2,859 research outputs found
Iterative Estimation of Solutions to Noisy Nonlinear Operator Equations in Nonparametric Instrumental Regression
This paper discusses the solution of nonlinear integral equations with noisy
integral kernels as they appear in nonparametric instrumental regression. We
propose a regularized Newton-type iteration and establish convergence and
convergence rate results. A particular emphasis is on instrumental regression
models where the usual conditional mean assumption is replaced by a stronger
independence assumption. We demonstrate for the case of a binary instrument
that our approach allows the correct estimation of regression functions which
are not identifiable with the standard model. This is illustrated in computed
examples with simulated data
Nonlinear Time Series Modeling: A Unified Perspective, Algorithm, and Application
A new comprehensive approach to nonlinear time series analysis and modeling
is developed in the present paper. We introduce novel data-specific
mid-distribution based Legendre Polynomial (LP) like nonlinear transformations
of the original time series Y(t) that enables us to adapt all the existing
stationary linear Gaussian time series modeling strategy and made it applicable
for non-Gaussian and nonlinear processes in a robust fashion. The emphasis of
the present paper is on empirical time series modeling via the algorithm
LPTime. We demonstrate the effectiveness of our theoretical framework using
daily S&P 500 return data between Jan/2/1963 - Dec/31/2009. Our proposed LPTime
algorithm systematically discovers all the `stylized facts' of the financial
time series automatically all at once, which were previously noted by many
researchers one at a time.Comment: Major restructuring has been don
Global sensitivity analysis for stochastic simulators based on generalized lambda surrogate models
Global sensitivity analysis aims at quantifying the impact of input
variability onto the variation of the response of a computational model. It has
been widely applied to deterministic simulators, for which a set of input
parameters has a unique corresponding output value. Stochastic simulators,
however, have intrinsic randomness due to their use of (pseudo)random numbers,
so they give different results when run twice with the same input parameters
but non-common random numbers. Due to this random nature, conventional Sobol'
indices, used in global sensitivity analysis, can be extended to stochastic
simulators in different ways. In this paper, we discuss three possible
extensions and focus on those that depend only on the statistical dependence
between input and output. This choice ignores the detailed data generating
process involving the internal randomness, and can thus be applied to a wider
class of problems. We propose to use the generalized lambda model to emulate
the response distribution of stochastic simulators. Such a surrogate can be
constructed without the need for replications. The proposed method is applied
to three examples including two case studies in finance and epidemiology. The
results confirm the convergence of the approach for estimating the sensitivity
indices even with the presence of strong heteroskedasticity and small
signal-to-noise ratio
An X-ray survey of low-mass stars in Trumpler 16 with Chandra
We identify and characterize low-mass stars in the ~3 Myr old Trumpler 16
(Tr16) region by means of a deep Chandra X-ray observation, and study their
optical and near-IR properties. We compare X-ray activity of Tr16 stars with
known characteristics of Orion and Cygnus OB2 stars. We analyzed a 88.4 ksec
Chandra ACIS-I observation pointed at the center of Tr16. Because of diffuse
X-ray emission, source detection was performed using the PWDetect code for two
different energy ranges: 0.5-8.0 keV and 0.9-8.0 keV. Results were merged into
a single final list. We positionally correlate X-ray sources with optical and
2MASS catalogues. Source events were extracted with the IDL-based routine
ACIS-Extract. X-ray variability was characterized using the Kolmogorov-Smirnov
test and spectra were fitted by using XSPEC. X-ray spectra of early-type,
massive stars were analyzed individually. Our list of X-ray sources consists of
1035 entries, 660 of which have near-IR counterparts and are probably
associated with Tr16 members. From near-IR color-color and color-magnitudes
diagrams we compute individual masses of stars and their Av values. About 15%
of the near-IR counterparts show disk-induced excesses. X-ray variability is
found in 77 sources. X-ray emission from OB stars appear softer than the
low-mass stars. The Tr16 region has a very rich population of low-mass X-ray
emitting stars. An important fraction of its circumstellar disks survive the
intense radiation field of its massive stars. Stars with masses 1.5-2.5 Mo
display X-ray activity similar to that of stars in Cyg OB2 but much less
intense than observed for Orion Nebula Cluster members.Comment: 19 pages, 3 ellectronic tables and 19 figures. Accepted for
publication at the A&
A local statistic for the spatial extent of extreme threshold exceedances
We introduce the extremal range, a local statistic for studying the spatial
extent of extreme events in random fields on . Conditioned on
exceedance of a high threshold at a location , the extremal range at is
the random variable defined as the smallest distance from to a location
where there is a non-exceedance. We leverage tools from excursion-set theory to
study distributional properties of the extremal range, propose parametric
models and predict the median extremal range at extreme threshold levels. The
extremal range captures the rate at which the spatial extent of conditional
extreme events scales for increasingly high thresholds, and we relate its
distributional properties with the bivariate tail dependence coefficient and
the extremal index of time series in classical Extreme-Value Theory. Consistent
estimation of the distribution function of the extremal range for stationary
random fields is proven. For non-stationary random fields, we implement
generalized additive median regression to predict extremal-range maps at very
high threshold levels. An application to two large daily temperature datasets,
namely reanalyses and climate-model simulations for France, highlights
decreasing extremal dependence for increasing threshold levels and reveals
strong differences in joint tail decay rates between reanalyses and
simulations.Comment: 32 pages, 5 figure
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