9,690 research outputs found
Modified Renormalization Strategy for Sandpile Models
Following the Renormalization Group scheme recently developed by Pietronero
{\it et al}, we introduce a simplifying strategy for the renormalization of the
relaxation dynamics of sandpile models. In our scheme, five sub-cells at a
generic scale form the renormalized cell at the next larger scale. Now the
fixed point has a unique nonzero dynamical component that allows for a great
simplification in the computation of the critical exponent . The values
obtained are in good agreement with both numerical and theoretical results
previously reported.Comment: APS style, 9 pages and 3 figures. To be published in Phys. Rev.
Discrete-time Markov chain approach to contact-based disease spreading in complex networks
Many epidemic processes in networks spread by stochastic contacts among their
connected vertices. There are two limiting cases widely analyzed in the physics
literature, the so-called contact process (CP) where the contagion is expanded
at a certain rate from an infected vertex to one neighbor at a time, and the
reactive process (RP) in which an infected individual effectively contacts all
its neighbors to expand the epidemics. However, a more realistic scenario is
obtained from the interpolation between these two cases, considering a certain
number of stochastic contacts per unit time. Here we propose a discrete-time
formulation of the problem of contact-based epidemic spreading. We resolve a
family of models, parameterized by the number of stochastic contact trials per
unit time, that range from the CP to the RP. In contrast to the common
heterogeneous mean-field approach, we focus on the probability of infection of
individual nodes. Using this formulation, we can construct the whole phase
diagram of the different infection models and determine their critical
properties.Comment: 6 pages, 4 figures. Europhys Lett (in press 2010
Thermal Instability in a Cooling and Expanding Medium Including Self-Gravity and Conduction
A systematic study of the linear thermal stability of a medium subject to
cooling, self-gravity and thermal conduction is carried out for the case when
the unperturbed state is subject to global cooling and expansion. A general,
recursive WKB solution for the perturbation problem is obtained which can be
applied to a large variety of situations in which there is a separation of
time-scales for the different physical processes. Solutions are explicitly
given and discussed for the case when sound propagation and/or self-gravity are
the fastest processes, with cooling, expansion and thermal conduction operating
on slower time-scales. A brief discussion is also added for the solutions in
the cases in which cooling or conduction operate on the fastest time-scale. The
general WKB solution obtained in this paper permits solving the problem of the
effect of thermal conduction and self-gravity on the thermal stability of a
globally cooling and expanding medium. As a result of the analysis, the
critical wavelength (often called Field length) above which cooling makes the
perturbations unstable against the action of thermal conduction is generalized
to the case of an unperturbed background with net cooling. As an astrophysical
application, the generalized Field length is calculated for a hot (10^4 - 10^8
K), optically thin medium (as pertains, for instance, for the hot interstellar
medium of SNRs or superbubbles) using a realistic cooling function and
including a weak magnetic field. The stability domains are compared with the
predictions made on the basis of models for which the background is in thermal
equilibrium. The instability domain of the sound waves, in particular, is seen
to be much larger in the case with net global cooling.Comment: 36 pages, 6 figures, accepted by ApJ, probable publication date:
April 20, 200
Spreading of sexually transmitted diseases in heterosexual populations
The spread of sexually transmitted diseases (e.g. Chlamydia, Syphilis,
Gonorrhea, HIV) across populations is a major concern for scientists and health
agencies. In this context, both data collection on sexual contact networks and
the modeling of disease spreading, are intensively contributing to the search
for effective immunization policies. Here, the spreading of sexually
transmitted diseases on bipartite scale-free graphs, representing heterosexual
contact networks, is considered. We analytically derive the expression for the
epidemic threshold and its dependence with the system size in finite
populations. We show that the epidemic outbreak in bipartite populations, with
number of sexual partners distributed as in empirical observations from
national sex surveys, takes place for larger spreading rates than for the case
in which the bipartite nature of the network is not taken into account.
Numerical simulations confirm the validity of the theoretical results. Our
findings indicate that the restriction to crossed infections between the two
classes of individuals (males and females) has to be taken into account in the
design of efficient immunization strategies for sexually transmitted diseases.Comment: 7 pages, 3 figures and 2 table
Reproducibility and clinical relevance of the ocular response analyzer in nonoperated eyes: corneal biomechanical and tonometric implications
To assess the reproducibility of the ocular response analyzer (ORA) in
nonoperated eyes and the impact of corneal biomechanical properties on
intraocular pressure (IOP) measurements in normal and glaucomatous eyes. METHODS:
In the reliability study, two independent examiners obtained repeated ORA
measurements in 30 eyes. In the clinical study, the examiners analyzed ORA and
IOP-Goldmann values from 220 normal and 42 glaucomatous eyes. In both studies,
Goldmann-correlated IOP measurement (IOP-ORAg), corneal-compensated IOP
(IOP-ORAc), corneal hysteresis (CH), and corneal resistance factor (CRF) were
evaluated. IOP differences of 3 mm Hg or greater between the IOP-ORAc and
IOP-ORAg were considered outcome significant. RESULTS: Intraexaminer intraclass
correlation coefficients and interexaminer concordance correlation coefficients
ranged from 0.78 to 0.93 and from 0.81 to 0.93, respectively, for all parameters.
CH reproducibility was highest, and the IOP-ORAg readings were lowest. The median
IOP was 16 mm Hg with the Goldmann tonometer, 14.5 mm Hg with IOP-ORAg (P <
0.001), and 15.7 mm Hg with IOP-ORAc (P < 0.001). Outcome-significant results
were found in 77 eyes (29.38%). The IOP-ORAc, CH, and CRF were correlated with
age (r = 0.22, P = 0.001; r = -0.23, P = 0.001; r = -0.14, P = 0.02,
respectively), but not the IOP-ORAg or IOP-Goldmann. CONCLUSIONS: The ORA
provides reproducible corneal biomechanical and IOP measurements in nonoperated
eyes. Considering the effect of ORA, corneal biomechanical metrics produces an
outcome-significant IOP adjustment in at least one quarter of glaucomatous and
normal eyes undergoing noncontact tonometry. Corneal viscoelasticity (CH) and
resistance (CRF) appear to decrease minimally with increasing age in healthy
adults
Lower Cretaceous (Hauterivian-Albian) ammonite biostratigraphy in the Maestrat Basin (E Spain)
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