1,288 research outputs found
Scaling Behavior of Driven Interfaces Above the Depinning Transition
We study the depinning transition for models representative of each of the
two universality classes of interface roughening with quenched disorder. For
one of the universality classes, the roughness exponent changes value at the
transition, while the dynamical exponent remains unchanged. We also find that
the prefactor of the width scales with the driving force. We propose several
scaling relations connecting the values of the exponents on both sides of the
transition, and discuss some experimental results in light of these findings.Comment: Revtex 3.0, 4 pages in PRL format + 5 figures (available at
ftp://jhilad.bu.edu/pub/abbhhss/ma-figures.tar.Z ) submitted to Phys Rev Let
Comment on: Kinetic Roughening in Slow Combustion of Paper
We comment on a recent Letter by Maunuksela et al. [Phys. Rev. Lett. 79, 1515
(1997)].Comment: 1 page, 1 figure, http://polymer.bu.edu/~hmakse/Home.htm
Emergence of Complex Dynamics in a Simple Model of Signaling Networks
A variety of physical, social and biological systems generate complex
fluctuations with correlations across multiple time scales. In physiologic
systems, these long-range correlations are altered with disease and aging. Such
correlated fluctuations in living systems have been attributed to the
interaction of multiple control systems; however, the mechanisms underlying
this behavior remain unknown. Here, we show that a number of distinct classes
of dynamical behaviors, including correlated fluctuations characterized by
-scaling of their power spectra, can emerge in networks of simple
signaling units. We find that under general conditions, complex dynamics can be
generated by systems fulfilling two requirements: i) a ``small-world'' topology
and ii) the presence of noise. Our findings support two notable conclusions:
first, complex physiologic-like signals can be modeled with a minimal set of
components; and second, systems fulfilling conditions (i) and (ii) are robust
to some degree of degradation, i.e., they will still be able to generate
-dynamics
Module identification in bipartite and directed networks
Modularity is one of the most prominent properties of real-world complex
networks. Here, we address the issue of module identification in two important
classes of networks: bipartite networks and directed unipartite networks. Nodes
in bipartite networks are divided into two non-overlapping sets, and the links
must have one end node from each set. Directed unipartite networks only have
one type of nodes, but links have an origin and an end. We show that directed
unipartite networks can be conviniently represented as bipartite networks for
module identification purposes. We report a novel approach especially suited
for module detection in bipartite networks, and define a set of random networks
that enable us to validate the new approach
Behavioral-Independent Features of Complex Heartbeat Dynamics
We test whether the complexity of cardiac interbeat interval time series is
simply a consequence of the wide range of scales characterizing human behavior,
especially physical activity, by analyzing data taken from healthy adult
subjects under three conditions with controls: (i) a ``constant routine''
protocol where physical activity and postural changes are kept to a minimum,
(ii) sympathetic blockade, and (iii) parasympathetic blockade. We find that
when fluctuations in physical activity and other behavioral modifiers are
minimized, a remarkable level of complexity of heartbeat dynamics remains,
while for neuroautonomic blockade the multifractal complexity decreases.Comment: 4 pages with 6 eps figures. Latex file. For more details or for
downloading the PDF file of the published article see
http://polymer.bu.edu/~amaral/Heart.html and
http://polymer.bu.edu/~amaral/Multifractal.htm
A Poissonian explanation for heavy-tails in e-mail communication
Patterns of deliberate human activity and behavior are of utmost importance
in areas as diverse as disease spread, resource allocation, and emergency
response. Because of its widespread availability and use, e-mail correspondence
provides an attractive proxy for studying human activity. Recently, it was
reported that the probability density for the inter-event time between
consecutively sent e-mails decays asymptotically as , with
. The slower than exponential decay of the inter-event time
distribution suggests that deliberate human activity is inherently
non-Poissonian. Here, we demonstrate that the approximate power-law scaling of
the inter-event time distribution is a consequence of circadian and weekly
cycles of human activity. We propose a cascading non-homogeneous Poisson
process which explicitly integrates these periodic patterns in activity with an
individual's tendency to continue participating in an activity. Using standard
statistical techniques, we show that our model is consistent with the empirical
data. Our findings may also provide insight into the origins of heavy-tailed
distributions in other complex systems.Comment: 9 pages, 5 figure
Universality Classes for Interface Growth with Quenched Disorder
We present numerical evidence that there are two distinct universality
classes characterizing driven interface roughening in the presence of quenched
disorder. The evidence is based on the behavior of , the coefficient
of the nonlinear term in the growth equation. Specifically, for three of the
models studied, at the depinning transition, while
for the two other models, .Comment: 11 pages and 3 figures (upon request), REVTeX 3.0, (submitted to PRL
Stochastic Feedback and the Regulation of Biological Rhythms
We propose a general approach to the question of how biological rhythms
spontaneously self-regulate, based on the concept of ``stochastic feedback''.
We illustrate this approach by considering the neuroautonomic regulation of the
heart rate. The model generates complex dynamics and successfully accounts for
key characteristics of cardiac variability, including the power spectrum,
the functional form and scaling of the distribution of variations, and
correlations in the Fourier phases. Our results suggest that in healthy systems
the control mechanisms operate to drive the system away from extreme values
while not allowing it to settle down to a constant output.Comment: 15 pages, latex2e using rotate and epsf, with 4 ps figures. Submitted
to PR
Analytical solution of a model for complex food webs
We investigate numerically and analytically a recently proposed model for
food webs [Nature {\bf 404}, 180 (2000)] in the limit of large web sizes and
sparse interaction matrices. We obtain analytical expressions for several
quantities with ecological interest, in particular the probability
distributions for the number of prey and the number of predators. We find that
these distributions have fast-decaying exponential and Gaussian tails,
respectively. We also find that our analytical expressions are robust to
changes in the details of the model.Comment: 4 pages (RevTeX). Final versio
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