4,722 research outputs found
Community detection in complex networks using Extremal Optimization
We propose a novel method to find the community structure in complex networks
based on an extremal optimization of the value of modularity. The method
outperforms the optimal modularity found by the existing algorithms in the
literature. We present the results of the algorithm for computer simulated and
real networks and compare them with other approaches. The efficiency and
accuracy of the method make it feasible to be used for the accurate
identification of community structure in large complex networks.Comment: 4 pages, 4 figure
FmocâRGDS based fibrils: atomistic details of their hierarchical assembly
We describe the 3D supramolecular structure of FmocâRGDS fibrils, where Fmoc and RGDS refer to the hydrophobic N-(fluorenyl-9-methoxycarbonyl) group and the hydrophilic Arg-Gly-Asp-Ser peptide sequence, respectively. For this purpose, we performed atomistic all-atom molecular dynamics simulations of a wide variety of packing modes derived from both parallel and antiparallel Ă-sheet configurations. The proposed model, which closely resembles the cross-Ă core structure of amyloids, is stabilized by pâp stacking interactions between hydrophobic Fmoc groups. More specifically, in this organization, the Fmoc-groups of Ă-strands belonging to the same Ă-sheet form columns of p-stacked aromatic rings arranged in a parallel fashion. Eight of such columns pack laterally forming a compact and dense hydrophobic core, in which two central columns are surrounded by three adjacent columns on each side. In addition to such FmocÂżFmoc interactions, the hierarchical assembly of the constituent Ă-strands involves a rich variety of intra- and inter-strand interactions. Accordingly, hydrogen bonding, salt bridges and pâp stacking interactions coexist in the highly ordered packing network proposed for the FmocâRGDS amphiphile. Quantum mechanical calculations, which have been performed to quantify the above referred interactions, confirm the decisive role played by the pâp stacking interactions between the rings of the Fmoc groups, even though both inter-strand and intra-strand hydrogen bonds and salt bridges also play a non-negligible role. Overall, these results provide a solid reference to complement the available experimental data, which are not precise enough to determine the fibril structure, and reconcile previous independent observations.Peer ReviewedPostprint (published version
Numerical model of solid phase transformations governed by nucleation and growth. Microstructure development during isothermal crystallization
A simple numerical model which calculates the kinetics of crystallization
involving randomly distributed nucleation and isotropic growth is presented.
The model can be applied to different thermal histories and no restrictions are
imposed on the time and the temperature dependencies of the nucleation and
growth rates. We also develop an algorithm which evaluates the corresponding
emerging grain size distribution. The algorithm is easy to implement and
particularly flexible making it possible to simulate several experimental
conditions. Its simplicity and minimal computer requirements allow high
accuracy for two- and three-dimensional growth simulations. The algorithm is
applied to explore the grain morphology development during isothermal
treatments for several nucleation regimes. In particular, thermal nucleation,
pre-existing nuclei and the combination of both nucleation mechanisms are
analyzed. For the first two cases, the universal grain size distribution is
obtained. The high accuracy of the model is stated from its comparison to
analytical predictions. Finally, the validity of the
Kolmogorov-Johnson-Mehl-Avrami model is verified for all the cases studied
Is the distant globular cluster Pal 14 in a deep-freeze?
We investigate the velocity dispersion of Pal 14, an outer Milky-Way globular
cluster at Galactocentric distance of 71 kpc with a very low stellar density
(central density 0.1-0.2 Msun/pc^3). Due to this low stellar density the binary
population of Pal 14 is likely to be close to the primordial binary population.
Artificial clusters are generated with the observed properties of Pal 14 and
the velocity dispersion within these clusters is measured as Jordi et al.
(2009) have done with 17 observed stars of Pal 14. We discuss the effect of the
binary population on these measurements and find that the small velocity
dispersion of 0.38 km/s which has been found by Jordi et al. (2009) would imply
a binary fraction of less than 0.1, even though from the stellar density of Pal
14 we would expect a binary fraction of more than 0.5. We also discuss the
effect of mass segregation on the velocity dispersion as possible explanation
for this discrepancy, but find that it would increase the velocity dispersion
further. Thus, either Pal 14 has a very unusual stellar population and its
birth process was significantly different than we see in today's star forming
regions, or the binary population is regular and we would have to correct the
observed 0.38 km/s for binarity. In this case the true velocity dispersion of
Pal 14 would be much smaller than this value and the cluster would have to be
considered as "kinematically frigid", thereby possibly posing a challenge for
Newtonian dynamics but in the opposite sense to MOND.Comment: 8 pages, 4 figures, accepted for publication in Ap
File-Sharing and Film Revenues: An Empirical Analysis
This study examines the impact of peer-to-peer (P2P) file-sharing on the Australian theatrical film industry. Using a large data set of torrent downloads observed on three popular P2P networks, we find evidence of a sales displacement effect on box office revenues. However, although statistically significant, the economic significance of this displacement appears relatively small. To establish causality, we make use of two precedent-setting Australian Federal Court case rulings, as well as observed levels of contemporaneous downloading in geographically separated markets within Australia. We observe that the release gap between the US and Australian markets is a key contributor to piracy early in a film's theatrical life; this finding provides a partial explanation for the industry's move toward coordinated worldwide releases
The Most Massive Black Holes in the Universe: Effects of Mergers in Massive Galaxy Clusters
Recent observations support the idea that nuclear black holes grew by gas
accretion while shining as luminous quasars at high redshift, and they
establish a relation of the black hole mass with the host galaxy's spheroidal
stellar system. We develop an analytic model to calculate the expected impact
of mergers on the masses of black holes in massive clusters of galaxies. We use
the extended Press-Schechter formalism to generate Monte Carlo merger histories
of halos with a mass 10^{15} h^{-1} Msun. We assume that the black hole mass
function at z=2 is similar to that inferred from observations at z=0 (since
quasar activity declines markedly at z<2), and we assign black holes to the
progenitor halos assuming a monotonic relation between halo mass and black hole
mass. We follow the dynamical evolution of subhalos within larger halos,
allowing for tidal stripping, the loss of orbital energy by dynamical friction,
and random orbital perturbations in gravitational encounters with subhalos, and
we assume that mergers of subhalos are followed by mergers of their central
black holes. Our analytic model reproduces numerical estimates of the subhalo
mass function. We find that the most massive black holes in massive clusters
typically grow by a factor ~ 2 by mergers after gas accretion has stopped. In
our ten realizations of 10^{15} h^{-1} Msun clusters, the highest initial (z=2)
black hole masses are 5-7 x 10^9 Msun, but four of the clusters contain black
holes in the range 1-1.5 x 10^{10} Msun at z=0. Satellite galaxies may host
black holes whose mass is comparable to, or even greater than, that of the
central galaxy. Thus, black hole mergers can significantly extend the very high
end of the black hole mass function.Comment: 13 pages, 7 figures, accepted for publication in The Astrophysical
Journa
A Fair and Secure Cluster Formation Process for Ad Hoc Networks
An efficient approach for organizing large ad hoc networks is to divide the nodes
into multiple clusters and designate, for each cluster, a clusterhead which is responsible for
holding intercluster control information. The role of a clusterhead entails rights and duties.
On the one hand, it has a dominant position in front of the others because it manages the
connectivity and has access to other nodeÂżs sensitive information. But on the other hand, the
clusterhead role also has some associated costs. Hence, in order to prevent malicious nodes
from taking control of the group in a fraudulent way and avoid selfish attacks from suitable
nodes, the clusterhead needs to be elected in a secure way. In this paper we present a novel
solution that guarantees the clusterhead is elected in a cheat-proof manner
Measuring the Cosmological Geometry from the Lyman Alpha Forest along Parallel Lines of Sight
We discuss the feasibility of measuring the cosmological metric using the
redshift space correlation function of the Lya forest in multiple lines of
sight, as a function of angular and velocity separation. The geometric
parameter that is measured is f(z) = H(z) D(z)/c, where H(z) is the Hubble
constant and D(z) the angular diameter distance at redshift z. The correlation
function is computed in linear theory. We describe a method to measure it from
observations with the Gaussianization procedure of Croft et al (1998) to map
the Lya forest transmitted flux to an approximation of the linear density
field. The effect of peculiar velocities on the shape of the recovered power
spectrum is pointed out. We estimate the error in recovering the f(z) factor
from observations due to the variance in the Lya absorbers. We show that ~ 20
pairs of quasars (separations < 3') are needed to distinguish a flat \Omega_0=1
universe from a universe with \Omega_0=0.2, \Omega_\Lambda=0.8. A second
parameter that is obtained from the correlation function of the Lya forest is
\beta \simeq \Omega(z)^{0.6}/b (affecting the magnitude of the peculiar
velocities), where b is a linear theory bias of the Lya forest. The statistical
error of f(z) is reduced if b can be determined independently from numerical
simulations, reducing the number of quasar pairs needed for constraining
cosmology to approximately six. On small scales, where the correlation function
is higher, f(z) should be measurable with fewer quasars, but non-linear effects
must then be taken into account. The anisotropy of the non-linear redshift
space correlation function as a function of scale should also provide a precise
quantitative test of the gravitational instability theory of the Lya forest.Comment: submitted to Ap
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