193,185 research outputs found
Measuring the likelihood of models for network evolution
Many researchers have hypothesised models which explain the evolution of the topology of a target network. The framework described in this paper gives the likelihood that the target network arose from the hypothesised model. This allows rival hypothesised models to be compared for their ability to explain the target network. A null model (of random evolution) is proposed as a baseline for comparison. The framework also considers models made from linear combinations of model components. A method is given for the automatic optimisation of component weights. The framework is tested on simulated networks with known parameters and also on real data
Measuring dark energy with the correlation of gamma-ray bursts using model-independent methods
In this paper, we use two model-independent methods to standardize long
gamma-ray bursts (GRBs) using the correlation, where
is the isotropic-equivalent gamma-ray energy and is
the spectral peak energy. We update 42 long GRBs and try to make constraint on
cosmological parameters. The full sample contains 151 long GRBs with redshifts
from 0.0331 to 8.2. The first method is the simultaneous fitting method. The
extrinsic scatter is taken into account and assigned to the
parameter . The best-fitting values are ,
, and in the flat
CDM model. The constraint on is at the
1 confidence level. If reduced method is used, the best-fit
results are , and . The
second method is using type Ia supernovae (SNe Ia) to calibrate the correlation. We calibrate 90 high-redshift GRBs in the redshift
range from 1.44 to 8.1. The cosmological constraints from these 90 GRBs are
for flat CDM, and
and for non-flat
CDM. For the combination of GRB and SNe Ia sample, we obtain
and for the flat CDM, and
for the non-flat CDM, the results are ,
and . These results from
calibrated GRBs are consistent with that of SNe Ia. Meanwhile, the combined
data can improve cosmological constraints significantly, comparing to SNe Ia
alone. Our results show that the correlation is
promising to probe the high-redshift universe.Comment: 10 pages, 6 figures, 4 table, accepted by A&A. Table 4 contains
calibrated distance moduli of GRB
A high-reproducibility and high-accuracy method for automated topic classification
Much of human knowledge sits in large databases of unstructured text.
Leveraging this knowledge requires algorithms that extract and record metadata
on unstructured text documents. Assigning topics to documents will enable
intelligent search, statistical characterization, and meaningful
classification. Latent Dirichlet allocation (LDA) is the state-of-the-art in
topic classification. Here, we perform a systematic theoretical and numerical
analysis that demonstrates that current optimization techniques for LDA often
yield results which are not accurate in inferring the most suitable model
parameters. Adapting approaches for community detection in networks, we propose
a new algorithm which displays high-reproducibility and high-accuracy, and also
has high computational efficiency. We apply it to a large set of documents in
the English Wikipedia and reveal its hierarchical structure. Our algorithm
promises to make "big data" text analysis systems more reliable.Comment: 23 pages, 24 figure
Early-warning signals of topological collapse in interbank networks
The financial crisis clearly illustrated the importance of characterizing the
level of 'systemic' risk associated with an entire credit network, rather than
with single institutions. However, the interplay between financial distress and
topological changes is still poorly understood. Here we analyze the quarterly
interbank exposures among Dutch banks over the period 1998-2008, ending with
the crisis. After controlling for the link density, many topological properties
display an abrupt change in 2008, providing a clear - but unpredictable -
signature of the crisis. By contrast, if the heterogeneity of banks'
connectivity is controlled for, the same properties show a gradual transition
to the crisis, starting in 2005 and preceded by an even earlier period during
which anomalous debt loops could have led to the underestimation of
counter-party risk. These early-warning signals are undetectable if the network
is reconstructed from partial bank-specific data, as routinely done. We discuss
important implications for bank regulatory policies.Comment: 28 pages, 23 figures, 1 tabl
Failure dynamics of the global risk network
Risks threatening modern societies form an intricately interconnected network
that often underlies crisis situations. Yet, little is known about how risk
materializations in distinct domains influence each other. Here we present an
approach in which expert assessments of risks likelihoods and influence
underlie a quantitative model of the global risk network dynamics. The modeled
risks range from environmental to economic and technological and include
difficult to quantify risks, such as geo-political or social. Using the maximum
likelihood estimation, we find the optimal model parameters and demonstrate
that the model including network effects significantly outperforms the others,
uncovering full value of the expert collected data. We analyze the model
dynamics and study its resilience and stability. Our findings include such risk
properties as contagion potential, persistence, roles in cascades of failures
and the identity of risks most detrimental to system stability. The model
provides quantitative means for measuring the adverse effects of risk
interdependence and the materialization of risks in the network
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