78 research outputs found
Damage spreading and coupling in Markov chains
In this paper, we relate the coupling of Markov chains, at the basis of
perfect sampling methods, with damage spreading, which captures the chaotic
nature of stochastic dynamics. For two-dimensional spin glasses and hard
spheres we point out that the obstacle to the application of perfect-sampling
schemes is posed by damage spreading rather than by the survey problem of the
entire configuration space. We find dynamical damage-spreading transitions
deeply inside the paramagnetic and liquid phases, and show that critical values
of the transition temperatures and densities depend on the coupling scheme. We
discuss our findings in the light of a classic proof that for arbitrary Monte
Carlo algorithms damage spreading can be avoided through non-Markovian coupling
schemes.Comment: 6 pages, 8 figure
Transmission of packets on a hierarchical network: Statistics and explosive percolation
We analyze an idealized model for the transmission or flow of particles, or
discrete packets of information, in a weight bearing branching hierarchical 2-D
networks, and its variants. The capacities add hierarchically down the
clusters. Each node can accommodate a limited number of packets, depending on
its capacity and the packets hop from node to node, following the links between
the nodes. The statistical properties of this system are given by the Maxwell -
Boltzmann distribution. We obtain analytical expressions for the mean
occupation numbers as functions of capacity, for different network topologies.
The analytical results are shown to be in agreement with the numerical
simulations. The traffic flow in these models can be represented by the site
percolation problem. It is seen that the percolation transitions in the 2-D
model and in its variant lattices are continuous transitions, whereas the
transition is found to be explosive (discontinuous) for the V- lattice, the
critical case of the 2-D lattice. We discuss the implications of our analysis.Comment: 24 pages, 41 figure
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
Maximum Likelihood Estimator for Hidden Markov Models in continuous time
The paper studies large sample asymptotic properties of the Maximum
Likelihood Estimator (MLE) for the parameter of a continuous time Markov chain,
observed in white noise. Using the method of weak convergence of likelihoods
due to I.Ibragimov and R.Khasminskii, consistency, asymptotic normality and
convergence of moments are established for MLE under certain strong ergodicity
conditions of the chain.Comment: Warning: due to a flaw in the publishing process, some of the
references in the published version of the article are confuse
Growth Kinetics in Systems with Local Symmetry
The phase transition kinetics of Ising gauge models are investigated. Despite
the absence of a local order parameter, relevant topological excitations that
control the ordering kinetics can be identified. Dynamical scaling holds in the
approach to equilibrium, and the growth of typical length scale is
characteristic of a new universality class with . We suggest that the asymptotic kinetics of the 2D Ising gauge
model is dual to that of the 2D annihilating random walks, a process also known
as the diffusion-reaction .Comment: 10 pages in Tex, 2 Postscript figures appended, NSF-ITP-93-4
Automatic Filters for the Detection of Coherent Structure in Spatiotemporal Systems
Most current methods for identifying coherent structures in
spatially-extended systems rely on prior information about the form which those
structures take. Here we present two new approaches to automatically filter the
changing configurations of spatial dynamical systems and extract coherent
structures. One, local sensitivity filtering, is a modification of the local
Lyapunov exponent approach suitable to cellular automata and other discrete
spatial systems. The other, local statistical complexity filtering, calculates
the amount of information needed for optimal prediction of the system's
behavior in the vicinity of a given point. By examining the changing
spatiotemporal distributions of these quantities, we can find the coherent
structures in a variety of pattern-forming cellular automata, without needing
to guess or postulate the form of that structure. We apply both filters to
elementary and cyclical cellular automata (ECA and CCA) and find that they
readily identify particles, domains and other more complicated structures. We
compare the results from ECA with earlier ones based upon the theory of formal
languages, and the results from CCA with a more traditional approach based on
an order parameter and free energy. While sensitivity and statistical
complexity are equally adept at uncovering structure, they are based on
different system properties (dynamical and probabilistic, respectively), and
provide complementary information.Comment: 16 pages, 21 figures. Figures considerably compressed to fit arxiv
requirements; write first author for higher-resolution version
Localization dynamics in a binary two-dimensional cellular automaton: the Diffusion Rule
We study a two-dimensional cellular automaton (CA), called Diffusion Rule
(DR), which exhibits diffusion-like dynamics of propagating patterns. In
computational experiments we discover a wide range of mobile and stationary
localizations (gliders, oscillators, glider guns, puffer trains, etc), analyze
spatio-temporal dynamics of collisions between localizations, and discuss
possible applications in unconventional computing.Comment: Accepted to Journal of Cellular Automat
The time to extinction for an SIS-household-epidemic model
We analyse a stochastic SIS epidemic amongst a finite population partitioned
into households. Since the population is finite, the epidemic will eventually
go extinct, i.e., have no more infectives in the population. We study the
effects of population size and within household transmission upon the time to
extinction. This is done through two approximations. The first approximation is
suitable for all levels of within household transmission and is based upon an
Ornstein-Uhlenbeck process approximation for the diseases fluctuations about an
endemic level relying on a large population. The second approximation is
suitable for high levels of within household transmission and approximates the
number of infectious households by a simple homogeneously mixing SIS model with
the households replaced by individuals. The analysis, supported by a simulation
study, shows that the mean time to extinction is minimized by moderate levels
of within household transmission
Randomly Evolving Idiotypic Networks: Structural Properties and Architecture
We consider a minimalistic dynamic model of the idiotypic network of
B-lymphocytes. A network node represents a population of B-lymphocytes of the
same specificity (idiotype), which is encoded by a bitstring. The links of the
network connect nodes with complementary and nearly complementary bitstrings,
allowing for a few mismatches. A node is occupied if a lymphocyte clone of the
corresponding idiotype exists, otherwise it is empty. There is a continuous
influx of new B-lymphocytes of random idiotype from the bone marrow.
B-lymphocytes are stimulated by cross-linking their receptors with
complementary structures. If there are too many complementary structures,
steric hindrance prevents cross-linking. Stimulated cells proliferate and
secrete antibodies of the same idiotype as their receptors, unstimulated
lymphocytes die.
Depending on few parameters, the autonomous system evolves randomly towards
patterns of highly organized architecture, where the nodes can be classified
into groups according to their statistical properties. We observe and describe
analytically the building principles of these patterns, which allow to
calculate number and size of the node groups and the number of links between
them. The architecture of all patterns observed so far in simulations can be
explained this way. A tool for real-time pattern identification is proposed.Comment: 19 pages, 15 figures, 4 table
- …