2,748 research outputs found
Extreme value statistics and recurrence intervals of NYMEX energy futures volatility
Energy markets and the associated energy futures markets play a crucial role
in global economies. We investigate the statistical properties of the
recurrence intervals of daily volatility time series of four NYMEX energy
futures, which are defined as the waiting times between consecutive
volatilities exceeding a given threshold . We find that the recurrence
intervals are distributed as a stretched exponential , where the exponent decreases with increasing
, and there is no scaling behavior in the distributions for different
thresholds after the recurrence intervals are scaled with the mean
recurrence interval . These findings are significant under the
Kolmogorov-Smirnov test and the Cram{\'e}r-von Mises test. We show that
empirical estimations are in nice agreement with the numerical integration
results for the occurrence probability of a next event above
the threshold within a (short) time interval after an elapsed time from
the last event above . We also investigate the memory effects of the
recurrence intervals. It is found that the conditional distributions of large
and small recurrence intervals differ from each other and the conditional mean
of the recurrence intervals scales as a power law of the preceding interval
, indicating that the
recurrence intervals have short-term correlations. Detrended fluctuation
analysis and detrending moving average analysis further uncover that the
recurrence intervals possess long-term correlations. We confirm that the
"clustering" of the volatility recurrence intervals is caused by the long-term
correlations well known to be present in the volatility.Comment: 11 pages, 8 figures and 3 table
Testing the weak-form efficiency of the WTI crude oil futures market
We perform detrending moving average analysis (DMA) and detrended fluctuation
analysis (DFA) of the WTI crude oil futures prices (1983-2012) to investigate
its efficiency. We further put forward a strict statistical test in the spirit
of bootstrapping to verify the weak-form market efficiency hypothesis by
employing the DMA (or DFA) exponent as the statistic. We verify the weak-form
efficiency of the crude oil futures market when the whole period is considered.
When we break the whole series into three sub-series separated by the outbreaks
of the Gulf War and the Iraq War, our statistical tests uncover that only the
Gulf War has the impact of reducing the efficiency of the crude oil market. If
we split the whole time series into two sub-series based on the signing date of
the North American Free Trade Agreement, we find that the market is inefficient
in the sub-periods during which the Gulf War broke out. We also perform the
same analysis on short time series in moving windows and find that the market
is inefficient only when some turbulent events occur, such as the oil price
crash in 1985, the Gulf war, and the oil price crash in 2008. Our analysis may
offer a new understanding of the efficiency of the crude oil futures market and
shed new lights on the investigation of the efficiency in other financial
markets.Comment: 9 pages and 5 figure
Circular orbit of a test particle and phase transition of a black hole
The radius of the circular orbit for the time-like or light-like test
particle in a background of general spherically symmetric spacetime is viewed
as a characterized quantity for the thermodynamic phase transition of the
corresponding black hole. We generally show that the phase transition
information of a black hole can be reflected by its surrounding particle's
circular orbit.Comment: 6 page
Photon sphere and phase transition of -dimensional () charged Gauss-Bonnet AdS black holes
Motivated by recent work, nonmonotonic behaviours of photon sphere radius can
be used to reflect black hole phase transition for
Reissner-Nordstrm-AdS (RN-AdS) black holes, we study the case of
five-dimensional charged Gauss-Bonnet-AdS (GB-AdS) black holes in the reduced
parameter space. We find that the nonmonotonic behaviours of photon sphere
radius still exist. Using the coexistence line calculated from plane, we
capture the photon sphere radius of saturated small and large black holes (the
boundary of the coexistence phase), then illustrate the reduced coexistence
region. The results show that, reduced coexistence region decreases with charge
but increases with Gauss-Bonnet coefficient . When the charge
vanishes, reduced coexistence region doesn't vary with Gauss-Bonnet coefficient
any more. In this case, the Gauss-Bonnet coefficient plays
the same role as the charge of five-dimensional RN-AdS black holes. Also, the
situation of higher dimension is studied in the end.Comment: 8 pages, 15 figure
Communication cliques in mobile phone calling networks
People in modern societies form different social networks through numerous
means of communication. These communication networks reflect different aspects
of human's societal structure. The billing records of calls among mobile phone
users enable us to construct a directed calling network (DCN) and its
Bonferroni network (SVDCN) in which the preferential communications are
statistically validated. Here we perform a comparative investigation of the
cliques of the original DCN and its SVDCN constructed from the calling records
of more than nine million individuals in Shanghai over a period of 110 days. We
find that the statistical properties of the cliques of the two calling networks
are qualitatively similar and the clique members in the DCN and the SVDCN
exhibit idiosyncratic behaviors quantitatively. Members in large cliques are
found to be spatially close to each other. Based on the clique degree profile
of each mobile phone user, the most active users in the two calling networks
can be classified in to several groups. The users in different groups are found
to have different calling behaviors. Our study unveils interesting
communication behaviors among mobile phone users that are densely connected to
each other.Comment: 18 pages, 10 figure
An End-to-End Compression Framework Based on Convolutional Neural Networks
Deep learning, e.g., convolutional neural networks (CNNs), has achieved great
success in image processing and computer vision especially in high level vision
applications such as recognition and understanding. However, it is rarely used
to solve low-level vision problems such as image compression studied in this
paper. Here, we move forward a step and propose a novel compression framework
based on CNNs. To achieve high-quality image compression at low bit rates, two
CNNs are seamlessly integrated into an end-to-end compression framework. The
first CNN, named compact convolutional neural network (ComCNN), learns an
optimal compact representation from an input image, which preserves the
structural information and is then encoded using an image codec (e.g., JPEG,
JPEG2000 or BPG). The second CNN, named reconstruction convolutional neural
network (RecCNN), is used to reconstruct the decoded image with high-quality in
the decoding end. To make two CNNs effectively collaborate, we develop a
unified end-to-end learning algorithm to simultaneously learn ComCNN and
RecCNN, which facilitates the accurate reconstruction of the decoded image
using RecCNN. Such a design also makes the proposed compression framework
compatible with existing image coding standards. Experimental results validate
that the proposed compression framework greatly outperforms several compression
frameworks that use existing image coding standards with state-of-the-art
deblocking or denoising post-processing methods.Comment: Submitted to IEEE Transactions on Circuits and Systems for Video
Technolog
Triadic motifs in the dependence networks of virtual societies
In friendship networks, individuals have different numbers of friends, and
the closeness or intimacy between an individual and her friends is
heterogeneous. Using a statistical filtering method to identify relationships
about who depends on whom, we construct dependence networks (which are
directed) from weighted friendship networks of avatars in more than two hundred
virtual societies of a massively multiplayer online role-playing game (MMORPG).
We investigate the evolution of triadic motifs in dependence networks. Several
metrics show that the virtual societies evolved through a transient stage in
the first two to three weeks and reached a relatively stable stage. We find
that the unidirectional loop motif () is underrepresented and does
not appear, open motifs are also underrepresented, while other close motifs are
overrepresented. We also find that, for most motifs, the overall level
difference of the three avatars in the same motif is significantly lower than
average, whereas the sum of ranks is only slightly larger than average. Our
findings show that avatars' social status plays an important role in the
formation of triadic motifs.Comment: 8 Latex pages + 5 figure
A unified phase transition picture of the charged topological black hole in Horava-Lifshitz gravity
Aiming at a unified phase transition picture of the charged topological black
hole in Ho\v{r}ava-Lifshitz gravity, we investigate this issue not only in
canonical ensemble with the fixed charge case but also in grand-canonical
ensemble with the fixed potential case. We firstly perform the standard
analysis of the specific heat, the free energy and the Gibbs potential, and
then study its geometrothermodynamics. It is shown that the local phase
transition points not only witness the divergence of the specific heat, but
also witness the minimum temperature and the maximum free energy or Gibbs
potential. They also witness the divergence of the corresponding thermodynamic
scalar curvature. No matter which ensemble is chosen, the metric constructed
can successfully produce the behavior of the thermodynamic interaction and
phase transition structure while other metrics failed to predict the phase
transition point of the charged topological black hole in former literature. In
grand-canonical ensemble, we have discovered the phase transition which has not
been reported before. It is similar to the canonical ensemble in which the
phase transition only takes place when . But it also has its unique
characteristics that the location of the phase transition point depends on the
value of potential, which is different from the canonical ensemble where the
phase transition point is independent of the parameters. After an analytical
check of Ehrenfest scheme, we find that the new phase transition is a second
order one. It is also found that the thermodynamics of the black hole in
Horava-Lifshitz gravity is quite different from that in Einstein gravity.Comment: 24pages,11figure
Two-state Markov-chain Poisson nature of individual cellphone call statistics
Humans are heterogenous and the behaviors of individuals could be different
from that at the population level. We conduct an in-depth study of the temporal
patterns of cellphone conversation activities of 73'339 anonymous cellphone
users with the same truncated Weibull distribution of inter-call durations. We
find that the individual call events exhibit a pattern of bursts, in which high
activity periods are alternated with low activity periods. Surprisingly, the
number of events in high activity periods are found to conform to a power-law
distribution at the population level, but follow an exponential distribution at
the individual level, which is a hallmark of absence of memory in individual
call activities. Such exponential distribution is also observed for the number
of events in low activity periods. Together with the exponential distributions
of inter-call durations within bursts and of the intervals between consecutive
bursts, we demonstrate that the individual call activities are driven by two
independent Poisson processes, which can be combined within a minimal model in
terms of a two-state first-order Markov chain giving very good agreement with
the empirical distributions using the parameters estimated from real data for
about half of the individuals in our sample. By measuring directly the
distributions of call rates across the population, which exhibit power-law
tails, we explain the difference with previous population level studies,
purporting the existence of power-law distributions, via the "Superposition of
Distributions" mechanism: The superposition of many exponential distributions
of activities with a power-law distribution of their characteristic scales
leads to a power-law distribution of the activities at the population level.Comment: 16 pages, 7 figure
Joint multifractal analysis based on the partition function approach: Analytical analysis, numerical simulation and empirical application
Many complex systems generate multifractal time series which are long-range
cross-correlated. Numerous methods have been proposed to characterize the
multifractal nature of these long-range cross correlations. However, several
important issues about these methods are not well understood and most methods
consider only one moment order. We study the joint multifractal analysis based
on partition function with two moment orders, which was initially invented to
investigate fluid fields, and derive analytically several important properties.
We apply the method numerically to binomial measures with multifractal cross
correlations and bivariate fractional Brownian motions without multifractal
cross correlations. For binomial multifractal measures, the explicit
expressions of mass function, singularity strength and multifractal spectrum of
the cross correlations are derived, which agree excellently with the numerical
results. We also apply the method to stock market indexes and unveil intriguing
multifractality in the cross correlations of index volatilities.Comment: 19 pages, 5 figure
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