Divide and conquer: resonance induced by competitive interactions

We study an Ising model in a network with disorder induced by the presence of both attractive and repulsive links. This system is subjected to a subthreshold signal, and the goal is to see how the response is enhanced for a given fraction of repulsive links. This can model a network of spin-like neurons with excitatory and inhibitory couplings. By means of numerical simulations and analytical calculations we find that there is an optimal probability, such that the coherent response is maximal

Divergent Time Scale in Axelrod Model Dynamics

We study the evolution of the Axelrod model for cultural diversity. We consider a simple version of the model in which each individual is characterized by two features, each of which can assume q possibilities. Within a mean-field description, we find a transition at a critical value q_c between an active state of diversity and a frozen state. For q just below q_c, the density of active links between interaction partners is non-monotonic in time and the asymptotic approach to the steady state is controlled by a time scale that diverges as (q-q_c)^{-1/2}.Comment: 4 pages, 5 figures, 2-column revtex4 forma

Noisy continuous--opinion dynamics

We study the Deffuant et al. model for continuous--opinion dynamics under the influence of noise. In the original version of this model, individuals meet in random pairwise encounters after which they compromise or not depending of a confidence parameter. Free will is introduced in the form of noisy perturbations: individuals are given the opportunity to change their opinion, with a given probability, to a randomly selected opinion inside the whole opinion space. We derive the master equation of this process. One of the main effects of noise is to induce an order-disorder transition. In the disordered state the opinion distribution tends to be uniform, while for the ordered state a set of well defined opinion groups are formed, although with some opinion spread inside them. Using a linear stability analysis we can derive approximate conditions for the transition between opinion groups and the disordered state. The master equation analysis is compared with direct Monte-Carlo simulations. We find that the master equation and the Monte-Carlo simulations do not always agree due to finite-size induced fluctuations that we analyze in some detail

Phase Separation Driven by External Fluctuations

The influence of external fluctuations in phase separation processes is analysed. These fluctuations arise from random variations of an external control parameter. A linear stability analysis of the homogeneous state shows that phase separation dynamics can be induced by external noise. The spatial structure of the noise is found to have a relevant role in this phenomenon. Numerical simulations confirm these results. A comparison with order-disorder noise induced phase transitions is also made.Comment: 4 pages, 4 Postscript figures included in text. LaTeX (with Revtex macros

Two species coagulation approach to consensus by group level interactions

We explore the self-organization dynamics of a set of entities by considering the interactions that affect the different subgroups conforming the whole. To this end, we employ the widespread example of coagulation kinetics, and characterize which interaction types lead to consensus formation and which do not, as well as the corresponding different macroscopic patterns. The crucial technical point is extending the usual one species coagulation dynamics to the two species one. This is achieved by means of introducing explicitly solvable kernels which have a clear physical meaning. The corresponding solutions are calculated in the long time limit, in which consensus may or may not be reached. The lack of consensus is characterized by means of scaling limits of the solutions. The possible applications of our results to some topics in which consensus reaching is fundamental, like collective animal motion and opinion spreading dynamics, are also outlined

Resonance induced by repulsive interactions in a model of globally-coupled bistable systems

We show the existence of a competition-induced resonance effect for a generic globally coupled bistable system. In particular, we demonstrate that the response of the macroscopic variable to an external signal is optimal for a particular proportion of repulsive links. Furthermore, we show that a resonance also occurs for other system parameters, like the coupling strength and the number of elements. We relate this resonance to the appearance of a multistable region, and we predict the location of the resonance peaks, by a simple spectral analysis of the Laplacian matrix

Herdabilidades e correlações genéticas de características de machos e fêmeas, em um rebanho bovino da raça Canchim.

O objetivo foi estimar as herdabilidades e as correlações genéticas do peso (P12) e da circunferência escrotal (CE12) de machos aos 12 meses de idade, da idade de descarte (TPR, tempo de permanência no rebanho) de fêmeas e do número (ND10) e quilogramas (QD10) de bezerros desmamados pelas fêmeas em até dez anos de idade, em um rebanho da raça Canchim (5/8 Charolês + 3/8 Zebu). As estimativas dos componentes de (co)variâncias foram obtidas pelo método bayesiano e amostrador de Gibbs. O modelo estatístico, além do efeito médio global, incluiu os efeitos fixos de ano de nascimento do animal para todas as características, de mês de nascimento para P12 e CE12, e da covariável idade do animal para CE12, e os efeitos aleatórios genético aditivo direto e residual. As estimativas de herdabilidade, obtidas por meio de análises unicaráter, foram iguais a 0,38; 0,52; 0,24; 0,33 e 0,34 para P12, CE12, TPR, ND10 e QD10, respectivamente, indicando que as características possuem variação genética aditiva suficiente para apresentar boa resposta à seleção. As correlações genéticas de TPR (0,33 e 0,33, respectivamente), ND10 (0,38 e 0,30, respectivamente) e QD10 (0,61 e 0,41, respectivamente) com P12 e CE12, obtidas pelas análises bicaráter, sugerem que a seleção com base no peso e na circunferência escrotal dos machos não deve resultar em decréscimo no tempo de permanência das fêmeas no rebanho e no número e quilogramas de bezerros produzidos pelas fêmeas em até dez anos de idade

Escape rate from a metastable state weakly interacting with a heat bath driven by an external noise

Based on a system-reservoir model, where the reservoir is driven by an external stationary, Gaussian noise with arbitrary decaying correlation function, we study the escape rate from a metastable state in the energy diffusion regime. For the open system we derive the Fokker-Planck equation in the energy space and subsequently calculate the generalized non-Markovian escape rate from a metastable well in the energy diffusion domain. By considering the dynamics in a model cubic potential we show that the results obtained from numerical simulation are in good agreement with the theoretical prediction. It has been also shown numerically that the well known turnover feature can be restored from our model.Comment: 11 pages, 2 figure

Estimação de parâmetros genéticos para peso ao nascer em bovinos da raça Canchim avaliados sob diferentes modelos animais.

Foram utilizadas 18.215 observações de peso ao nascer de bovinos da raça Canchim criados em todo o Brasil, para estimar parâmetros genéticos para essa caracteristica utilizando-se quatro diferentes modelos animais. Nestes modelos foram considerados os efeitos fixos de grupo de contemporâneos (fazenda, ano e época de nascimento e sexo) e as covariáveis porcentagens de Charolês no animal e na mãe e porcentagem de heterozigose na mãe (efeito linear) e idade da vaca ao parto (efeitos linear e quadrático). Os efeitos aleatórios considerados foram os efeitos genéticos aditivo direto e materno e de ambiente permanente materno em diferentes combinações. Para a estimação dos parâmetros, os dados foram analisados pelo método da máxima verossimilhança restrita livre de derivadas e o teste de razão de verossimilhança (LRT) foi utilizado para a comparação dos modelos. O modelo completo (efeitos genéticos aditivo direto e materno e de ambiente permanente) foi o mais adequado, resultando nas seguintes estimativas de parâmetros: 0,30; 0,05 e 0,05 para a herdabilidade direta, herdabilidade materna e fração da variância atribuída ao ambiente permanente, respectivamente. A correlação entre os efeitos aditivos direto e materno foi próxima de zero

Apparent phase transitions in finite one-dimensional sine-Gordon lattices

We study the one-dimensional sine-Gordon model as a prototype of roughening phenomena. In spite of the fact that it has been recently proven that this model can not have any phase transition [J. A. Cuesta and A. Sanchez, J. Phys. A 35, 2373 (2002)], Langevin as well as Monte Carlo simulations strongly suggest the existence of a finite temperature separating a flat from a rough phase. We explain this result by means of the transfer operator formalism and show as a consequence that sine-Gordon lattices of any practically achievable size will exhibit this apparent phase transition at unexpectedly large temperatures.Comment: 7 pages, 4 figure