7,397 research outputs found
Limiting distributions of continuous-time random walks with superheavy-tailed waiting times
We study the long-time behavior of the scaled walker (particle) position
associated with decoupled continuous-time random walk which is characterized by
superheavy-tailed distribution of waiting times and asymmetric heavy-tailed
distribution of jump lengths. Both the scaling function and the corresponding
limiting probability density are determined for all admissible values of tail
indexes describing the jump distribution. To analytically investigate the
limiting density function, we derive a number of different representations of
this function and, by this way, establish its main properties. We also develop
an efficient numerical method for computing the limiting probability density
and compare our analytical and numerical results.Comment: 35 pages, 4 figure
Big jump principle for heavy-tailed random walks with correlated increments
The big jump principle explains the emergence of extreme events for physical
quantities modelled by a sum of independent and identically distributed random
variables which are heavy-tailed. Extreme events are large values of the sum
and they are solely dominated by the largest summand called the big jump.
Recently, the principle was introduced into physical sciences where systems
usually exhibit correlations. Here, we study the principle for a random walk
with correlated increments. Examples are the autoregressive model of first
order and the discretized Ornstein-Uhlenbeck process both with heavy-tailed
noise. The correlation leads to the dependence of large values of the sum not
only on the big jump but also on the following increments. We describe this
behaviour by two big jump principles, namely unconditioned and conditioned on
the step number when the big jump occurs. The unconditional big jump principle
is described by a correlation dependent shift between the sum and maximum
distribution tails. For the conditional big jump principle, the shift depends
also on the step number of the big jump
Some non-standard approaches to the study of sums of heavy-tailed distributions
Heavy-tailed phenomena arise whenever events with very low probability have sufficiently
large consequences that these events cannot be treated as negligible. These are
sometimes described as low intensity, high impact events. Sums of heavy-tailed random
variables play a major role in many areas of applied probability, for instance in
risk theory, insurance mathematics, financial mathematics, queueing theory, telecommunications
and computing, to name but a few areas. The theory of the asymptotic
behaviour of a sum of independent heavy-tailed random variables is well-understood.
We give a review of known results in this area, stressing the importance of some insensitivity
properties of the class of long-tailed distributions. We introduce the new
concept of the Boundary Class for a long-tailed distribution, and describe some of its
properties and uses. We give examples of calculating the boundary class.
Geometric sums of random variables are a useful model in their own right, for instance
in reliability theory, but are also useful because they model the maximum of a random
walk, which is itself a model that occurs in many applications. When the summands
are heavy-tailed and independent then the asymptotic behaviour has been known
since the 1970s. The asymptotic expression for the geometric sum is often used
as an approximation to the actual distribution, owing to the (usually) analytically
intractable form of the exact distribution. However the accuracy of this asymptotic
approximation can be very poor, as we demonstrate. Following and further developing
work by Kalashnikov and Tsitsiashvili we construct an upper bound for the relative
accuracy of this approximation. We then develop new techniques for the application
of our analytical results, and apply these in practice to several examples. Source code
viii
for the computer algorithms used in these calculations is given.
As we have said, the asymptotic behaviour of a sum of heavy-tailed random variables
is well-understood when the random variables are independent, the main characteristic
being the principle of the single big jump. However, the case when the random
variables are dependent is much less clear. We study this case for both deterministic
and random sums using a novel approach, by considering conditional independence
structures on the random variables. We seek sufficient conditions for the results of
the theory with independent random variables still to hold. We give several examples
to show how to apply and check our conditions, and the examples demonstrate a
variety of effects owing to the dependence, and are also interesting in their own right.
All the results we develop on this topic are entirely new. Some of the examples also
include results that are new and have not been obtainable through previously existing
techniques. For some examples we study the asymptotic behaviour is known, and this
allows us to contrast our approach with previous approaches
Modeling record-breaking stock prices
We study the statistics of record-breaking events in daily stock prices of
366 stocks from the Standard and Poors 500 stock index. Both the record events
in the daily stock prices themselves and the records in the daily returns are
discussed. In both cases we try to describe the record statistics of the stock
data with simple theoretical models. The daily returns are compared to i.i.d.
RV's and the stock prices are modeled using a biased random walk, for which the
record statistics are known. These models agree partly with the behavior of the
stock data, but we also identify several interesting deviations. Most
importantly, the number of records in the stocks appears to be systematically
decreased in comparison with the random walk model. Considering the
autoregressive AR(1) process, we can predict the record statistics of the daily
stock prices more accurately. We also compare the stock data with simulations
of the record statistics of the more complicated GARCH(1,1) model, which, in
combination with the AR(1) model, gives the best agreement with the
observational data. To better understand our findings, we discuss the survival
and first-passage times of stock prices on certain intervals and analyze the
correlations between the individual record events. After recapitulating some
recent results for the record statistics of ensembles of N stocks, we also
present some new observations for the weekly distributions of record events.Comment: 20 pages, 28 figure
Persistence of heavy-tailed sample averages occurs by infinitely many jumps
We consider the sample average of a centered random walk in
with regularly varying step size distribution. For the first exit time from a
compact convex set not containing the origin, we show that its tail is of
lognormal type. Moreover, we show that the typical way for a large exit time to
occur is by having a number of jumps growing logarithmically in the scaling
parameter.Comment: 25 pages, 2 figure
Efficient Simulation and Conditional Functional Limit Theorems for Ruinous Heavy-tailed Random Walks
The contribution of this paper is to introduce change of measure based
techniques for the rare-event analysis of heavy-tailed stochastic processes.
Our changes-of-measure are parameterized by a family of distributions admitting
a mixture form. We exploit our methodology to achieve two types of results.
First, we construct Monte Carlo estimators that are strongly efficient (i.e.
have bounded relative mean squared error as the event of interest becomes
rare). These estimators are used to estimate both rare-event probabilities of
interest and associated conditional expectations. We emphasize that our
techniques allow us to control the expected termination time of the Monte Carlo
algorithm even if the conditional expected stopping time (under the original
distribution) given the event of interest is infinity -- a situation that
sometimes occurs in heavy-tailed settings. Second, the mixture family serves as
a good approximation (in total variation) of the conditional distribution of
the whole process given the rare event of interest. The convenient form of the
mixture family allows us to obtain, as a corollary, functional conditional
central limit theorems that extend classical results in the literature. We
illustrate our methodology in the context of the ruin probability , where is a random walk with heavy-tailed increments that have
negative drift. Our techniques are based on the use of Lyapunov inequalities
for variance control and termination time. The conditional limit theorems
combine the application of Lyapunov bounds with coupling arguments
Correlated continuous-time random walks: combining scale-invariance with long-range memory for spatial and temporal dynamics
Standard continuous time random walk (CTRW) models are renewal processes in
the sense that at each jump a new, independent pair of jump length and waiting
time are chosen. Globally, anomalous diffusion emerges through action of the
generalized central limit theorem leading to scale-free forms of the jump
length or waiting time distributions. Here we present a modified version of
recently proposed correlated CTRW processes, where we incorporate a power-law
correlated noise on the level of both jump length and waiting time dynamics. We
obtain a very general stochastic model, that encompasses key features of
several paradigmatic models of anomalous diffusion: discontinuous, scale-free
displacements as in Levy flights, scale-free waiting times as in subdiffusive
CTRWs, and the long-range temporal correlations of fractional Brownian motion
(FBM). We derive the exact solutions for the single-time probability density
functions and extract the scaling behaviours. Interestingly, we find that
different combinations of the model parameters lead to indistinguishable shapes
of the emerging probability density functions and identical scaling laws. Our
model will be useful to describe recent experimental single particle tracking
data, that feature a combination of CTRW and FBM properties.Comment: 25 pages, IOP style, 5 figure
- …