142 research outputs found
Epidemic thresholds for bipartite networks
It is well known that sexually transmitted diseases (STD) spread across a network of human sexual contacts. This network is most often bipartite, as most STD are transmitted between men and women. Even though network models in epidemiology have quite a long history now, there are few general results about bipartite networks. One of them is the simple dependence, predicted using the mean field approximation, between the epidemic threshold and the average and variance of the degree distribution of the network. Here we show that going beyond thisapproximation can lead to qualitatively different results that are supported by numerical simulations. One of the new features, that can be relevant for applications, is the existence of a critical value for the infectivity of each population, below which no epidemics can arise, regardless of the value of the infectivity of the other population.Fil: Hernandez, Damián G.. Comision Nacional de Energia Atomica. Gerencia del _rea de Energía Nuclear. Instituto Balseiro; Argentina;Fil: Risau Gusman, Sebastian Luis. Comision Nacional de Energia Atomica. Gerencia del Area Investicaciones y Aplicaciones No Nucleares; Argentina
Analisis Faktor-faktor Eksternal Yang Mempengaruhi Keputusan Konsumen Dalam Memilih Belanja Di Assalaam Hypermarket
The purpose of this study was to determine the influence of cultural, reference groups, family and social class factors, either partially or simultan to consumer decision in choosing shopping in Assalaam Hypermarket. The analytical tool used in this study were: Multiple linear regression test, Test t, Test F and The coefficient of determination (R2) or Adjusted R Square. The results of data analysis concludes that the cultural, reference groups, family and social class variables influence the purchasing decisions of consumers on Assalam Hypermarket, either partially or simultan, Cultural Variables become the dominant factor in the purchase decision. Suggestions that emerged in this study were The management should pay more attention to factors such as cultural community grows in accordance with decisions of private consumption, the reference group is able to provide encouragement to consume, the number of family members and their effects on individual consumption decisions, and social class potential segments that many make purchases at Assalam hypermarket and the manager also acan develop a plan of activities to encourage consumers to get used to make purchases at Assalam hypermarket.
Keywords: Cultural, Reference Group, Family, Social Class, Consumer Decision Making
Statistical Mechanics of Soft Margin Classifiers
We study the typical learning properties of the recently introduced Soft
Margin Classifiers (SMCs), learning realizable and unrealizable tasks, with the
tools of Statistical Mechanics. We derive analytically the behaviour of the
learning curves in the regime of very large training sets. We obtain
exponential and power laws for the decay of the generalization error towards
the asymptotic value, depending on the task and on general characteristics of
the distribution of stabilities of the patterns to be learned. The optimal
learning curves of the SMCs, which give the minimal generalization error, are
obtained by tuning the coefficient controlling the trade-off between the error
and the regularization terms in the cost function. If the task is realizable by
the SMC, the optimal performance is better than that of a hard margin Support
Vector Machine and is very close to that of a Bayesian classifier.Comment: 26 pages, 12 figures, submitted to Physical Review
Temperature dependence of the dielectric permittivity of BaTiO3-CoFe2O4 ceramic composites
Two-phase ceramic composites of cobalt ferrite dispersed in a barium titanate matrix were prepared and their dielectric permittivity was characterized. A rounding of their dielectric constant near the ferroelectric transition temperature of the BaTiO3 phase was observed. A generalized brick-wall model, taking into account the BaTiO3 contribution, fitted with the Landau-Devonshire theory, and the CoFe2O4 permittivity, experimentally determined, was used in order to obtain their effective dielectric constant. The effective model was applied to the discussion and fitting of the temperature dependence of the dielectric permittivity of the composites. The Landau-Devonshire coefficients for the BaTiO3 phase were also obtained.This work has been financially supported by the Portuguese Foundation for Science and Technology (FCT), through the project POCI/CTM/60181/2004
Levy ratchets with dichotomic random flashing
Additive symmetric L\'evy noise can induce directed transport of overdamped
particles in a static asymmetric potential. We study, numerically and
analytically, the effect of an additional dichotomous random flashing in such
L\'evy ratchet system. For this purpose we analyze and solve the corresponding
fractional Fokker-Planck equations and we check the results with Langevin
simulations. We study the behavior of the current as function of the stability
index of the L\'evy noise, the noise intensity and the flashing parameters. We
find that flashing allows both to enhance and diminish in a broad range the
static L\'evy ratchet current, depending on the frequencies and asymmetry of
the multiplicative dichotomous noise, and on the additive L\'evy noise
parameters. Our results thus extend those for dichotomous flashing ratchets
with Gaussian noise to the case of broadly distributed noises.Comment: 15 pages, 6 figure
Influence of Grain Size Dispersion on the Magnetic Properties of Nanogranular BaTiO3-CoFe2O4 Thin Films
Thin film nanogranular composites of cobalt ferrite (CoFe2O4) dispersed in a barium titanate (BaTiO3) matrix were deposited by laser ablation with different cobalt ferrite concentrations (x). Their structural and magnetic properties were characterized. The films were polycrystalline and composed by a mixture of tetragonal-BaTiO3 and CoFe2O4 with the cubic spinel structure. A slight (111) barium titanate phase orientation and (311) CoFe2O4 phase orientation were observed. The lattice parameter of the CoFe2O4 was always smaller than the bulk value indicating that the cobalt ferrite was under compressive stress. From atomic force microscopy a broad distribution of grain sizes was observed in the nanocomposites, with a significant amount of smaller grains (<40nm) from the CoFe2O4 phase. The magnetic measurements show an increase of the magnetic moment from the low concentration region where the magnetic grains are more isolated and their magnetic interaction is small, towards the bulk value for higher CoFe2O4 content in the films. A corresponding decrease of coercive field with increasing cobalt ferrite concentration was also observed, due to the higher inter-particle magnetic interaction (and reduced stress) of the agglomerated grains.This work has been financially supported by the Portuguese Foundation for Science and Technology (FCT) and FEDER, through the project POCI/CTM/60181/2004
Ground-state topology of the Edwards-Anderson +/-J spin glass model
In the Edwards-Anderson model of spin glasses with a bimodal distribution of
bonds, the degeneracy of the ground state allows one to define a structure
called backbone, which can be characterized by the rigid lattice (RL),
consisting of the bonds that retain their frustration (or lack of it) in all
ground states. In this work we have performed a detailed numerical study of the
properties of the RL, both in two-dimensional (2D) and three-dimensional (3D)
lattices. Whereas in 3D we find strong evidence for percolation in the
thermodynamic limit, in 2D our results indicate that the most probable scenario
is that the RL does not percolate. On the other hand, both in 2D and 3D we find
that frustration is very unevenly distributed. Frustration is much lower in the
RL than in its complement. Using equilibrium simulations we observe that this
property can be found even above the critical temperature. This leads us to
propose that the RL should share many properties of ferromagnetic models, an
idea that recently has also been proposed in other contexts. We also suggest a
preliminary generalization of the definition of backbone for systems with
continuous distributions of bonds, and we argue that the study of this
structure could be useful for a better understanding of the low temperature
phase of those frustrated models.Comment: 16 pages and 21 figure
The dynamics of opinion in hierarchical organizations
We study the mutual influence of authority and persuasion in the flow of
opinion. Many social organizations are characterized by a hierarchical
structure where the propagation of opinion is asymmetric. In the normal flow of
opinion formation a high-rank agent uses its authority (or its persuasion when
necessary) to impose its opinion on others. However, agents with no authority
may only use the force of its persuasion to propagate their opinions. In this
contribution we describe a simple model with no social mobility, where each
agent belongs to a class in the hierarchy and has also a persuasion capability.
The model is studied numerically for a three levels case, and analytically
within a mean field approximation, with a very good agreement between the two
approaches. The stratum where the dominant opinion arises from is strongly
dependent on the percentage of agents in each hierarchy level, and we obtain a
phase diagram identifying the relative frequency of prevailing opinions. We
also find that the time evolution of the conflicting opinions polarizes after a
short transient.Comment: 6 pages, 5 figures, submitted to Phys. Rev.
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