731 research outputs found
Synchronised firing induced by network dynamics in excitable systems
We study the collective dynamics of an ensemble of coupled identical
FitzHugh--Nagumo elements in their excitable regime. We show that collective
firing, where all the elements perform their individual firing cycle
synchronously, can be induced by random changes in the interaction pattern.
Specifically, on a sparse evolving network where, at any time, each element is
connected with at most one partner, collective firing occurs for intermediate
values of the rewiring frequency. Thus, network dynamics can replace noise and
connectivity in inducing this kind of self-organised behaviour in highly
disconnected systems which, otherwise, wouldn't allow for the spreading of
coherent evolution.Comment: 5 pages, 5 figure
Zipf Law for Brazilian Cities
This work studies the Zipf Law for cities in Brazil. Data from censuses of
1970, 1980, 1991 and 2000 were used to select a sample containing only cities
with 30,000 inhabitants or more. The results show that the population
distribution in Brazilian cities does follow a power law similar to the ones
found in other countries. Estimates of the power law exponent were found to be
2.22 +/- 0.34 for the 1970 and 1980 censuses, and 2.26 +/- 0.11 for censuses of
1991 and 2000. More accurate results were obtained with the maximum likelihood
estimator, showing an exponent equal to 2.41 for 1970 and 2.36 for the other
three years.Comment: 12 pages, 6 figures, 3 tables, Elsevier LaTeX, accepted for
publication in "Physica A". Correction of minor mistyping (eq. 8
Propagation of small perturbations in synchronized oscillator networks
We study the propagation of a harmonic perturbation of small amplitude on a
network of coupled identical phase oscillators prepared in a state of full
synchronization. The perturbation is externally applied to a single oscillator,
and is transmitted to the other oscillators through coupling. Numerical results
and an approximate analytical treatment, valid for random and ordered networks,
show that the response of each oscillator is a rather well-defined function of
its distance from the oscillator where the external perturbation is applied.
For small distances, the system behaves as a dissipative linear medium: the
perturbation amplitude decreases exponentially with the distance, while
propagating at constant speed. We suggest that the pattern of interactions may
be deduced from measurements of the response of individual oscillators to
perturbations applied at different nodes of the network
Global firing induced by network disorder in ensembles of activerotators
Abstract.: We study the influence of repulsive interactions on an ensemble of coupled excitable rotators. We find that a moderate fraction of repulsive interactions can trigger global firing of the ensemble. The regime of global firing, however, is suppressed in sufficiently large systems if the network of repulsive interactions is fully random, due to self-averaging in its degree distribution. We thus introduce a model of partially random networks with a broad degree distribution, where self-averaging due to size growth is absent. In this case, the regime of global firing persists for large sizes. Our results extend previous work on the constructive effects of diversity in the collective dynamics of complex system
Dependences of the Casimir-Polder interaction between an atom and a cavity wall on atomic and material properties
The Casimir-Polder and van der Waals interactions between an atom and a flat
cavity wall are investigated under the influence of real conditions including
the dynamic polarizability of the atom, actual conductivity of the wall
material and nonzero temperature of the wall. The cases of different atoms near
metal and dielectric walls are considered. It is shown that to obtain accurate
results for the atom-wall interaction at short separations, one should use the
complete tabulated optical data for the complex refractive index of the wall
material and the accurate dynamic polarizability of an atom. At relatively
large separations in the case of a metal wall, one may use the plasma model
dielectric function to describe the dielectric properties of wall material. The
obtained results are important for the theoretical interpretation of
experiments on quantum reflection and Bose-Einstein condensation.Comment: 5 pages, 1 figure, iopart.cls is used, to appear in J. Phys. A
(special issue: Proceedings of QFEXT05, Barcelona, Sept. 5-9, 2005
Global firing induced by network disorder in ensembles of active rotators
We study the influence of repulsive interactions on an ensemble of coupled
excitable rotators. We find that a moderate fraction of repulsive interactions
can trigger global firing of the ensemble. The regime of global firing,
however, is suppressed in sufficiently large systems if the network of
repulsive interactions is fully random, due to self-averaging in its degree
distribution. We thus introduce a model of partially random networks with a
broad degree distribution, where self-averaging due to size growth is absent.
In this case, the regime of global firing persists for large sizes. Our results
extend previous work on the constructive effects of diversity in the collective
dynamics of complex systems.Comment: 8 pages, 6 figure
Measuring the evolution of contemporary western popular music
Popular music is a key cultural expression that has captured listeners'
attention for ages. Many of the structural regularities underlying musical
discourse are yet to be discovered and, accordingly, their historical evolution
remains formally unknown. Here we unveil a number of patterns and metrics
characterizing the generic usage of primary musical facets such as pitch,
timbre, and loudness in contemporary western popular music. Many of these
patterns and metrics have been consistently stable for a period of more than
fifty years, thus pointing towards a great degree of conventionalism.
Nonetheless, we prove important changes or trends related to the restriction of
pitch transitions, the homogenization of the timbral palette, and the growing
loudness levels. This suggests that our perception of the new would be rooted
on these changing characteristics. Hence, an old tune could perfectly sound
novel and fashionable, provided that it consisted of common harmonic
progressions, changed the instrumentation, and increased the average loudness.Comment: Supplementary materials not included. Please see the journal
reference or contact the author
Optimization as a result of the interplay between dynamics and structure
In this work we study the interplay between the dynamics of a model of
diffusion governed by a mechanism of imitation and its underlying structure.
The dynamics of the model can be quantified by a macroscopic observable which
permits the characterization of an optimal regime. We show that dynamics and
underlying network cannot be considered as separated ingredients in order to
achieve an optimal behavior.Comment: 12 pages, 4 figures, to appear in Physica
- …