6,091 research outputs found
Dynamical Properties of Interaction Data
Network dynamics are typically presented as a time series of network
properties captured at each period. The current approach examines the dynamical
properties of transmission via novel measures on an integrated, temporally
extended network representation of interaction data across time. Because it
encodes time and interactions as network connections, static network measures
can be applied to this "temporal web" to reveal features of the dynamics
themselves. Here we provide the technical details and apply it to agent-based
implementations of the well-known SEIR and SEIS epidemiological models.Comment: 29 pages, 15 figure
X-ray sources as tracers of the large-scale structure in the Universe
We review the current status of studies of large-scale structure in the X-ray
Universe. After motivating the use X-rays for cosmological purposes, we discuss
the various approaches used on different angular scales including X-ray
background multipoles, cross-correlations of the X-ray background with galaxy
catalogues, clustering of X-ray selected sources and small-scale fluctuations
and anisotropies in the X-ray background. We discuss the implications of the
above studies for the bias parameter of X-ray sources, which is likely to be
moderate for X-ray selected AGN and the X-ray background (~1-2). We finally
outline how all-sky X-ray maps at hard X-rays and medium surveys with large sky
coverage could provide important tests for the cosmological models.Comment: Invited review presented at the Workshop X-ray Astronomy'99: Stellar
endpoints, AGN and the diffuse X-ray background (Astrophys Lett and Comm
Bulk Viscosity driven clusterization of quark-gluon plasma and early freeze-out in relativistic heavy-ion collisions
We introduce a new scenario for heavy ion collisions that could solve the
lingering problems associated with the so-called HBT puzzle. We postulate that
the system starts expansion as the perfect quark-gluon fluid but close to
freeze-out it splits into clusters, due to a sharp rise of bulk viscosity in
the vicinity of the hadronization transition. We then argue that the
characteristic cluster size is determined by the viscosity coefficient and the
expansion rate. Typically it is much smaller and independent of the total
system volume. These clusters maintain the pre-existing outward-going flow, as
a spray of droplets, but develop no flow of their own, and hadronize by
evaporation. We provide an ansatz for converting the hydrodynamic output into
clusters.Comment: Accepted for publication, Phys. Rev. C. Arguments considerably
expanded, refined and reworde
Router-level community structure of the Internet Autonomous Systems
The Internet is composed of routing devices connected between them and
organized into independent administrative entities: the Autonomous Systems. The
existence of different types of Autonomous Systems (like large connectivity
providers, Internet Service Providers or universities) together with
geographical and economical constraints, turns the Internet into a complex
modular and hierarchical network. This organization is reflected in many
properties of the Internet topology, like its high degree of clustering and its
robustness.
In this work, we study the modular structure of the Internet router-level
graph in order to assess to what extent the Autonomous Systems satisfy some of
the known notions of community structure. We show that the modular structure of
the Internet is much richer than what can be captured by the current community
detection methods, which are severely affected by resolution limits and by the
heterogeneity of the Autonomous Systems. Here we overcome this issue by using a
multiresolution detection algorithm combined with a small sample of nodes. We
also discuss recent work on community structure in the light of our results
Complex networks of earthquakes and aftershocks
We invoke a metric to quantify the correlation between any two earthquakes.
This provides a simple and straightforward alternative to using space-time
windows to detect aftershock sequences and obviates the need to distinguish
main shocks from aftershocks. Directed networks of earthquakes are constructed
by placing a link, directed from the past to the future, between pairs of
events that are strongly correlated. Each link has a weight giving the relative
strength of correlation such that the sum over the incoming links to any node
equals unity for aftershocks, or zero if the event had no correlated
predecessors. A correlation threshold is set to drastically reduce the size of
the data set without losing significant information. Events can be aftershocks
of many previous events, and also generate many aftershocks. The probability
distribution for the number of incoming and outgoing links are both scale free,
and the networks are highly clustered. The Omori law holds for aftershock rates
up to a decorrelation time that scales with the magnitude, , of the
initiating shock as with .
Another scaling law relates distances between earthquakes and their aftershocks
to the magnitude of the initiating shock. Our results are inconsistent with the
hypothesis of finite aftershock zones. We also find evidence that seismicity is
dominantly triggered by small earthquakes. Our approach, using concepts from
the modern theory of complex networks, together with a metric to estimate
correlations, opens up new avenues of research, as well as new tools to
understand seismicity.Comment: 12 pages, 12 figures, revtex
Effects of receptor clustering on ligand dissociation: Theory and simulations
Receptor-ligand binding is a critical first step in signal transduction and
the duration of the interaction can impact signal generation. In mammalian
cells, clustering of receptors may be facilitated by heterogeneous zones of
lipids, known as lipid rafts. In vitro experiments show that disruption of
rafts significantly alters the dissociation of fibroblast growth factor-2
(FGF-2) from heparan sulfate proteoglycans, co-receptors for FGF-2. In this
paper, we develop a continuum stochastic formalism in order to (i) study how
rebinding affects the dissociation of ligands from a planar substrate, and (ii)
address the question of how receptor clustering influences ligand rebinding. We
find that clusters reduce the effective dissociation rate dramatically when the
clusters are dense and the overall surface density of receptors is low. The
effect is much less pronounced in the case of high receptor density and shows
non-monotonic behavior with time. These predictions are verified via lattice
Monte Carlo simulations. Comparison with experimental results suggests that the
theory does not capture the complete biological system. We speculate that
additional co-operative mechanisms might be present in order to increase ligand
retention, and present one possible ``internal diffusion'' model.Comment: Expanded text and added figures, revised version to appear in
Biophys.
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