270 research outputs found
Stochastic Effects in Physical Systems
A tutorial review is given of some developments and applications of
stochastic processes from the point of view of the practicioner physicist. The
index is the following: 1.- Introduction 2.- Stochastic Processes 3.- Transient
Stochastic Dynamics 4.- Noise in Dynamical Systems 5.- Noise Effects in
Spatially Extended Systems 6.- Fluctuations, Phase Transitions and
Noise-Induced Transitions.Comment: 93 pages, 36 figures, LaTeX. To appear in Instabilities and
Nonequilibrium Structures VI, E. Tirapegui and W. Zeller,eds. Kluwer Academi
Zealots in the mean-field noisy voter model
The influence of zealots on the noisy voter model is studied theoretically
and numerically at the mean-field level. The noisy voter model is a
modification of the voter model that includes a second mechanism for
transitions between states: apart from the original herding processes, voters
may change their states because of an intrinsic, noisy in origin source. By
increasing the importance of the noise with respect to the herding, the system
exhibits a finite-size phase transition from a quasi-consensus state, where
most of the voters share the same opinion, to a one with coexistence. Upon
introducing some zealots, or voters with fixed opinion, the latter scenario may
change significantly. We unveil the new situations by carrying out a systematic
numerical and analytical study of a fully connected network for voters, but
allowing different voters to be directly influenced by different zealots. We
show that this general system is equivalent to a system of voters without
zealots, but with heterogeneous values of their parameters characterizing
herding and noisy dynamics. We find excellent agreement between our analytical
and numerical results. Noise and herding/zealotry acting together in the voter
model yields not a trivial mixture of the scenarios with the two mechanisms
acting alone: it represents a situation where the global-local (noise-herding)
competitions is coupled to a symmetry breaking (zealots). In general, the
zealotry enhances the effective noise of the system, which may destroy the
original quasi--consensus state, and can introduce a bias towards the opinion
of the majority of zealots, hence breaking the symmetry of the system and
giving rise to new phases ...Comment: 13 pages, 15 figure
Resistance to learning and the evolution of cooperation
In many evolutionary algorithms, crossover is the main operator used in generating new individuals from old ones. However, the usual mechanism for generating offsprings in spatially structured evolutionary games has to date been clonation. Here we study the effect of incorporating crossover on these models. Our framework is the spatial Continuous Prisoner's Dilemma. For this evolutionary game, it has been reported that occasional errors (mutations) in the clonal process can explain the emergence of cooperation from a non-cooperative initial state. First, we show that this only occurs for particular regimes of low costs of cooperation. Then, we display how crossover gets greater the range of scenarios where cooperative mutants can invade selfish populations. In a social context, where crossover involves a general rule of gradual learning, our results show that the less that is learnt in a single step, the larger the degree of global cooperation finally attained. In general, the effect of step-by-step learning can be more efficient for the evolution of cooperation than a full blast one.Evolutionary games, Continuous prisoner's dilemma, Spatially structured, Crossover, Learning
First-passage distributions for the one-dimensional Fokker-Planck equation
We present an analytical framework to study the first-passage (FP) and
first-return (FR) distributions for the broad family of models described by the
one-dimensional Fokker-Planck equation in finite domains, identifying general
properties of these distributions for different classes of models. When in the
Fokker-Planck equation the diffusion coefficient is positive (nonzero) and the
drift term is bounded, as in the case of a Brownian walker, both distributions
may exhibit a power-law decay with exponent -3/2 for intermediate times. We
discuss how the influence of an absorbing state changes this exponent. The
absorbing state is characterized by a vanishing diffusion coefficient and/or a
diverging drift term. Remarkably, the exponent of the Brownian walker class of
models is still found, as long as the departure and arrival regions are far
enough from the absorbing state, but the range of times where the power law is
observed narrows. Close enough to the absorbing point, though, a new exponent
may appear. The particular value of the exponent depends on the behavior of the
diffusion and the drift terms of the Fokker-Planck equation. We focus on the
case of a diffusion term vanishing linearly at the absorbing point. In this
case, the FP and FR distributions are similar to those of the voter model,
characterized by a power law with exponent -2. As an illustration of the
general theory, we compare it with exact analytical solutions and extensive
numerical simulations of a two-parameter voter-like family models. We study the
behavior of the FP and FR distributions by tuning the importance of the
absorbing points throughout changes of the parameters. Finally, the possibility
of inferring relevant information about the steady-sate probability
distribution of a model from the FP and FR distributions is addressed.Comment: 17 pages, 8 figure
Life After Amputation: A Case Study
In the Philippines, the National Council on Disability made a survey revealing a prevalence of43.367 disabled who lost one or both legs and/or feet. This study explored the challengesencountered and coping mechanisms an amputee manifests, and the assistance that they getfrom their expected support system. This case study was anchored on Dorothea Orem’s SelfCare Theory of Nursing. Four purposively sampled amputee informants were interviewed andobserved. Primary and secondary data were gathered and triangulation with the relatives wasdone to ensure the validity and depth of the results. Data were encoded and analyzed usingthematic analysis. Results revealed that the informants experienced physical, psychological,emotional, socio-economic, and spiritual challenges. Physical challenges involved mobilityproblem and lack of gait balance, physical deformity, adjustment to the new body, alteredphysical appearance, a sedentary behavior, and phantom pain. Psychological challengesinvolved suicidal tendency, loss of libido, self-pity, and depression. Emotional challengesincluded fear and hopelessness. Socio-economic challenges involved the development of antisocial behavior, dissociative behavior, fear of losing a job, problem with money, and fear ofrejection. Spiritual challenges involved loss of faith. Coping mechanism included support fromfamily and friends, mastery of gait and balance, proper practice in using assistive devices,hastened adjustment to the new body, wearing of prosthesis, medication and mobility, and trustin God. Although they get strong support from the family and friends, there is very limitedassistance from the government and the community. There is a need to increase familyawareness in anticipating the needs of the amputees. Likewise, full support must be given tothem. Health education campaign may be formulated by the local health leaders and provisionof assistive devices and equipment to achieve the equalization and opportunities for personswith disabilities may also be done
Absorbing transition in a coevolution model with node and link states in an adaptive network: Network fragmentation transition at criticality
We consider a general model in which there is a coupled dynamics of node
states and links states in a network. This coupled dynamics coevolves with
dynamical changes of the topology of the network caused by a link rewiring
mechanism. Such coevolution model features the interaction of the local
dynamics of node and link states with the nonlocal dynamics of link-rewiring in
a random network. The coupled dynamics of the states of the nodes and the links
produces by itself an absorbing phase transition which is shown to be robust
against the link rewiring mechanism. However, the dynamics of the network gives
rise to significant physical changes, specially in the limit in which some
links do not change state but are always rewired: First a network fragmentation
occurs at the critical line of the absorbing transition, and only at this line,
so that fragmentation is a manifestation of criticality. Secondly, in the
active phase of the absorbing transition, finite-size fluctuations take the
system to a single network component consensus phase, while other
configurations are possible in the absence of rewiring. In addition, this phase
is reached after a survival time that scales linearly with system size, while
the survival time scales exponentially with system size when there is no
rewiring. A social interpretation of our results contribute to the description
of processes of emergence of social fragmentation and polarization
Partisan Voter Model: Quasi-stationary properties and noise-induced transitions
We revisit the Partisan Voter Model (PVM) reporting analytical results for
the quasi-stationary distribution, exit probabilities and fixation times.
Similarly to the Noisy Voter Model (NVM) we introduce a Noisy version of the
PVM (NPVM). We find that the finite size noise induced transition of the NVM is
modified in the NPVM so that there exists a new intermediate phase
Absorbing phase transition in the coupled dynamics of node and link states in random networks
We present a stochastic dynamics model of coupled evolution for the binary
states of nodes and links in a complex network. In the context of opinion
formation node states represent two possible opinions and link states a
positive or negative relation. Dynamics proceeds via node and link state update
towards pairwise satisfactory relations in which nodes in the same state are
connected by positive links or nodes in different states are connected by
negative links. By a mean-field rate equations analysis and Monte Carlo
simulations in random networks we find an absorbing phase transition from a
dynamically active phase to an absorbing phase. The transition occurs for a
critical value of the relative time scale for node and link state updates. In
the absorbing phase the order parameter, measuring global order, approaches
exponentially the final frozen configuration. Finite size effects are such that
in the absorbing phase the final configuration is reached in a characteristic
time that scales logarithmically with system size, while in the active phase,
finite-size fluctuation take the system to a frozen configuration in a
characteristic time that grows exponentially with system size. There is also a
finite-size topological transition associated with group splitting in the
network of these final frozen configurations
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