61 research outputs found
Forecasting time series by means of evolutionary algorithms
Proceeding of: 8th International Conference in Parallel Problem Solving from Nature - PPSN VIII , Birmingham, UK, September 18-22, 2004.The time series forecast is a very complex problem, consisting in predicting the behaviour of a data series with only the information of the previous sequence. There is many physical and artificial phenomenon that can be described by time series. The prediction of such phenomenon could be very complex. For instance, in the case of tide forecast, unusually high tides, or sea surges, result from a combination of chaotic climatic elements in conjunction with the more normal, periodic, tidal systems associated with a particular area. Too much variables influence the behaviour of the water level. Our problem is not only to find prediction rules, we also need to discard the noise and select the representative data. Our objective is to generate a set of prediction rules. There are many methods tying to achieve good predictions. In most of the cases this methods look for general rules that are able to predict the whole series. The problem is that usually the time series has local behaviours that dont allow a good level of prediction when using general rules. In this work we present a method for finding local rules able to predict only some zones of the series but achieving better level prediction. This method is based on the evolution of set of rules genetically codified, and following the Michigan approach. For evaluating the proposal, two different domains have been used: an artificial domain widely use in the bibliography (Mackey-Glass series) and a time series corresponding to a natural phenomenon, the water level in Venice Lagoon.Investigation supported by the Spanish Ministry of Science and Technology through the TRACER project under contract TIC2002-04498-C05-
Electromagnetic superconductivity of vacuum induced by strong magnetic field: numerical evidence in lattice gauge theory
Using numerical simulations of quenched SU(2) gauge theory we demonstrate
that an external magnetic field leads to spontaneous generation of quark
condensates with quantum numbers of electrically charged rho mesons if the
strength of the magnetic field exceeds the critical value eBc = 0.927(77) GeV^2
or Bc =(1.56 \pm 0.13) 10^{16} Tesla. The condensation of the charged rho
mesons in strong magnetic field is a key feature of the magnetic-field-induced
electromagnetic superconductivity of the vacuum.Comment: 14 pages, 5 figures, 2 tables, elsarticle style; continuum limit is
analyzed, best fit parameters are presented in Table 2, published versio
Density classification on infinite lattices and trees
Consider an infinite graph with nodes initially labeled by independent
Bernoulli random variables of parameter p. We address the density
classification problem, that is, we want to design a (probabilistic or
deterministic) cellular automaton or a finite-range interacting particle system
that evolves on this graph and decides whether p is smaller or larger than 1/2.
Precisely, the trajectories should converge to the uniform configuration with
only 0's if p1/2. We present solutions to that problem
on the d-dimensional lattice, for any d>1, and on the regular infinite trees.
For Z, we propose some candidates that we back up with numerical simulations
Experiments on the Forced Wake of an Airfoil
Presented here is an experimental effort which attempts to
understand the nature of the wake of an airfoil in a controlled
environment. The frequency of oscillation in the wake (the vortex
shedding frequency) is controlled through the introduction
of an external perturbation. Strip heaters are used to introduce
waves into the top and bottom boundary layers of a thin symmetric
airfoil which are amplified and introduced to the wake.
The linear and nonlinear interactions of these waves in the wake
are studied in detail. Three modes of interaction have been
observed through flow visualization and velocity measurements:
frequency locking in which the vortex shedding frequency is the
same as the forcing frequency, quasiperiodic vortex interaction
in which periodic clusters of vortices are observed in the wake,
and chaotic vortex interaction in which the vortices in the wake
have a three dimensional random structure
Growth and Decay in Life-Like Cellular Automata
We propose a four-way classification of two-dimensional semi-totalistic
cellular automata that is different than Wolfram's, based on two questions with
yes-or-no answers: do there exist patterns that eventually escape any finite
bounding box placed around them? And do there exist patterns that die out
completely? If both of these conditions are true, then a cellular automaton
rule is likely to support spaceships, small patterns that move and that form
the building blocks of many of the more complex patterns that are known for
Life. If one or both of these conditions is not true, then there may still be
phenomena of interest supported by the given cellular automaton rule, but we
will have to look harder for them. Although our classification is very crude,
we argue that it is more objective than Wolfram's (due to the greater ease of
determining a rigorous answer to these questions), more predictive (as we can
classify large groups of rules without observing them individually), and more
accurate in focusing attention on rules likely to support patterns with complex
behavior. We support these assertions by surveying a number of known cellular
automaton rules.Comment: 30 pages, 23 figure
Using Topological Statistics to Detect Determinism in Time Series
Statistical differentiability of the measure along the reconstructed
trajectory is a good candidate to quantify determinism in time series. The
procedure is based upon a formula that explicitly shows the sensitivity of the
measure to stochasticity. Numerical results for partially surrogated time
series and series derived from several stochastic models, illustrate the
usefulness of the method proposed here. The method is shown to work also for
high--dimensional systems and experimental time seriesComment: 23 RevTeX pages, 14 eps figures. To appear in Physical Review
Temperature Dependence of the Dynamics of Portevin-Le Chatelier Effect in Al-2.5%Mg alloy
Tensile tests were carried out by deforming polycrystalline samples of
Al-2.5%Mg alloy at four different temperatures in an intermediate strain rate
regime of 2x10-4s-1 to 2x10-3s-1. The Portevin-Le Chatelier (PLC) effect was
observed throughout the strain rate and temperature region. The mean cumulative
stress drop magnitude and the mean reloading time exhibit an increasing trend
with temperature which is attributed to the enhanced solute diffusion at higher
temperature. The observed stress-time series data were analyzed using the
nonlinear dynamical methods. From the analyses, we could establish the presence
of deterministic chaos in the PLC effect throughout the temperature regime. The
dynamics goes to higher dimension at a sufficiently high temperature of 425K
but the complexity of the dynamics is not affected by the temperature.Comment: 18 pages, 8 figures; accepted in Met. Mater. Trans.
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