517 research outputs found
Correlation effects in a simple model of small-world network
We analyze the effect of correlations in a simple model of small world
network by obtaining exact analytical expressions for the distribution of
shortest paths in the network. We enter correlations into a simple model with a
distinguished site, by taking the random connections to this site from an Ising
distribution. Our method shows how the transfer matrix technique can be used in
the new context of small world networks.Comment: 10 pages, 3 figure
Opinion and community formation in coevolving networks
In human societies opinion formation is mediated by social interactions,
consequently taking place on a network of relationships and at the same time
influencing the structure of the network and its evolution. To investigate this
coevolution of opinions and social interaction structure we develop a dynamic
agent-based network model, by taking into account short range interactions like
discussions between individuals, long range interactions like a sense for
overall mood modulated by the attitudes of individuals, and external field
corresponding to outside influence. Moreover, individual biases can be
naturally taken into account. In addition the model includes the opinion
dependent link-rewiring scheme to describe network topology coevolution with a
slower time scale than that of the opinion formation. With this model
comprehensive numerical simulations and mean field calculations have been
carried out and they show the importance of the separation between fast and
slow time scales resulting in the network to organize as well-connected small
communities of agents with the same opinion.Comment: 10 pages, 5 figures. New inset for Fig. 1 and references added.
Submitted to Physical Review
State of the science and the way forward for the ecotoxicological assessment of contaminated land.
Durante as últimas duas décadas, ecotoxicologistas de solo têm feito progressos ao utilizar conceitos básicos e avanços da zoologia e ecologia do solo. Os métodos existentes têm sido aplicados, e têm-se desenvolvido novas ferramentas para avaliar de que modo a contaminação química pode afetar o ecossistema terrestre, inclusive pela degradação ou destruição da qualidade do solo e dos habitats ou pela redução da biodiversidade edáfica. Os ecotoxicologistas de solo utilizam um conjunto de protocolos padronizados, originalmente desenvolvidos como testes de laboratório com compostos químicos simples como os pesticidas e, posteriormente, adaptados em termos de abordagens e métodos, para a avaliação de áreas contaminadas. No entanto, a relevância ecológica de algumas abordagens permanece questionável. Neste artigo, os autores discutem os recentes desafios para uma avaliação ecotoxicológica coerente do ecossistema solo em áreas contaminadas e apresentam recomendações de como integrar os efeitos das propriedades físicoquímicas do solo, as variações na diversidade de invertebrados do solo e, as interações entre organismos dos vários níveis tróficos. São analisadas novas abordagens e métodos de avaliação, usando-se exemplos de três continentes (particularmente o trabalho desenvolvido no Brasil), e são dadas recomendações de como aumentar a relevância ecológica na avaliação ecotoxicológica de áreas contaminadas
Experimental evidence of stochastic resonance without tuning due to non Gaussian noises
In order to test theoretical predictions, we have studied the phenomenon of
stochastic resonance in an electronic experimental system driven by white non
Gaussian noise. In agreement with the theoretical predictions our main findings
are: an enhancement of the sensibility of the system together with a remarkable
widening of the response (robustness). This implies that even a single resonant
unit can reach a marked reduction in the need of noise tuning.Comment: 4 pages, 3 figure
Enhancement of Stochastic Resonance in distributed systems due to a selective coupling
Recent massive numerical simulations have shown that the response of a
"stochastic resonator" is enhanced as a consequence of spatial coupling.
Similar results have been analytically obtained in a reaction-diffusion model,
using "nonequilibrium potential" techniques. We now consider a field-dependent
diffusivity and show that the "selectivity" of the coupling is more efficient
for achieving stochastic-resonance enhancement than its overall value in the
constant-diffusivity case.Comment: 10 pgs (RevTex), 4 figures, submitted to Phys.Rev.Let
Exact expression for the diffusion propagator in a family of time-dependent anharmonic potentials
We have obtained the exact expression of the diffusion propagator in the
time-dependent anharmonic potential . The
underlying Euclidean metric of the problem allows us to obtain analytical
solutions for a whole family of the elastic parameter a(t), exploiting the
relation between the path integral representation of the short time propagator
and the modified Bessel functions. We have also analyzed the conditions for the
appearance of a non-zero flow of particles through the infinite barrier located
at the origin (b<0).Comment: RevTex, 19 pgs. Accepted in Physical Review
Exploring the measurement of markedness and its relationship with other linguistic variables
Antonym pair members can be differentiated by each word's markedness-that distinction attributable to the presence or absence of features at morphological or semantic levels. Morphologically marked words incorporate their unmarked counterpart with additional morphs (e.g., "unlucky" vs. "lucky"); properties used to determine semantically marked words (e.g., "short" vs. "long") are less clearly defined. Despite extensive theoretical scrutiny, the lexical properties of markedness have received scant empirical study. The current paper employs an antonym sequencing approach to measure markedness: establishing markedness probabilities for individual words and evaluating their relationship with other lexical properties (e.g., length, frequency, valence). Regression analyses reveal that markedness probability is, as predicted, related to affixation and also strongly related to valence. Our results support the suggestion that antonym sequence is reflected in discourse, and further analysis demonstrates that markedness probabilities, derived from the antonym sequencing task, reflect the ordering of antonyms within natural language. In line with the Pollyanna Hypothesis, we argue that markedness is closely related to valence; language users demonstrate a tendency to present words evaluated positively ahead of those evaluated negatively if given the choice. Future research should consider the relationship of markedness and valence, and the influence of contextual information in determining which member of an antonym pair is marked or unmarked within discourse
Topology and Computational Performance of Attractor Neural Networks
To explore the relation between network structure and function, we studied
the computational performance of Hopfield-type attractor neural nets with
regular lattice, random, small-world and scale-free topologies. The random net
is the most efficient for storage and retrieval of patterns by the entire
network. However, in the scale-free case retrieval errors are not distributed
uniformly: the portion of a pattern encoded by the subset of highly connected
nodes is more robust and efficiently recognized than the rest of the pattern.
The scale-free network thus achieves a very strong partial recognition.
Implications for brain function and social dynamics are suggestive.Comment: 2 figures included. Submitted to Phys. Rev. Letter
Optimal interdependence between networks for the evolution of cooperation
Recent research has identified interactions between networks as crucial for the outcome of evolutionary
games taking place on them. While the consensus is that interdependence does promote cooperation by
means of organizational complexity and enhanced reciprocity that is out of reach on isolated networks, we
here address the question just how much interdependence there should be. Intuitively, one might assume
the more the better. However, we show that in fact only an intermediate density of sufficiently strong
interactions between networks warrants an optimal resolution of social dilemmas. This is due to an intricate
interplay between the heterogeneity that causes an asymmetric strategy flow because of the additional links
between the networks, and the independent formation of cooperative patterns on each individual network.
Presented results are robust to variations of the strategy updating rule, the topology of interdependent
networks, and the governing social dilemma, thus suggesting a high degree of universality
Statistical mechanics of complex networks
Complex networks describe a wide range of systems in nature and society, much
quoted examples including the cell, a network of chemicals linked by chemical
reactions, or the Internet, a network of routers and computers connected by
physical links. While traditionally these systems were modeled as random
graphs, it is increasingly recognized that the topology and evolution of real
networks is governed by robust organizing principles. Here we review the recent
advances in the field of complex networks, focusing on the statistical
mechanics of network topology and dynamics. After reviewing the empirical data
that motivated the recent interest in networks, we discuss the main models and
analytical tools, covering random graphs, small-world and scale-free networks,
as well as the interplay between topology and the network's robustness against
failures and attacks.Comment: 54 pages, submitted to Reviews of Modern Physic
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