7,482 research outputs found
Comparison of mean distance in superposed networks
AbstractIn this paper, we consider the problem of the superposition of two networks in the structure of local telecommunication networks: the physical one, determined by the ways of the means of communication, and the interconnection one. We compare the mean-distance of the graphs associated to these two networks
Achieving the Optimal Steaming Capacity and Delay Using Random Regular Digraphs in P2P Networks
In earlier work, we showed that it is possible to achieve
streaming delay with high probability in a peer-to-peer network, where each
peer has as little as four neighbors, while achieving any arbitrary fraction of
the maximum possible streaming rate. However, the constant in the
delay term becomes rather large as we get closer to the maximum streaming rate.
In this paper, we design an alternative pairing and chunk dissemination
algorithm that allows us to transmit at the maximum streaming rate while
ensuring that all, but a negligible fraction of the peers, receive the data
stream with delay with high probability. The result is established
by examining the properties of graph formed by the union of two or more random
1-regular digraphs, i.e., directed graphs in which each node has an incoming
and an outgoing node degree both equal to one
Characteristics of broadband lightning emissions associated with terrestrial gamma ray flashes
To characterize lightning processes that produce terrestrial gamma ray flashes (TGFs), we have analyzed broadband (<1 Hz to 30 kHz) lightning magnetic fields for TGFs detected by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite in 2004-2009. The majority (96%) of 56 TGF-associated lightning signals contain single or multiple VLF impulses superposed on a slow pulse that reflects a process raising considerable negative charge within 2-6 ms. Some TGF lightning emissions also contain VLF signals that precede any appreciable slow pulse and that we term precursor sferics. The analyses of 9 TGFs related to lightning discharges with location uncertainty <100 km consistently indicate that TGFs are temporally linked to the early portion of the slow process and associated VLF impulses, and not to precursor sferics. The nearly universal presence of a slow pulse suggests that the slow process plays an important role in gamma ray production. In all cases the slow process raises negative charge with a typical mean current moment of +30 kA km. The resulting charge moment change ranges from small values below +10 C km to a maximum of +200 C km, with an average of +64 C km. The current moment waveform extracted from TGF sferics with single or multiple VLF impulses also shows that the slow process initiates shortly before the major TGF-associated fast discharge. These features are generally consistent with the TGF-lightning sequence reported by Lu et al. (2010), suggesting that the majority of RHESSI TGFs are produced during the upward negative leader progression prevalent in normal polarity intracloud flashes
Information content based model for the topological properties of the gene regulatory network of Escherichia coli
Gene regulatory networks (GRN) are being studied with increasingly precise
quantitative tools and can provide a testing ground for ideas regarding the
emergence and evolution of complex biological networks. We analyze the global
statistical properties of the transcriptional regulatory network of the
prokaryote Escherichia coli, identifying each operon with a node of the
network. We propose a null model for this network using the content-based
approach applied earlier to the eukaryote Saccharomyces cerevisiae. (Balcan et
al., 2007) Random sequences that represent promoter regions and binding
sequences are associated with the nodes. The length distributions of these
sequences are extracted from the relevant databases. The network is constructed
by testing for the occurrence of binding sequences within the promoter regions.
The ensemble of emergent networks yields an exponentially decaying in-degree
distribution and a putative power law dependence for the out-degree
distribution with a flat tail, in agreement with the data. The clustering
coefficient, degree-degree correlation, rich club coefficient and k-core
visualization all agree qualitatively with the empirical network to an extent
not yet achieved by any other computational model, to our knowledge. The
significant statistical differences can point the way to further research into
non-adaptive and adaptive processes in the evolution of the E. coli GRN.Comment: 58 pages, 3 tables, 22 figures. In press, Journal of Theoretical
Biology (2009)
Modeling Ferro- and Antiferromagnetic Interactions in Metal-Organic Coordination Networks
Magnetization curves of two rectangular metal-organic coordination networks
formed by the organic ligand TCNQ (7,7,8,8-tetracyanoquinodimethane) and two
different (Mn and Ni) 3d transition metal atoms [M(3d)] show marked differences
that are explained using first principles density functional theory and model
calculations. We find that the existence of a weakly dispersive hybrid band
with M(3d) and TCNQ character crossing the Fermi level is determinant for the
appearance of ferromagnetic coupling between metal centers, as it is the case
of the metallic system Ni-TCNQ but not of the insulating system Mn-TCNQ. The
spin magnetic moment localized at the Ni atoms induces a significant spin
polarization in the organic molecule; the corresponding spin density being
delocalized along the whole system. The exchange interaction between localized
spins at Ni centers and the itinerant spin density is ferromagnetic. Based on
two different model Hamiltonians, we estimate the strength of exchange
couplings between magnetic atoms for both Ni- and Mn-TCNQ networks that results
in weak ferromagnetic and very weak antiferromagnetic correlations for Ni- and
Mn-TCNQ networks, respectively.Comment: 27 pages, 6 figures, accepted for publication; Journal of Physical
Chemistry C (2014
- âŠ