International Journal of Modern Physics C

Self-Organized Criticality (SOC) phenomena could have a significant effect on the dynamics of ecosystems. The Bak–Sneppen (BS) model is a simple and robust model of biological evolution that exhibits punctuated equilibrium behavior. Here, we will introduce random version of BS model. We also generalize the single objective BS model to a multiobjective one.

A new two-dimensional lattice kinetic scheme on the uniform mesh was recently proposed by Inamuro, based on the standard lattice Boltzmann method (LBM). Compared with the standard LBM, this scheme can easily implement the boundary condition and save computer memory. In order to remove the shortcoming of a relatively large viscosity at a high Reynolds number, a first-order derivative term is introduced in the equilibrium density distribution function. However, the parameter associated with the derivative term is very sensitive and was chosen in a narrow range for a high Reynolds number case. To avoid the use of the derivative term while removing the shortcoming of a relatively large viscosity, new lattice kinetic schemes are proposed in this work following the original lattice kinetic scheme. In these new lattice kinetic schemes, the derivative term is dropped out and the difficulty of the relatively large viscosity is eased by controlling the time step δt or sonic speed c_s. To validate these new lattice kinetic schemes, the numerical simulations of the two-dimensional square driven cavity flow at Reynolds numbers from 100 to 1000 are carried out. The results using the new lattice kinetic schemes are compared with the benchmark data.

We have investigated the effects of the Eley–Rideal (ER) process on the phase diagram of the Langmuir–Hinshelwood (LH) type monomer–dimer (CO–O₂) catalytic reaction on (001) surface and subsurface of a simple cubic structure. With the consideration of a new pathway in which a dimer is adsorbed in such a way that it takes one surface site whereas second site may be from a surface or from a subsurface, a situation close to experiments is observed, i.e., with slight introduction of the monomer (CO), the production of AB₂ starts. In this case, the qualitative trend of surface oxygen coverage is consistent with the experimental situation.

A social group is represented by a graph, where each pair of nodes is connected by two oppositely directed links. At the beginning, a given amount p(i) of resources is assigned randomly to each node i. Also, each link r(i,j) is initially represented by a random positive value, which means the percentage of resources of node i which is offered to node j. Initially, the graph is fully connected, i.e., all nondiagonal matrix elements r(i,j) are different from zero. During the simulation, the amounts of resources p(i) change according to the balance equation. The nodes reorganize their activity with time to give more resources to those which give them more. This is the rule of varying the coefficients r(i,j). The result is that after some transient time, only some pairs (m,n) of nodes survive with nonzero p(m) and p(n), each pair with symmetric and positive r(m,n)=r(n,m). Other coefficients r(m,i≠n) vanish. Unpaired nodes remain with no resources, i.e., their p(i)=0, and they cease to be active as they have nothing to offer. The percentage of survivors (i.e., those with p(i) positive) increases with the velocity of varying the numbers r(i,j), and it slightly decreases with the size of the group. The picture and the results can be interpreted as a description of a social algorithm leading to marriages.

The eligibility trace is the most important mechanism used so far in reinforcement learning to handle delayed reward. Here, we introduce a new kind of eligibility trace, the goal-directed trace, and show that it results in more reliable learning than the conventional trace. In addition, we also propose a new efficient algorithm for solving the goal-directed reinforcement learning problem.

Self-organized Monte Carlo simulations of 2D Ising ferromagnet on the square lattice are performed. The essence of the suggested simulation method is an artificial dynamics consisting of the well-known single-spin-flip Metropolis algorithm supplemented by a random walk on the temperature axis. The walk is biased towards the critical region through a feedback based on instantaneous energy and magnetization cumulants, which are updated at every Monte Carlo step and filtered through a special recursion algorithm. The simulations revealed the invariance of the temperature probability distribution function, once some self-organized critical steady regime is reached, which is called here noncanonical equilibrium. The mean value of this distribution approximates the pseudocritical temperature of canonical equilibrium. In order to suppress finite-size effects, the self-organized approach is extended to multi-lattice systems, where the feedback basis on pairs of instantaneous estimates of the fourth-order magnetization cumulant on two systems of different size. These replica-based simulations resemble, in Monte Carlo lattice systems, some of the invariant statistical distributions of standard self-organized critical systems.

We investigate the frustration effects on small-world networks by studying antiferromagnetic Ising model in two dimensions. When the rewiring is constrained to those sites such that the interaction still occurs between spins in distinct sublattices and frustration does not take place, we observe that the system behaves as in previous investigations of ferromagnetic Ising model. However, when the rewiring procedure does not only produce interactions between spins in distinct sublattices, small-world configurations can effectively produce geometrical frustration and we attain a different critical behavior. In the frustrated case, the critical temperature decreases with the augment of the rewiring probability and the magnetic ordering presents two different regimes for low and high p.

In this work, we study the critical behavior of an epidemic propagation model that considers individuals that can develop drug resistance. In our lattice model, each site can be found in one of the four states: empty, healthy, normally infected (not drug resistant) and strain infected (drug resistant) states. The most relevant parameters in our model are related to the mortality, cure and mutation rates. This model presents two distinct stationary active phases: a phase with co-existing normal and drug resistant infected individuals, and an intermediate active phase with only drug resistant individuals. We employed a finite-size scaling analysis to compute the critical points and the critical exponents, β/ν and 1/ν, governing the phase transitions between these active states and the absorbing inactive state. Our results are consistent with the hypothesis that these transitions belong to the directed percolation universality class.

In the present paper, we have briefly reviewed Sznajd's sociophysics model and its variants, and we also have proposed a simple Sznajd-like sociophysics model based on Ising spin system to explain the time evaluation of resistance probability of a closed community against occupation. Using a numerical method, we have shown that the time evaluation of resistance probability of community has a nonexponential character, which decays as stretched exponential, independent of the number of soldiers in one-dimensional model. Furthermore, it has been astonishingly found that our simple sociophysics model belongs to the same universality class of random walk process on the trapping space.

In the compromise model of Deffuant et al., opinions are real numbers between 0 and 1, and two agents are compatible if the difference of their opinions is smaller than the confidence bound parameter ∊. The opinions of a randomly chosen pair of compatible agents get closer to each other. We provide strong numerical evidence that the threshold value of ∊ above which all agents share the same opinion in the final configuration is 1/2, independently of the underlying social topology.

The skin is the largest (and the most exposed) organ of the body both in terms of surface area and weight. Its care is of great importance for both aesthetics and health issues. Often, the skin appearance gives us information about the skin health status as well as hints at the biological age. Therefore, the skin surface characterization is of great significance for dermatologists as well as for cosmetic scientists in order to evaluate the effectiveness of medical or cosmetic treatments. So far, no in vivo measurements regarding skin topography characterization could be achieved routinely to evaluate skin aging. This work describes how a portable capacitive device, normally used for fingerprint acquisition, can be utilized to achieve measures of skin aging routinely. The capacitive images give a high resolution (50 μm) representation of skin topography, in terms of wrinkles and cells. In this work, we have addressed the latter: through image segmentation techniques, cells have been localized and identified and a feature related to their area distribution has been generated. Accurate experiments accomplished in vivo show how the feature we conceived is linearly related to skin aging. Besides, since this finding has been achieved using a low cost portable device, this could boost research in this field as well as open doors to an application based on an embedded system.

Numerical investigation of critical exponents on a hypercubic lattice with L^d random sites with L up to 33 and d up to 7 showed that above the critical dimension the phase transitions in Ising model and percolation are not alike.