5,320 research outputs found
Generic Absorbing Transition in Coevolution Dynamics
We study a coevolution voter model on a network that evolves according to the
state of the nodes. In a single update, a link between opposite-state nodes is
rewired with probability , while with probability one of the nodes
takes its neighbor's state. A mean-field approximation reveals an absorbing
transition from an active to a frozen phase at a critical value
that only depends on the average degree of the
network. The approach to the final state is characterized by a time scale that
diverges at the critical point as . We find that the
active and frozen phases correspond to a connected and a fragmented network
respectively. We show that the transition in finite-size systems can be seen as
the sudden change in the trajectory of an equivalent random walk at the
critical rewiring rate , highlighting the fact that the mechanism behind
the transition is a competition between the rates at which the network and the
state of the nodes evolve.Comment: 5 pages, 4 figure
Lensing of Fast Radio Bursts by Plasma Structures in Host Galaxies
Plasma lenses in the host galaxies of fast radio bursts (FRBs) can strongly
modulate FRB amplitudes for a wide range of distances, including the
Gpc distance of the repeater FRB121102. To produce caustics, the lens'
dispersion-measure depth (), scale size (), and distance
from the source () must satisfy . Caustics produce strong
magnifications () on short time scales ( hours to days and
perhaps shorter) along with narrow, epoch dependent spectral peaks (0.1 to
1~GHz). However, strong suppression also occurs in long-duration (
months) troughs. For geometries that produce multiple images, the resulting
burst components will arrive differentially by s to tens of ms and
they will show different apparent dispersion measures, pc cm. Arrival time perturbations may mask any
underlying periodicity with period s. When arrival times differ by
less than the burst width, interference effects in dynamic spectra are
expected. Strong lensing requires source sizes smaller than , which can be satisfied by compact objects such as
neutron star magnetospheres but not by AGNs. Much of the phenomenology of the
repeating fast radio burst source FRB121102 is similar to lensing effects. The
overall picture can be tested by obtaining wideband spectra of bursts (from
to 10 GHz and possibly higher), which can also be used to characterize the
plasma environment near FRB sources. A rich variety of phenomena is expected
from an ensemble of lenses near the FRB source. We discuss constraints on
densities, magnetic fields, and locations of plasma lenses related to
requirements for lensing to occur.Comment: 11 pages, 7 figures, submitted to the Astrophysical Journa
Post-mission Viking data anaysis
Three Mars data analysis projects from the Viking Mars program were identified initially, and three more came into being as the work proceeded. All together, these six pertained to: (1) the veritical distribution of scattering particles in the Martian atmosphere at various locations in various seasons, (2) the physical parameters that define photometric properties of the Martian surface and atmosphere, (3) patterns of dust-cloud and global dust-storm development, (4) a direct comparison of near-simultaneous Viking and ground-based observations, (5) the annual formation and dissipation of polar frost caps, and (6) evidence concerning possible present-day volcanism or venting. A list of publications pertaining to the appropriate projects is included
Understanding the language of gene regulation
A report on the Cold Spring Harbor Laboratory meeting 'Systems Biology: genomic approaches to transcriptional regulation', Cold Spring Harbor, USA, 6-9 March 2003
Finding local community structure in networks
Although the inference of global community structure in networks has recently
become a topic of great interest in the physics community, all such algorithms
require that the graph be completely known. Here, we define both a measure of
local community structure and an algorithm that infers the hierarchy of
communities that enclose a given vertex by exploring the graph one vertex at a
time. This algorithm runs in time O(d*k^2) for general graphs when is the
mean degree and k is the number of vertices to be explored. For graphs where
exploring a new vertex is time-consuming, the running time is linear, O(k). We
show that on computer-generated graphs this technique compares favorably to
algorithms that require global knowledge. We also use this algorithm to extract
meaningful local clustering information in the large recommender network of an
online retailer and show the existence of mesoscopic structure.Comment: 7 pages, 6 figure
Semiannual Status Report, 1 July 1975 Through 31 December 1975
No abstract availabl
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
Continuum states from time-dependent density functional theory
Linear response time-dependent density functional theory is used to study
low-lying electronic continuum states of targets that can bind an extra
electron. Exact formulas to extract scattering amplitudes from the
susceptibility are derived in one dimension. A single-pole approximation for
scattering phase shifts in three dimensions is shown to be more accurate than
static exchange for singlet electron-He scattering.Comment: 5 pages, 2 figures, J. Chem. Phys. accepte
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