1,721,071 research outputs found
Voting behavior in proportional elections from agent-based models
No full textIn the talk we reviewed universal aspects of voting behavior in proportional elections and universality breaking patterns, as established in the existing literature. Focus was made on the Brazilian elections, which are characterized by compulsory voting. We showed how agent-based models can qualitatively and/or quantitatively reproduce the observed empirical distributions. As an example, we discussed the multi-state voter model over a network based on interacting cliques and zealot candidates. © 2015 The Authors. Published by Elsevier B.V
Stochastic Dynamics of the Multi-State Voter Model Over a Network Based on Interacting Cliques and Zealot Candidates
No full textThe stochastic dynamics of the multi-state voter model is investigated on a class of complex networks made of non-overlapping cliques, each hosting a political candidate and interacting with the others via Erdo{double acute}s-Rényi links. Numerical simulations of the model are interpreted in terms of an ad-hoc mean field theory, specifically tuned to resolve the inter/intra-clique interactions. Under a proper definition of the thermodynamic limit (with the average degree of the agents kept fixed while increasing the network size), the model is found to display the empirical scaling discovered by Fortunato and Castellano (Phys Rev Lett 99(13):138701, 2007), while the vote distribution resembles roughly that observed in Brazilian elections. © 2014 Springer Science+Business Media New York
Influence of periodic external fields in multiagent models with language dynamics
No full textWe investigate large-scale effects induced by external fields, phenomenologically interpreted as mass media, in multiagent models evolving with the microscopic dynamics of the binary naming game. In particular, we show that a single external field, broadcasting information at regular time intervals, can reverse the majority opinion of the population, provided the frequency and the effectiveness of the sent messages lie above well-defined thresholds. We study the phase structure of the model in the mean field approximation and in numerical simulations with several network topologies. We also investigate the influence on the agent dynamics of two competing external fields, periodically broadcasting different messages. In finite regions of the parameter space we observe periodic equilibrium states in which the average opinion densities are reversed with respect to naive expectations. Such equilibria occur in two cases: (i) when the frequencies of the competing messages are different but close to each other; (ii) when the frequencies are equal and the relative time shift of the messages does not exceed half a period. We interpret the observed phenomena as a result of the interplay between the external fields and the internal dynamics of the agents and conclude that, depending on the model parameters, the naming game is consistent with scenarios of first- or second-mover advantage (to borrow an expression from the jargon of business strategy). © 2017 American Physical Society
Topological aspects of the multi-language phases of the naming game on community-based networks
No full texthe Naming Game is an agent-based model where individuals communicate to name an initially unnamed object. On a large class of networks continual pairwise interactions lead the system to an ultimate consensus state, in which agents converge on a globally shared name. Soon after the introduction of the model, it was observed in literature that on community-based networks the path to consensus passes through metastable multi-language states. Subsequently, it was proposed to use this feature as a mean to discover communities in a given network. In this paper we show that metastable states correspond to genuine multi-language phases, emerging in the thermodynamic limit when the fraction of links connecting communities drops below critical thresholds. In particular, we study the transition to multi-language states in the stochastic block model and on networks with community overlap. We also examine the scaling of critical thresholds under variations of topological properties of the network, such as the number and relative size of communities and the structure of intra-/inter-community links. Our results provide a theoretical justification for the proposed use of the model as a community-detection algorithm. © 2017 by the authors; licensee MDPI, Basel, Switzerland
Numerical Reconstruction of the Covariance Matrix of a Spherically Truncated Multinormal Distribution
No full textWe relate the matrix SB of the second moments of a spherically truncated normal multivariate to its full covariance matrix Σ and present an algorithm to invert the relation and reconstruct Σ from SB. While the eigenvectors of Σ are left invariant by the truncation, its eigenvalues are nonuniformly damped. We show that the eigenvalues of Σ can be reconstructed from their truncated counterparts via a fixed point iteration, whose convergence we prove analytically. The procedure requires the computation of multidimensional Gaussian integrals over an Euclidean ball, for which we extend a numerical technique, originally proposed by Ruben in 1962, based on a series expansion in chi-square distributions. In order to study the feasibility of our approach, we examine the convergence rate of some iterative schemes on suitably chosen ensembles of Wishart matrices. We finally discuss the practical difficulties arising in sample space and outline a regularization of the problem based on perturbation theory. © 2017 Filippo Palombi et al
Use of dirichlet distributions and orthogonal projection techniques for the fluctuation analysis of steady-state multivariate birth-death systems
No full textApproximate weak solutions of the Fokker-Planck equation represent a useful tool to analyze the equilibrium fluctuations of birth-death systems, as they provide a quantitative knowledge lying in between numerical simulations and exact analytic arguments. In this paper, we adapt the general mathematical formalism known as the Ritz-Galerkin method for partial differential equations to the Fokker-Planck equation with time-independent polynomial drift and diffusion coefficients on the simplex. Then, we show how the method works in two examples, namely the binary and multi-state voter models with zealots. © 2015 World Scientific Publishing Company
A perturbative approach to the reconstruction of the eigenvalue spectrum of a normal covariance matrix from a spherically truncated counterpart
No full textIn this paper we propose a perturbative method for the reconstruction of the covariance matrix of a multinormal distribution, under the assumption that the only available information amounts to the covariance matrix of a spherically truncated counterpart of the same distribution. We expand the relevant equations up to the fourth perturbative order and discuss the analytic properties of the first few perturbative terms. We finally compare the proposed approach with an exact iterative algorithm (discussed in Palombi et al. (2017)) in the hypothesis that the spherically truncated covariance matrix is estimated from samples of various sizes
Coevolutionary dynamics of a variant of the cyclic Lotka–Volterra model with three-agent interactions
Full text linkAbstract: We study a variant of the cyclic Lotka–Volterra model with three-agent interactions. Inspired by a multiplayer variation of the Rock–Paper–Scissors game, the model describes an ideal ecosystem in which cyclic competition among three species develops through cooperative predation. Its rate equations in a well-mixed environment display a degenerate Hopf bifurcation, occurring as reactions involving two predators plus one prey have the same rate as reactions involving two prey plus one predator. We estimate the magnitude of the stochastic noise at the bifurcation point, where finite size effects turn neutrally stable orbits into erratically diverging trajectories. In particular, we compare analytic predictions for the extinction probability, derived in the Fokker–Planck approximation, with numerical simulations based on the Gillespie stochastic algorithm. We then extend the analysis of the phase portrait to heterogeneous rates. In a well-mixed environment, we observe a continuum of degenerate Hopf bifurcations, generalizing the above one. Neutral stability ensues from a complex equilibrium between different reactions. Remarkably, on a two-dimensional lattice, all bifurcations disappear as a consequence of the spatial locality of the interactions. In the second part of the paper, we investigate the effects of mobility in a lattice metapopulation model with patches hosting several agents. We find that strategies propagate along the arms of rotating spirals, as they usually do in models of cyclic dominance. We observe propagation instabilities in the regime of large wavelengths. We also examine three-agent interactions inducing nonlinear diffusion.“Three at play. That’ll be the day!”(a child in Wings of desire [W. Wenders, 1987]) Graphical abstract: [Figure not available: see fulltext.]
f(B) and two-scales problems in lattice QCD
No full textA novel method to calculate f(B) on the lattice is introduced, based on
the study of the dependence of finite size effects upon the heavy quark
mass of flavoured mesons and on a non-perturbative recursive finite size
technique. We avoid the systematic errors related to extrapolations from
the static limit or to the tuning of the coefficients of effective
Lagrangian and the results admit an extrapolation to the continuum
limit. We perform a first estimate at finite lattice spacing, but close
to the continuum limit, giving f(B) = 170(11)(5)(22) MeV We also obtain
f(Bs) = 192(9)(5)(24) MeV. The first error is statistical, the second is
our estimate of the systematic error from the method and the third the
systematic error from the specific approximations adopted in this first
exploratory calculation. The method can be generalized to two-scale
problems in lattice QCD. (C) 2002 Elsevier Science B.V. All rights
reserved
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