arXiv subjects

Z. Neda

Publications and source records attributed to Z. Neda.

At least 19 recordsLinked to original sources

arXiv2012

Income distribution patterns from a complete social security database

We analyze the income distribution of employees for 9 consecutive years (2001-2009) using a complete social security database for an economically important district of Romania. The database contains detailed information on more than half million taxpayers, including their monthly salaries from all employers where they worked. Besides studying the characteristic distribution functions in the high and low/medium income limits, the database allows us a detailed dynamical study by following the time-evolution of the taxpayers income. To our knowledge, this is the first extensive study of this kind (a previous japanese taxpayers survey was limited to two years). In the high income limit we prove once again the validity of Pareto's law, obtaining a perfect scaling on four orders of magnitude in the rank for all the studied years. The obtained Pareto exponents are quite stable with values around $α\approx 2.5$, in spite of the fact that during this period the economy developed rapidly and also a financial-economic crisis hit Romania in 2007-2008. For the low and medium income category we confirmed the exponential-type income distribution. Following the income of employees in time, we have found that the top limit of the income distribution is a highly dynamical region with strong fluctuations in the rank. In this region, the observed dynamics is consistent with a multiplicative random growth hypothesis. Contrarily with previous results obtained for the japanese employees, we find that the logarithmic growth-rate is not independent of the income.

physics.soc-ph↗

arXiv2012

A seed-diffusion model for tropical tree diversity patterns

Diversity patterns of tree species in a tropical forest community are approached by a simple lattice model and investigated by Monte Carlo simulations using a backtracking method. Our spatially explicit neutral model is based on a simple statistical physics process, namely the diffusion of seeds. The model has three parameters: the speciation rate, the size of the meta-community in which the studied tree-community is embedded, and the average surviving time of the seeds. By extensive computer simulations we aim the reproduction of relevant statistical measures derived from the experimental data of the Barro Colorado Island tree census in year 1995. The first two parameters of the model are fixed to known values, characteristic of the studied community, thus obtaining a model with only one freely adjustable parameter. As a result of this, the average number of species in the considered territory, the relative species abundance distribution, the species-area relationship and the spatial auto-correlation function of the individuals in abundant species are simultaneously fitted with only one parameter which is the average surviving time of the seeds.

q-bio.PE↗

arXiv2008

A spatially explicit model for tropical tree diversity patterns

A complex two-parameter model resembling the classical voter model is introduced to describe macroecological properties of tropical tree communities. Monte-Carlo type computer simulations are performed on the model, investigating species abundances and the spatial distribution of individuals and species. Simulation results are critically compared with the experimental data obtained from a tree census on a 50 hectares area of the Barro Colorado Island (BCI), Panama. The model parameters are optimized for reproducing quantitatively the experimental results from the BCI dataset.

q-bio.PE↗

arXiv2008

On the size-distribution of Poisson Voronoi cells

Poisson Voronoi diagrams are useful for modeling and describing various natural patterns and for generating random lattices. Although this particular space tessellation is intensively studied by mathematicians, in two- and three dimensional spaces there is no exact result known for the size-distribution of Voronoi cells. Motivated by the simple form of the distribution function in the one-dimensional case, a simple and compact analytical formula is proposed for approximating the Voronoi cell's size distribution function in the practically important two- and three dimensional cases as well. Denoting the dimensionality of the space by d (d=1,2,3) the $f(y)=Const*y^{(3d-1)/2}exp(-(3d+1)y/2)$ compact form is suggested for the normalized cell-size distribution function. By using large-scale computer simulations the validity of the proposed distribution function is studied and critically discussed.

cond-mat.soft↗

arXiv2008

Collective behavior of "electronic fireflies"

A simple system composed of electronic oscillators capable of emitting and detecting light-pulses is studied. The oscillators are biologically inspired, their behavior is designed for keeping a desired light intensity, W, in the system. From another perspective, the system behaves like modified integrate and fire type neurons that are pulse-coupled with inhibitory type interactions: the firing of one oscillator delays the firing of all the others. Experimental and computational studies reveal that although no driving force favoring synchronization is considered, for a given interval of W phase-locking appears. This weak synchronization is sometimes accompanied by complex dynamical patterns in the flashing sequence of the oscillators.

nlin.AO↗

arXiv2008

Shake-induced order in nanosphere systems

Self-assembled patterns obtained from a drying nanosphere suspension are investigated by computer simulations and simple experiments. Motivated by the earlier experimental results of Sasaki and Hane and Schope, we confirm that more ordered triangular lattice structures can be obtained whenever a moderate intensity random shaking is applied on the drying system. Computer simulations are realized on an improved version of a recently elaborated Burridge-Knopoff-type model. Experiments are made following the setup of Sasaki and Hane, using ultrasonic radiation as source for controlled shaking.

cond-mat.mtrl-sci↗

arXiv2006

Perspectives for Monte Carlo simulations on the CNN Universal Machine

Possibilities for performing stochastic simulations on the analog and fully parallelized Cellular Neural Network Universal Machine (CNN-UM) are investigated. By using a chaotic cellular automaton perturbed with the natural noise of the CNN-UM chip, a realistic binary random number generator is built. As a specific example for Monte Carlo type simulations, we use this random number generator and a CNN template to study the classical site-percolation problem on the ACE16K chip. The study reveals that the analog and parallel architecture of the CNN-UM is very appropriate for stochastic simulations on lattice models. The natural trend for increasing the number of cells and local memories on the CNN-UM chip will definitely favor in the near future the CNN-UM architecture for such problems.

physics.comp-ph↗

arXiv2005

Wealth distribution and Pareto's law in the Hungarian medieval society

The distribution of wealth in the Hungarian medieval aristocratic society is reported and studied. The number of serf families belonging to a noble is taken as a measure of the corresponding wealth. Our results reveal the power-law nature of this distribution function, confirming the validity of the Pareto law for such a society. The obtained Pareto index $α=0.92$ is however smaller than the values currently reported in the literature. We argue that the value close to 1, of the Pareto index is a consequence of the absence of a relevant economic life in the targeted society, in agreement with the prediction of existing wealth distribution models for the idealized case of independently acting agents. Models developed to explain city populations may also be adapted to justify our results.

physics.soc-ph↗

arXiv2005

Species Abundances Distribution in Neutral Community Models

An analytical approximation is derived for the Zero Sum Multinomial distribution which gives the Species Abundance Distribution in Neutral Community Models. The obtained distribution function describes well computer simulation results on the model, and leads to an interesting relation between the total number of individuals, total number of species and the size of the most abundant species of the considered metacommunity. Computer simulations on neutral community models, proves also the validity of this scaling relation.

q-bio.PE↗

arXiv2005

The Species Abundances Distribution in a new perspective

Studies on distribution, abundance and diversity of species revealed fascinating universalities in macroecology. Many of these patterns, like the species-area and range-abundance relationship or the year-to-year fluctuations in population sizes are expressed as power-law distributions, and indicate thus scale-invariance. The species abundance distribution (SAD) apparently shows this scale-free nature only for rare species, and its mathematical form is much debated. In the present work we propose a new mathematical expression for SAD which describes reasonable well most of the presently available large-scale experimental data and the results of the neutral models. This distribution function leads to an interesting relation between the total number of individuals, total number of species and the size of the most abundant species of the meta-community. This novel scaling relation is confirmed by computer simulations on neutral models.

q-bio.PE↗

arXiv2005

Understanding self-assembled nanosphere patterns

Patterns generated by a colloidal suspension of nanospheres drying on a frictional substrate are studied by experiments and computer simulations. The obtained two-dimensional self-assembled structures are commonly used for nanosphere lithography. A spring-block stick-slip model is introduced for simulating the phenomenon and the influence of several controllable parameters on the final structure is investigated. The model successfully reproduces the experimentally observed patterns and the dynamics leading to pattern formation is revealed.

cond-mat.soft↗

arXiv2004

A family-network model for wealth distribution in societies

A model based on first-degree family relations network is used to describe the wealth distribution in societies. The network structure is not a-priori introduced in the model, it is generated in parallel with the wealth values through simple and realistic dynamical rules. The model has two main parameters, governing the wealth exchange in the network. Choosing their values realistically, leads to wealth distributions in good agreement with measured data. The cumulative wealth distribution function has an exponential behavior in the low and medium wealth limit, and shows the Pareto-like power-law tail for the upper 5% of the society. The obtained Pareto indexes are in good agreement with the measured ones. The generated family networks also converges to a statistically stable topology with a simple Poissonian degree distribution. On this family-network many interesting correlations are studied, and the main factors leading to wealth-diversification and the formation of the Pareto law are identified.

cond-mat.other↗

arXiv2004

A spring-block model for Barkhausen noise

A simple mechanical spring-block model is introduced for studying magnetization phenomena and in particularly the Barkhausen noise. The model captures and reproduces the accepted microscopic picture of domain wall movement and pinning. Computer simulations suggest that this model is able to reproduce the main characteristics of hysteresis loops and Barkhausen jumps. In the thermodynamic limit the statistics of the obtained Barkhausen jumps follows several scaling laws, in qualitative agreement with the experimental results. The simplicity of the model and the invoked mechanical analogies makes it attractive for computer simulations and pedagogical purposes.

cond-mat.soft↗

arXiv2003

Networks in life: Scaling properties and eigenvalue spectra

We analyse growing networks ranging from collaboration graphs of scientists to the network of similarities defined among the various transcriptional profiles of living cells. For the explicit demonstration of the scale-free nature and hierarchical organization of these graphs, a deterministic construction is also used. We demonstrate the use of determining the eigenvalue spectra of sparse random graph models for the categorization of small measured networks.

cond-mat.stat-mech↗

arXiv2002

Percolation-like behavior of some optimal coalition formation models

The ground-state of an infinite-range Potts glass-type model with +/- J bonds and unrestricted number of states is used to investigate coalition formation. As a function of the q probability of +J bonds in the system it is found that the r relative size of the largest cluster (a cluster being the group of elements in the same state) shows a percolation like behavior. By a simple renormalization approach and several optimization methods we investigate the r(q) curves for finite systems sizes. Non-trivial consequences for social percolation problems are discussed.

cond-mat.stat-mech↗

arXiv2002

Flatness of the setting Sun

Atmospheric refraction is responsible for the bending of light-rays in the atmosphere. It is a result of the continuous decrease in the refractive index of the air as a function of altitude. A well-known consequence of this phenomenon is the apparently elliptic shape of the setting or rising Sun (or full-Moon). In the present paper we systematically investigate this phenomenon in a standard atmosphere. Theoretical and numerical calculations are compared with experimental data. The asymmetric rim of the Sun is computed as a function of its inclination angle, observational height and meteorological conditions characterized by pressure, temperature and lapse-rate. We reveal and illustrate some extreme and highly unusual situations.

physics.pop-ph↗

arXiv2002

Spiral cracks in drying precipitates

We investigate the formation of spiral crack patterns during the desiccation of thin layers of precipitates in contact with a substrate. This symmetry-breaking fracturing mode is found to arise naturally not from torsion forces, but from a propagating stress front induced by the fold-up of the fragments. We model their formation mechanism using a coarse-grain model for fragmentation and successfully reproduce the spiral cracks. Fittings of experimental and simulation data show that the spirals are logarithmic, corresponding to constant deviation from a circular crack path. Theoretical aspects of the logarithmic spirals are discussed. In particular we show that this occurs generally when the crack speed is proportional to the propagating speed of stress front.

cond-mat.soft↗

arXiv2001

Improved percolation thresholds for rods in three-dimensional boxes

We improve our previous results for the percolation thresholds of isotropically oriented rods in three dimensional boxes. We prove again the applicability of the excluded volume rule in the slender-rod limit (radius/length -> 0). Other limits for the rod sizes are discussed and important finite-size effects are revealed.

cond-mat.stat-mech↗