32,281 research outputs found
Threshold effects for two pathogens spreading on a network
Diseases spread through host populations over the networks of contacts
between individuals, and a number of results about this process have been
derived in recent years by exploiting connections between epidemic processes
and bond percolation on networks. Here we investigate the case of two pathogens
in a single population, which has been the subject of recent interest among
epidemiologists. We demonstrate that two pathogens competing for the same hosts
can both spread through a population only for intermediate values of the bond
occupation probability that lie above the classic epidemic threshold and below
a second higher value, which we call the coexistence threshold, corresponding
to a distinct topological phase transition in networked systems.Comment: 5 pages, 2 figure
Fitness-dependent topological properties of the World Trade Web
Among the proposed network models, the hidden variable (or good get richer)
one is particularly interesting, even if an explicit empirical test of its
hypotheses has not yet been performed on a real network. Here we provide the
first empirical test of this mechanism on the world trade web, the network
defined by the trade relationships between world countries. We find that the
power-law distributed gross domestic product can be successfully identified
with the hidden variable (or fitness) determining the topology of the world
trade web: all previously studied properties up to third-order correlation
structure (degree distribution, degree correlations and hierarchy) are found to
be in excellent agreement with the predictions of the model. The choice of the
connection probability is such that all realizations of the network with the
same degree sequence are equiprobable.Comment: 4 Pages, 4 Figures. Final version accepted for publication on
Physical Review Letter
Statistics of Certain Models of Evolution
In a recent paper, Newman surveys the literature on power law spectra in
evolution, self-organised criticality and presents a model of his own to arrive
at a conclusion that self-organised criticality is not necessary for evolution.
Not only did he miss a key model (Ecolab) that has a clear self-organised
critical mechanism, but also Newman's model exhibits the same mechanism that
gives rise to power law behaviour as does Ecolab. Newman's model is, in fact, a
``mean field'' approximation of a self-organised critical system. In this
paper, I have also implemented Newman's model using the Ecolab software,
removing the restriction that the number of species remains constant. It turns
out that the requirement of constant species number is non-trivial, leading to
a global coupling between species that is similar in effect to the species
interactions seen in Ecolab. In fact, the model must self-organise to a state
where the long time average of speciations balances that of the extinctions,
otherwise the system either collapses or explodes. In view of this, Newman's
model does not provide the hoped-for counter example to the presence of
self-organised criticality in evolution, but does provide a simple, almost
analytic model that can used to understand more intricate models such as
Ecolab.Comment: accepted in Phys Rev E.; RevTeX; See
http://parallel.hpc.unsw.edu.au/rks/ecolab.html for more informatio
Population Annealing with Weighted Averages: A Monte Carlo Method for Rough Free Energy Landscapes
The population annealing algorithm introduced by Hukushima and Iba is
described. Population annealing combines simulated annealing and Boltzmann
weighted differential reproduction within a population of replicas to sample
equilibrium states. Population annealing gives direct access to the free
energy. It is shown that unbiased measurements of observables can be obtained
by weighted averages over many runs with weight factors related to the free
energy estimate from the run. Population annealing is well suited to
parallelization and may be a useful alternative to parallel tempering for
systems with rough free energy landscapes such as spin glasses. The method is
demonstrated for spin glasses.Comment: 9 pages, 5 figures; version 2 has improved figure 5 and new titl
ZIP3D: An elastic and elastic-plastic finite-element analysis program for cracked bodies
ZIP3D is an elastic and an elastic-plastic finite element program to analyze cracks in three dimensional solids. The program may also be used to analyze uncracked bodies or multi-body problems involving contacting surfaces. For crack problems, the program has several unique features including the calculation of mixed-mode strain energy release rates using the three dimensional virtual crack closure technique, the calculation of the J integral using the equivalent domain integral method, the capability to extend the crack front under monotonic or cyclic loading, and the capability to close or open the crack surfaces during cyclic loading. The theories behind the various aspects of the program are explained briefly. Line-by-line data preparation is presented. Input data and results for an elastic analysis of a surface crack in a plate and for an elastic-plastic analysis of a single-edge-crack-tension specimen are also presented
MgB2 tunnel junctions with native or thermal oxide barriers
MgB2 tunnel junctions (MgB2/barrier/MgB2) were fabricated using a native
oxide grown on the bottom MgB2 film as the tunnel barrier. Such barriers
therefore survive the deposition of the second electrode at 300oC, even over
junction areas of ~1 mm2. Studies of such junctions, and those of the type
MgB2/native or thermal oxide/metal (Pb, Au, or Ag) show that tunnel barriers
grown on MgB2 exhibit a wide range of barrier heights and widths.Comment: 9 pages, 3 figure
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