on nilpotent groups can be predicted in depth O(log t) or O(1) by circuits with binary or ``sum mod p'' gates respectively. We use these techniques to give an efficient algorithm for a CA rule which, like elementary CA rule 18, has diffusing defects that annihilate in pairs. This can be used to predict the motion of defects in rule 18 in O(log^2 t) parallel time.

patt-sol↗
arXiv1998

Aftershocks in Coherent-Noise Models

The decay pattern of aftershocks in the so-called 'coherent-noise' models [M. E. J. Newman and K. Sneppen, Phys. Rev. E54, 6226 (1996)] is studied in detail. Analytical and numerical results show that the probability to find a large event at time $t$ after an initial major event decreases as $t^{-τ}$ for small $t$, with the exponent $τ$ ranging from 0 to values well above 1. This is in contrast to Sneppen und Newman, who stated that the exponent is about 1, independent of the microscopic details of the simulation. Numerical simulations of an extended model [C. Wilke, T. Martinetz, Phys. Rev. E56, 7128 (1997)] show that the power-law is only a generic feature of the original dynamics and does not necessarily appear in a more general context. Finally, the implications of the results to the modeling of earthquakes are discussed.

physics.data-an↗