40,977 research outputs found
Community structure in directed networks
We consider the problem of finding communities or modules in directed
networks. The most common approach to this problem in the previous literature
has been simply to ignore edge direction and apply methods developed for
community discovery in undirected networks, but this approach discards
potentially useful information contained in the edge directions. Here we show
how the widely used benefit function known as modularity can be generalized in
a principled fashion to incorporate the information contained in edge
directions. This in turn allows us to find communities by maximizing the
modularity over possible divisions of a network, which we do using an algorithm
based on the eigenvectors of the corresponding modularity matrix. This method
is shown to give demonstrably better results than previous methods on a variety
of test networks, both real and computer-generated.Comment: 5 pages, 3 figure
Clustering and preferential attachment in growing networks
We study empirically the time evolution of scientific collaboration networks
in physics and biology. In these networks, two scientists are considered
connected if they have coauthored one or more papers together. We show that the
probability of scientists collaborating increases with the number of other
collaborators they have in common, and that the probability of a particular
scientist acquiring new collaborators increases with the number of his or her
past collaborators. These results provide experimental evidence in favor of
previously conjectured mechanisms for clustering and power-law degree
distributions in networks.Comment: 13 pages, 2 figure
Characterizing the structure of small-world networks
We give exact relations which are valid for small-world networks (SWN's) with
a general `degree distribution', i.e the distribution of nearest-neighbor
connections. For the original SWN model, we illustrate how these exact
relations can be used to obtain approximations for the corresponding basic
probability distribution. In the limit of large system sizes and small
disorder, we use numerical studies to obtain a functional fit for this
distribution. Finally, we obtain the scaling properties for the mean-square
displacement of a random walker, which are determined by the scaling behavior
of the underlying SWN
Degree Correlations in Random Geometric Graphs
Spatially embedded networks are important in several disciplines. The
prototypical spatial net- work we assume is the Random Geometric Graph of which
many properties are known. Here we present new results for the two-point degree
correlation function in terms of the clustering coefficient of the graphs for
two-dimensional space in particular, with extensions to arbitrary finite
dimension
Mean-field solution of the small-world network model
The small-world network model is a simple model of the structure of social
networks, which simultaneously possesses characteristics of both regular
lattices and random graphs. The model consists of a one-dimensional lattice
with a low density of shortcuts added between randomly selected pairs of
points. These shortcuts greatly reduce the typical path length between any two
points on the lattice. We present a mean-field solution for the average path
length and for the distribution of path lengths in the model. This solution is
exact in the limit of large system size and either large or small number of
shortcuts.Comment: 14 pages, 2 postscript figure
Community Structure in the United States House of Representatives
We investigate the networks of committee and subcommittee assignments in the
United States House of Representatives from the 101st--108th Congresses, with
the committees connected by ``interlocks'' or common membership. We examine the
community structure in these networks using several methods, revealing strong
links between certain committees as well as an intrinsic hierarchical structure
in the House as a whole. We identify structural changes, including additional
hierarchical levels and higher modularity, resulting from the 1994 election, in
which the Republican party earned majority status in the House for the first
time in more than forty years. We also combine our network approach with
analysis of roll call votes using singular value decomposition to uncover
correlations between the political and organizational structure of House
committees.Comment: 44 pages, 13 figures (some with multiple parts and most in color), 9
tables, to appear in Physica A; new figures and revised discussion (including
extra introductory material) for this versio
Dynamics of Epidemics
This article examines how diseases on random networks spread in time. The
disease is described by a probability distribution function for the number of
infected and recovered individuals, and the probability distribution is
described by a generating function. The time development of the disease is
obtained by iterating the generating function. In cases where the disease can
expand to an epidemic, the probability distribution function is the sum of two
parts; one which is static at long times, and another whose mean grows
exponentially. The time development of the mean number of infected individuals
is obtained analytically. When epidemics occur, the probability distributions
are very broad, and the uncertainty in the number of infected individuals at
any given time is typically larger than the mean number of infected
individuals.Comment: 4 pages and 3 figure
Small-World Networks: Links with long-tailed distributions
Small-world networks (SWN), obtained by randomly adding to a regular
structure additional links (AL), are of current interest. In this article we
explore (based on physical models) a new variant of SWN, in which the
probability of realizing an AL depends on the chemical distance between the
connected sites. We assume a power-law probability distribution and study
random walkers on the network, focussing especially on their probability of
being at the origin. We connect the results to L\'evy Flights, which follow
from a mean field variant of our model.Comment: 11 pages, 4 figures, to appear in Phys.Rev.
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