5,534 research outputs found
Social dynamics in conferences: analyses of data from the Live Social Semantics application
Popularity and spread of online social networking in recent years has given a great momentum to the study of dynamics and patterns of social interactions. However, these studies have often been confined to the online world, neglecting its interdependencies with the offline world. This is mainly due to the lack of real data that spans across this divide. The Live Social Semantics application is a novel platform that dissolves this divide, by collecting and integrating data about people from (a) their online social networks and tagging activities from popular social networking sites, (b) their publications and co-authorship networks from semantic repositories, and (c) their real-world face-to-face contacts with other attendees collected via a network of wearable active sensors. This paper investigates the data collected by this application during its deployment at three major conferences, where it was used by more than 400 people. Our analyses show the robustness of the patterns of contacts at various conferences, and the influence of various personal properties (e.g. seniority, conference attendance) on social networking patterns
Reinforced communication and social navigation generate groups in model networks
To investigate the role of information flow in group formation, we introduce
a model of communication and social navigation. We let agents gather
information in an idealized network society, and demonstrate that heterogeneous
groups can evolve without presuming that individuals have different interests.
In our scenario, individuals' access to global information is constrained by
local communication with the nearest neighbors on a dynamic network. The result
is reinforced interests among like-minded agents in modular networks; the flow
of information works as a glue that keeps individuals together. The model
explains group formation in terms of limited information access and highlights
global broadcasting of information as a way to counterbalance this
fragmentation. To illustrate how the information constraints imposed by the
communication structure affects future development of real-world systems, we
extrapolate dynamics from the topology of four social networks.Comment: 7 pages, 3 figure
The 13 November 1984 occultation of BD +08 deg 0471 by (1) Ceres
The 13 November 1984 occultation of BD +08 deg 0471 was discovered during a photographic search carried out with the 0.5 meter Carnegie Double Astrograph at Lick Observatory and the Lowell Observatory PDS microdensitometer. Such a search was stimulated by the curious fact that few favorably located occultations of AGK3 or SAO catalog starts by Ceres will occur during the 1980s. The occultation on 13 November, however, is a particularly good event. The star is 1000 cubic M in V, yielding a predicted drop at occultation of about 10%. Such a drop can be detected by small telescopes equipped with photoelectric photometers, but is too small to be seen visually. The track was predicted to cross the Caribbean, Florida, southern Texas, and Mexico. Based on this prediction, preparations were made to observe the event in Mexico using four portable occultation data systems
Inferring the Spatial and Energy Distribution of Gamma Ray Burst Sources. II. Isotropic Models
We use Bayesian methods to analyze the distribution of gamma ray burst
intensities reported in the Third BATSE Catalog (3B catalog) of gamma ray
bursts, presuming the distribution of burst sources (``bursters'') is
isotropic. We study both phenomenological and cosmological source distribution
models, using Bayes's theorem both to infer unknown parameters in the models,
and to compare rival models. We analyze the distribution of the time-averaged
peak photon number flux, F, measured on both 64 ms and 1024 ms time scales,
performing the analysis of data based on each time scale independently. Several
of our findings differ from those of previous analyses that modeled burst
detection less completely. In particular, we find that the width of the
intrinsic luminosity function for bursters is unconstrained, and the luminosity
function of the actually observed bursts can be extremely broad, in contrast to
the findings of all previous studies. Useful constraints probably require
observation of bursts significantly fainter than those visible to BATSE. We
also find that the 3B peak flux data do not usefully constrain the redshifts of
burst sources; useful constraints require the analysis of data beyond that in
the 3B catalog (such as burst time histories), or data from brighter bursts
than have been seen by BATSE (such as those observed by the Pioneer Venus
Orbiter). In addition, we find that an accurate understanding of the peak flux
distributions reported in the 3B almost certainly requires consideration of
data on the temporal and spectral properties of bursts beyond that reported in
the 3B catalog, and more sophisticated modeling than has so far been attempted.Comment: 66 pages, 33 figures; submitted to The Astrophysical Journal, 12/9
A New Algorithm for Automatic History Matching
History-matching problems, in which reservoir parameters arc to be estimated from well pressure data, are formulated as optimal control problems. The necessary conditions for optimality lead naturally to gradient optimization methods for determining the optimal parameter estimates. The key feature of the approach is that reservoir properties are considered as continuous functions
of position rather than as uniform in a certain number of zones. The optimal control approach is illustrated on a hypothetical reservoir and on an actual Saudi Arabian reservoir, both characterized by single-phase flow. A significant saving in computing time over conventional constant-zone gradient optimization methods is demonstrated
Finding community structure in very large networks
The discovery and analysis of community structure in networks is a topic of
considerable recent interest within the physics community, but most methods
proposed so far are unsuitable for very large networks because of their
computational cost. Here we present a hierarchical agglomeration algorithm for
detecting community structure which is faster than many competing algorithms:
its running time on a network with n vertices and m edges is O(m d log n) where
d is the depth of the dendrogram describing the community structure. Many
real-world networks are sparse and hierarchical, with m ~ n and d ~ log n, in
which case our algorithm runs in essentially linear time, O(n log^2 n). As an
example of the application of this algorithm we use it to analyze a network of
items for sale on the web-site of a large online retailer, items in the network
being linked if they are frequently purchased by the same buyer. The network
has more than 400,000 vertices and 2 million edges. We show that our algorithm
can extract meaningful communities from this network, revealing large-scale
patterns present in the purchasing habits of customers
Electronic Control of Spin Alignment in pi-Conjugated Molecular Magnets
Intramolecular spin alignment in pi-conjugated molecules is studied
theoretically in a model of a Peierls-Hubbard chain coupled with two localized
spins. By means of the exact diagonalization technique, we demonstrate that a
spin singlet (S=0) to quartet (S=3/2) transition can be induced by electronic
doping, depending on the chain length, the positions of the localized spins,
and the sign of the electron-spin coupling. The calculated results provides a
theoretical basis for understanding the mechanism of spin alignment recently
observed in a diradical donor molecule.Comment: 4 pages, 4 figures, Physical Review Letters (in press
Time-dependent Density Functional calculation of e-H scattering
Phase shifts for single-channel elastic electron-atom scattering are derived
from time-dependent density functional theory. The H ion is placed in a
spherical box, its discrete spectrum found, and phase shifts deduced.
Exact-exchange yields an excellent approximation to the ground-state Kohn-Sham
potential, while the adiabatic local density approximation yields good singlet
and triplet phase shifts.Comment: 5 pages, 4 figures, 1 tabl
Modification to the Luminosity Distance Redshift Relation in Modified Gravity Theories
We derive an expression for the luminosity distance as a function of redshift
for a flat Robertson-Walker spacetime perturbed by arbitrary scalar
perturbations possibly produced by a modified gravity theory with two different
scalar perturbation potentials. Measurements of the luminosity distance as
function of redshift provide a constraint on a combination of the scalar
potentials and so they can complement weak lensing and other measurements in
trying to distinguish among the various alternative theories of gravity.Comment: 15 pages, we discuss in more detail how the luminosity distance
expression can be used to differentiate among various theories of gravit
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