777 research outputs found
All scale-free networks are sparse
We study the realizability of scale free-networks with a given degree
sequence, showing that the fraction of realizable sequences undergoes two
first-order transitions at the values 0 and 2 of the power-law exponent. We
substantiate this finding by analytical reasoning and by a numerical method,
proposed here, based on extreme value arguments, which can be applied to any
given degree distribution. Our results reveal a fundamental reason why large
scale-free networks without constraints on minimum and maximum degree must be
sparse.Comment: 4 pages, 2 figure
Degree correlations in directed scale-free networks
Scale-free networks, in which the distribution of the degrees obeys a
power-law, are ubiquitous in the study of complex systems. One basic network
property that relates to the structure of the links found is the degree
assortativity, which is a measure of the correlation between the degrees of the
nodes at the end of the links. Degree correlations are known to affect both the
structure of a network and the dynamics of the processes supported thereon,
including the resilience to damage, the spread of information and epidemics,
and the efficiency of defence mechanisms. Nonetheless, while many studies focus
on undirected scale-free networks, the interactions in real-world systems often
have a directionality. Here, we investigate the dependence of the degree
correlations on the power-law exponents in directed scale-free networks. To
perform our study, we consider the problem of building directed networks with a
prescribed degree distribution, providing a method for proper generation of
power-law-distributed directed degree sequences. Applying this new method, we
perform extensive numerical simulations, generating ensembles of directed
scale-free networks with exponents between~2 and~3, and measuring ensemble
averages of the Pearson correlation coefficients. Our results show that
scale-free networks are on average uncorrelated across directed links for three
of the four possible degree-degree correlations, namely in-degree to in-degree,
in-degree to out-degree, and out-degree to out-degree. However, they exhibit
anticorrelation between the number of outgoing connections and the number of
incoming ones. The findings are consistent with an entropic origin for the
observed disassortativity in biological and technological networks.Comment: 10 pages, 5 figure
Depth-dependent ordering, two-length-scale phenomena and crossover behavior in a crystal featuring a skin-layer with defects
Structural defects in a crystal are responsible for the "two length-scale"
behavior, in which a sharp central peak is superimposed over a broad peak in
critical diffuse X-ray scattering. We have previously measured the scaling
behavior of the central peak by scattering from a near-surface region of a V2H
crystal, which has a first-order transition in the bulk. As the temperature is
lowered toward the critical temperature, a crossover in critical behavior is
seen, with the temperature range nearest to the critical point being
characterized by mean field exponents. Near the transition, a small two-phase
coexistence region is observed. The values of transition and crossover
temperatures decay with depth. An explanation of these experimental results is
here proposed by means of a theory in which edge dislocations in the
near-surface region occur in walls oriented in the two directions normal to the
surface. The strain caused by the dislocation lines causes the ordering in the
crystal to occur as growth of roughly cylindrically shaped regions. After the
regions have reached a certain size, the crossover in the critical behavior
occurs, and mean field behavior prevails. At a still lower temperature, the
rest of the material between the cylindrical regions orders via a weak
first-order transition.Comment: 12 pages, 8 figure
Efficient and exact sampling of simple graphs with given arbitrary degree sequence
Uniform sampling from graphical realizations of a given degree sequence is a
fundamental component in simulation-based measurements of network observables,
with applications ranging from epidemics, through social networks to Internet
modeling. Existing graph sampling methods are either link-swap based
(Markov-Chain Monte Carlo algorithms) or stub-matching based (the Configuration
Model). Both types are ill-controlled, with typically unknown mixing times for
link-swap methods and uncontrolled rejections for the Configuration Model. Here
we propose an efficient, polynomial time algorithm that generates statistically
independent graph samples with a given, arbitrary, degree sequence. The
algorithm provides a weight associated with each sample, allowing the
observable to be measured either uniformly over the graph ensemble, or,
alternatively, with a desired distribution. Unlike other algorithms, this
method always produces a sample, without back-tracking or rejections. Using a
central limit theorem-based reasoning, we argue, that for large N, and for
degree sequences admitting many realizations, the sample weights are expected
to have a lognormal distribution. As examples, we apply our algorithm to
generate networks with degree sequences drawn from power-law distributions and
from binomial distributions.Comment: 8 pages, 3 figure
Mapping structural diversity in networks sharing a given degree distribution and global clustering: Adaptive resolution grid search evolution with Diophantine equation-based mutations
Methods that generate networks sharing a given degree distribution and global clustering can induce changes in structural properties other than that controlled for. Diversity in structural properties, in turn, can affect the outcomes of dynamical processes operating on those networks. Since exhaustive sampling is not possible, we propose a novel evolutionary framework for mapping this structural diversity. The three main features of this framework are: (a) subgraph-based encoding of networks, (b) exact mutations based on solving systems of Diophantine equations, and (c) heuristic diversity-driven mechanism to drive resolution changes in the MapElite algorithm.We show that our framework can elicit networks with diversity in their higher-order structure and that this diversity affects the behaviour of the complex contagion model. Through a comparison with state of the art clustered network generation methods, we demonstrate that our approach can uncover a comparably diverse range of networks without needing computationally unfeasible mixing times. Further, we suggest that the subgraph-based encoding provides greater confidence in the diversity of higher-order network structure for low numbers of samples and is the basis for explaining our results with complex contagion model. We believe that this framework could be applied to other complex landscapes that cannot be practically mapped via exhaustive sampling
Impact of Space Weather on Climate and Habitability of Terrestrial Type Exoplanets
The current progress in the detection of terrestrial type exoplanets has
opened a new avenue in the characterization of exoplanetary atmospheres and in
the search for biosignatures of life with the upcoming ground-based and space
missions. To specify the conditions favorable for the origin, development and
sustainment of life as we know it in other worlds, we need to understand the
nature of astrospheric, atmospheric and surface environments of exoplanets in
habitable zones around G-K-M dwarfs including our young Sun. Global environment
is formed by propagated disturbances from the planet-hosting stars in the form
of stellar flares, coronal mass ejections, energetic particles, and winds
collectively known as astrospheric space weather. Its characterization will
help in understanding how an exoplanetary ecosystem interacts with its host
star, as well as in the specification of the physical, chemical and biochemical
conditions that can create favorable and/or detrimental conditions for
planetary climate and habitability along with evolution of planetary internal
dynamics over geological timescales. A key linkage of (astro) physical,
chemical, and geological processes can only be understood in the framework of
interdisciplinary studies with the incorporation of progress in heliophysics,
astrophysics, planetary and Earth sciences. The assessment of the impacts of
host stars on the climate and habitability of terrestrial (exo)planets will
significantly expand the current definition of the habitable zone to the
biogenic zone and provide new observational strategies for searching for
signatures of life. The major goal of this paper is to describe and discuss the
current status and recent progress in this interdisciplinary field and to
provide a new roadmap for the future development of the emerging field of
exoplanetary science and astrobiology.Comment: 206 pages, 24 figures, 1 table; Review paper. International Journal
of Astrobiology (2019
Evaluating models' response of tropical low clouds to SST forcings using CALIPSO observations
Recent studies have shown that, in response to a surface warming,
the marine tropical low-cloud cover (LCC) as observed by passive-sensor
satellites substantially decreases, therefore generating a smaller negative
value of the top-of-the-atmosphere (TOA) cloud radiative effect (CRE). Here we
study the LCC and CRE interannual changes in response to sea surface
temperature (SST) forcings in the GISS model E2 climate model, a
developmental version of the GISS model E3 climate model, and in 12 other
climate models, as a function of their ability to represent the vertical
structure of the cloud response to SST change against 10Â years of CALIPSO (Cloud-Aerosol Lidar and Infrared
Pathfinder Satellite Observations) observations.
The more realistic models (those that satisfy the observational
constraint) capture the observed interannual LCC change quite well
(ÎLCC/ÎSST=-3.49±1.01 % Kâ1 vs.
ÎLCC/ÎSSTobs=-3.59±0.28 % Kâ1) while the
others largely underestimate it (ÎLCC/ÎSST=-1.32±1.28 % Kâ1). Consequently, the more realistic models simulate more
positive shortwave (SW) feedback (ÎCRE/ÎSST=2.60±1.13 W mâ2 Kâ1)
than the less realistic models (ÎCRE/ÎSST=0.87±2.63 W mâ2 Kâ1), in better agreement with the
observations (ÎCRE/ÎSSTobs=3±0.26 W mâ2 Kâ1),
although slightly underestimated. The ability of the models to
represent moist processes within the planetary boundary layer (PBL) and produce
persistent stratocumulus (Sc) decks appears crucial to replicating the observed
relationship between clouds, radiation and surface temperature. This
relationship is different depending on the type of low clouds in the
observations. Over stratocumulus regions, cloud-top height increases slightly
with SST, accompanied by a large decrease in cloud fraction, whereas over
trade cumulus (Cu) regions, cloud fraction decreases everywhere, to a smaller
extent.</p
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