108 research outputs found
Transport on weighted Networks: when correlations are independent of degree
Most real-world networks are weighted graphs with the weight of the edges
reflecting the relative importance of the connections. In this work, we study
non degree dependent correlations between edge weights, generalizing thus the
correlations beyond the degree dependent case. We propose a simple method to
introduce weight-weight correlations in topologically uncorrelated graphs. This
allows us to test different measures to discriminate between the different
correlation types and to quantify their intensity. We also discuss here the
effect of weight correlations on the transport properties of the networks,
showing that positive correlations dramatically improve transport. Finally, we
give two examples of real-world networks (social and transport graphs) in which
weight-weight correlations are present.Comment: 8 pages, 8 figure
Combinatorial approach to Modularity
Communities are clusters of nodes with a higher than average density of
internal connections. Their detection is of great relevance to better
understand the structure and hierarchies present in a network. Modularity has
become a standard tool in the area of community detection, providing at the
same time a way to evaluate partitions and, by maximizing it, a method to find
communities. In this work, we study the modularity from a combinatorial point
of view. Our analysis (as the modularity definition) relies on the use of the
configurational model, a technique that given a graph produces a series of
randomized copies keeping the degree sequence invariant. We develop an approach
that enumerates the null model partitions and can be used to calculate the
probability distribution function of the modularity. Our theory allows for a
deep inquiry of several interesting features characterizing modularity such as
its resolution limit and the statistics of the partitions that maximize it.
Additionally, the study of the probability of extremes of the modularity in the
random graph partitions opens the way for a definition of the statistical
significance of network partitions.Comment: 8 pages, 4 figure
Social inertia in collaboration networks
This work is a study of the properties of collaboration networks employing
the formalism of weighted graphs to represent their one-mode projection. The
weight of the edges is directly the number of times that a partnership has been
repeated. This representation allows us to define the concept of "social
inertia" that measures the tendency of authors to keep on collaborating with
previous partners. We use a collection of empirical datasets to analyze several
aspects of the social inertia: 1) its probability distribution, 2) its
correlation with other properties, and 3) the correlations of the inertia
between neighbors in the network. We also contrast these empirical results with
the predictions of a recently proposed theoretical model for the growth of
collaboration networks.Comment: 7 pages, 5 figure
Percolative phase transition on ferromagnetic insulator manganites: uncorrelated to correlated polaron clusters
In this work, we report an atomic scale study on the ferromagnetic insulator
manganite LaMnO using PAC spectroscopy. Data analysis
reveals a nanoscopic transition from an undistorted to a Jahn-Teller-distorted
local environment upon cooling. The percolation thresholds of the two local
environments enclose a macroscopic structural transition
(Rhombohedric-Orthorhombic). Two distinct regimes of JT-distortions were found:
a high temperature regime where uncorrelated polaron clusters with severe
distortions of the MnO octahedra survive up to
and a low temperature regime where correlated regions have a weaker
JT-distorted symmetry.Comment: 4 pages, 4 Figures, submitted to PRL, new version with more data,
text reformulate
Statistical significance of communities in networks
Nodes in real-world networks are usually organized in local modules. These
groups, called communities, are intuitively defined as sub-graphs with a larger
density of internal connections than of external links. In this work, we
introduce a new measure aimed at quantifying the statistical significance of
single communities. Extreme and Order Statistics are used to predict the
statistics associated with individual clusters in random graphs. These
distributions allows us to define one community significance as the probability
that a generic clustering algorithm finds such a group in a random graph. The
method is successfully applied in the case of real-world networks for the
evaluation of the significance of their communities.Comment: 9 pages, 8 figures, 2 tables. The software to calculate the C-score
can be found at http://filrad.homelinux.org/cscor
Human dynamics revealed through Web analytics
When the World Wide Web was first conceived as a way to facilitate the
sharing of scientific information at the CERN (European Center for Nuclear
Research) few could have imagined the role it would come to play in the
following decades. Since then, the increasing ubiquity of Internet access and
the frequency with which people interact with it raise the possibility of using
the Web to better observe, understand, and monitor several aspects of human
social behavior. Web sites with large numbers of frequently returning users are
ideal for this task. If these sites belong to companies or universities, their
usage patterns can furnish information about the working habits of entire
populations. In this work, we analyze the properly anonymized logs detailing
the access history to Emory University's Web site. Emory is a medium size
university located in Atlanta, Georgia. We find interesting structure in the
activity patterns of the domain and study in a systematic way the main forces
behind the dynamics of the traffic. In particular, we show that both linear
preferential linking and priority based queuing are essential ingredients to
understand the way users navigate the Web.Comment: 7 pages, 8 figure
Langevin theory of absorbing phase transitions with a conserved magnitude
The recently proposed Langevin equation, aimed to capture the relevant
critical features of stochastic sandpiles, and other self-organizing systems is
studied numerically. This equation is similar to the Reggeon field theory,
describing generic systems with absorbing states, but it is coupled linearly to
a second conserved and static (non-diffusive) field. It has been claimed to
represent a new universality class, including different discrete models: the
Manna as well as other sandpiles, reaction-diffusion systems, etc. In order to
integrate the equation, and surpass the difficulties associated with its
singular noise, we follow a numerical technique introduced by Dickman. Our
results coincide remarkably well with those of discrete models claimed to
belong to this universality class, in one, two, and three dimensions. This
provides a strong backing for the Langevin theory of stochastic sandpiles, and
to the very existence of this new, yet meagerly understood, universality class.Comment: 4 pages, 3 eps figs, submitted to PR
Sticky grains do not change the universality class of isotropic sandpiles
We revisit the sandpile model with ``sticky'' grains introduced by Mohanty
and Dhar [Phys. Rev. Lett. {\bf 89}, 104303 (2002)] whose scaling properties
were claimed to be in the universality class of directed percolation for both
isotropic and directed models. Simulations in the so-called fixed-energy
ensemble show that this conclusion is not valid for isotropic sandpiles and
that this model shares the same critical properties of other stochastic
sandpiles, such as the Manna model. %as expected from the existence of an extra
%conservation-law, absent in directed percolation. These results are
strengthened by the analysis of the Langevin equations proposed by the same
authors to account for this problem which we show to converge, upon
coarse-graining, to the well-established set of Langevin equations for the
Manna class. Therefore, the presence of a conservation law keeps isotropic
sandpiles, with or without stickiness, away from the directed percolation
class.Comment: 4 pages. 3 Figures. Subm. to PR
Self-organization of collaboration networks
We study collaboration networks in terms of evolving, self-organizing
bipartite graph models. We propose a model of a growing network, which combines
preferential edge attachment with the bipartite structure, generic for
collaboration networks. The model depends exclusively on basic properties of
the network, such as the total number of collaborators and acts of
collaboration, the mean size of collaborations, etc. The simplest model defined
within this framework already allows us to describe many of the main
topological characteristics (degree distribution, clustering coefficient, etc.)
of one-mode projections of several real collaboration networks, without
parameter fitting. We explain the observed dependence of the local clustering
on degree and the degree--degree correlations in terms of the ``aging'' of
collaborators and their physical impossibility to participate in an unlimited
number of collaborations.Comment: 10 pages, 8 figure
- …