42 research outputs found
Finite Domain Anomalous Spreading Consistent with First and Second Law
After reviewing the problematic behavior of some previously suggested finite
interval spatial operators of the symmetric Riesz type, we create a wish list
leading toward a new spatial operator suitable to use in the space-time
fractional differential equation of anomalous diffusion when the transport of
material is strictly restricted to a bounded domain. Based on recent studies of
wall effects, we introduce a new definition of the spatial operator and
illustrate its favorable characteristics. We provide two numerical methods to
solve the modified space-time fractional differential equation and show
particular results illustrating compliance to our established list of
requirements, most important to the conservation principle and the second law
of thermodynamics.Comment: 14 figure
Non-Markovian diffusion equations and processes: analysis and simulations
In this paper we introduce and analyze a class of diffusion type equations
related to certain non-Markovian stochastic processes. We start from the
forward drift equation which is made non-local in time by the introduction of a
suitable chosen memory kernel K(t). The resulting non-Markovian equation can be
interpreted in a natural way as the evolution equation of the marginal density
function of a random time process l(t). We then consider the subordinated
process Y(t)=X(l(t)) where X(t) is a Markovian diffusion. The corresponding
time evolution of the marginal density function of Y(t) is governed by a
non-Markovian Fokker-Planck equation which involves the memory kernel K(t). We
develop several applications and derive the exact solutions. We consider
different stochastic models for the given equations providing path simulations.Comment: 43 pages, 19 figures, in press on Physica A (2008
Arithmetic Brownian motion subordinated by tempered stable and inverse tempered stable processes
In the last decade the subordinated processes have become popular and found
many practical applications. Therefore in this paper we examine two processes
related to time-changed (subordinated) classical Brownian motion with drift
(called arithmetic Brownian motion). The first one, so called normal tempered
stable, is related to the tempered stable subordinator, while the second one -
to the inverse tempered stable process. We compare the main properties (such as
probability density functions, Laplace transforms, ensemble averaged mean
squared displacements) of such two subordinated processes and propose the
parameters' estimation procedures. Moreover we calibrate the analyzed systems
to real data related to indoor air quality
Theory of Earthquake Recurrence Times
The statistics of recurrence times in broad areas have been reported to obey
universal scaling laws, both for single homogeneous regions (Corral, 2003) and
when averaged over multiple regions (Bak et al.,2002). These unified scaling
laws are characterized by intermediate power law asymptotics. On the other
hand, Molchan (2005) has presented a mathematical proof that, if such a
universal law exists, it is necessarily an exponential, in obvious
contradiction with the data. First, we generalize Molchan's argument to show
that an approximate unified law can be found which is compatible with the
empirical observations when incorporating the impact of the Omori law of
earthquake triggering. We then develop the full theory of the statistics of
inter-event times in the framework of the ETAS model of triggered seismicity
and show that the empirical observations can be fully explained. Our
theoretical expression fits well the empirical statistics over the whole range
of recurrence times, accounting for different regimes by using only the physics
of triggering quantified by Omori's law. The description of the statistics of
recurrence times over multiple regions requires an additional subtle
statistical derivation that maps the fractal geometry of earthquake epicenters
onto the distribution of the average seismic rates in multiple regions. This
yields a prediction in excellent agreement with the empirical data for
reasonable values of the fractal dimension , the average
clustering ratio , and the productivity exponent times the -value of the Gutenberg-Richter law.Comment: 30 pages + 13 figure
Continuous time random walk and parametric subordination in fractional diffusion
The well-scaled transition to the diffusion limit in the framework of the
theory of continuous-time random walk (CTRW)is presented starting from its
representation as an infinite series that points out the subordinated character
of the CTRW itself. We treat the CTRW as a combination of a random walk on the
axis of physical time with a random walk in space, both walks happening in
discrete operational time. In the continuum limit we obtain a generally
non-Markovian diffusion process governed by a space-time fractional diffusion
equation. The essential assumption is that the probabilities for waiting times
and jump-widths behave asymptotically like powers with negative exponents
related to the orders of the fractional derivatives. By what we call parametric
subordination, applied to a combination of a Markov process with a positively
oriented L\'evy process, we generate and display sample paths for some special
cases.Comment: 28 pages, 18 figures. Workshop 'In Search of a Theory of Complexity'.
Denton, Texas, August 200
Some aspects of fractional diffusion equations of single and distributed order
The time fractional diffusion equation is obtained from the standard
diffusion equation by replacing the first-order time derivative with a
fractional derivative of order . The fundamental solution for
the Cauchy problem is interpreted as a probability density of a self-similar
non-Markovian stochastic process related to a phenomenon of sub-diffusion (the
variance grows in time sub-linearly). A further generalization is obtained by
considering a continuous or discrete distribution of fractional time
derivatives of order less than one.
Then the fundamental solution is still a probability density of a
non-Markovian process that, however, is no longer self-similar but exhibits a
corresponding distribution of time-scales.Comment: 14 pages. International Symposium on "Analytic Function Theory,
Fractional Calculus and Their Applications", University of Victoria (British
Columbia, Canada), 22-27 August 200
Generation-by-Generation Dissection of the Response Function in Long Memory Epidemic Processes
In a number of natural and social systems, the response to an exogenous shock
relaxes back to the average level according to a long-memory kernel with . In the presence of an epidemic-like
process of triggered shocks developing in a cascade of generations at or close
to criticality, this "bare" kernel is renormalized into an even slower decaying
response function . Surprisingly, this means that the
shorter the memory of the bare kernel (the larger ), the longer the
memory of the response function (the smaller ). Here, we present a
detailed investigation of this paradoxical behavior based on a
generation-by-generation decomposition of the total response function, the use
of Laplace transforms and of "anomalous" scaling arguments. The paradox is
explained by the fact that the number of triggered generations grows
anomalously with time at so that the contributions of active
generations up to time more than compensate the shorter memory associated
with a larger exponent . This anomalous scaling results fundamentally
from the property that the expected waiting time is infinite for . The techniques developed here are also applied to the case
and we find in this case that the total renormalized response is a {\bf
constant} for followed by a cross-over to
for .Comment: 27 pages, 4 figure
Deep learning for healthcare applications based on physiological signals: A review
Background and objective: We have cast the net into the ocean of knowledge to retrieve the latest scientific research on deep learning methods for physiological signals. We found 53 research papers on this topic, published from 01.01.2008 to 31.12.2017. Methods: An initial bibliometric analysis shows that the reviewed papers focused on Electromyogram(EMG), Electroencephalogram(EEG), Electrocardiogram(ECG), and Electrooculogram(EOG). These four categories were used to structure the subsequent content review. Results: During the content review, we understood that deep learning performs better for big and varied datasets than classic analysis and machine classification methods. Deep learning algorithms try to develop the model by using all the available input. Conclusions: This review paper depicts the application of various deep learning algorithms used till recently, but in future it will be used for more healthcare areas to improve the quality of diagnosi