1,490 research outputs found
Adaptive estimation of linear functionals in the convolution model and applications
We consider the model , for i.i.d. 's and
's and independent sequences and
. The density of
is assumed to be known, whereas the one of , denoted by
, is unknown. Our aim is to estimate linear functionals of ,
for a known function $\psi$. We propose a general estimator of and
study the rate of convergence of its quadratic risk as a function of the
smoothness of , and . Different contexts with
dependent data, such as stochastic volatility and AutoRegressive Conditionally
Heteroskedastic models, are also considered. An estimator which is adaptive to
the smoothness of unknown is then proposed, following a method studied by
Laurent et al. (Preprint (2006)) in the Gaussian white noise model. We give
upper bounds and asymptotic lower bounds of the quadratic risk of this
estimator. The results are applied to adaptive pointwise deconvolution, in
which context losses in the adaptive rates are shown to be optimal in the
minimax sense. They are also applied in the context of the stochastic
volatility model.Comment: Published in at http://dx.doi.org/10.3150/08-BEJ146 the Bernoulli
(http://isi.cbs.nl/bernoulli/) by the International Statistical
Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm
Variational approach to the excitonic phase transition in graphene
We analyze the Coulomb interacting problem in undoped graphene layers by
using an excitonic variational ansatz. By minimizing the energy, we derive a
gap equation which reproduces and extends known results. We show that a full
treatment of the exchange term, which includes the renormalization of the Fermi
velocity, tends to suppress the phase transition by increasing the critical
coupling at which the excitonic instability takes place.Comment: 4 page
Anisotropic adaptive kernel deconvolution
International audienceIn this paper, we consider a multidimensional convolution model for which we provide adaptive anisotropic kernel estimators of a signal density measured with additive error. For this, we generalize Fan's~(1991) estimators to multidimensional setting and use a bandwidth selection device in the spirit of Goldenschluger and Lepski's~(2011) proposal fr density estimation without noise. We consider first the pointwise setting and then, we study the integrated risk. Our estimators depend on an automatically selected random bandwidth. We assume both ordinary and super smooth components for measurement errors, which have known density. We also consider both anisotropic H\"{o}lder and Sobolev classes for . We provide non asymptotic risk bounds and asymptotic rates for the resulting data driven estimator, which is proved to be adaptive. We provide an illustrative simulation study, involving the use of Fast Fourier Transform algorithms. We conclude by a proposal of extension of the method to the case of unknown noise density, when a preliminary pure noise sample is available
The Marseille Schmidt survey for active star-forming galaxies. I. Data on 92 emission line objects in two fields
We present data from a moderately deep spectroscopic Schmidt survey
(Blim=17.5) of ``active galaxies'' selected by the presence of emission lines
in their spectra and/or their UV excess. The redshift, magnitudes, color and
diameter reduction methods have been discussed in a previous paper. Here we
explain the emission line equivalent width determination method. 92 emission
line objects have been found in two adjacent fields (approximately 50deg^2) in
the direction of the south extension of the Virgo cluster. We give a catalog
containing positions, photographic R and B magnitudes, U-R colors, effective
diameters, redshifts, equivalent widths and intensity ratios of the
[OIII]4959,5007, Hbeta and [OII] 3727 emission lines. On these fields, we
evaluate the completeness limit of the survey at a pseudo B magnitude value of
15.7.Comment: tar gzipped file 7 AA latex2e pages(including 7 figures) + 1 tex
table (2 pages) + 5 pages (color ps) charts will appear in A&A, available
also at http://hasso.mpia-hd.mpg.de/Surace/surace.html#article
Spin degree of freedom in two dimensional exciton condensates
We present a theoretical analysis of a spin-dependent multicomponent
condensate in two dimensions. The case of a condensate of resonantly
photoexcited excitons having two different spin orientations is studied in
detail. The energy and the chemical potentials of this system depend strongly
on the spin polarization . When electrons and holes are located in two
different planes, the condensate can be either totally spin polarized or spin
unpolarized, a property that is measurable. The phase diagram in terms of the
total density and electron-hole separation is discussed.Comment: 4 pages, 3 figures, Accepted for publication in Physical Review
Letter
Adaptive density estimation for stationary processes
We propose an algorithm to estimate the common density of a stationary
process . We suppose that the process is either or
-mixing. We provide a model selection procedure based on a generalization
of Mallows' and we prove oracle inequalities for the selected estimator
under a few prior assumptions on the collection of models and on the mixing
coefficients. We prove that our estimator is adaptive over a class of Besov
spaces, namely, we prove that it achieves the same rates of convergence as in
the i.i.d framework
Spin noise spectroscopy to probe quantum states of ultracold fermionic atomic gases
Ultracold alkali atoms provide experimentally accessible model systems for
probing quantum states that manifest themselves at the macroscopic scale.
Recent experimental realizations of superfluidity in dilute gases of ultracold
fermionic (half-integer spin) atoms offer exciting opportunities to directly
test theoretical models of related many-body fermion systems that are
inaccessible to experimental manipulation, such as neutron stars and
quark-gluon plasmas. However, the microscopic interactions between fermions are
potentially quite complex, and experiments in ultracold gases to date cannot
clearly distinguish between the qualitatively different microscopic models that
have been proposed. Here, we theoretically demonstrate that optical
measurements of electron spin noise -- the intrinsic, random fluctuations of
spin -- can probe the entangled quantum states of ultracold fermionic atomic
gases and unambiguously reveal the detailed nature of the interatomic
interactions. We show that different models predict different sets of
resonances in the noise spectrum, and once the correct effective interatomic
interaction model is identified, the line-shapes of the spin noise can be used
to constrain this model. Further, experimental measurements of spin noise in
classical (Boltzmann) alkali vapors are used to estimate the expected signal
magnitudes for spin noise measurements in ultracold atom systems and to show
that these measurements are feasible
Geodetic, teleseismic, and strong motion constraints on slip from recent southern Peru subduction zone earthquakes
We use seismic and geodetic data both jointly and separately to constrain coseismic slip from the 12 November 1996 M_w 7.7 and 23 June 2001 M_w 8.5 southern Peru subduction zone earthquakes, as well as two large aftershocks following the 2001 earthquake on 26 June and 7 July 2001. We use all available data in our inversions: GPS, interferometric synthetic aperture radar (InSAR) from the ERS-1, ERS-2, JERS, and RADARSAT-1 satellites, and seismic data from teleseismic and strong motion stations. Our two-dimensional slip models derived from only teleseismic body waves from South American subduction zone earthquakes with M_w > 7.5 do not reliably predict available geodetic data. In particular, we find significant differences in the distribution of slip for the 2001 earthquake from models that use only seismic (teleseismic and two strong motion stations) or geodetic (InSAR and GPS) data. The differences might be related to postseismic deformation or, more likely, the different sensitivities of the teleseismic and geodetic data to coseismic rupture properties. The earthquakes studied here follow the pattern of earthquake directivity along the coast of western South America, north of 5°S, earthquakes rupture to the north; south of about 12°S, directivity is southerly; and in between, earthquakes are bilateral. The predicted deformation at the Arequipa GPS station from the seismic-only slip model for the 7 July 2001 aftershock is not consistent with significant preseismic motion
Effect of volcanic dykes on coastal groundwater flow and saltwater intrusion : a field-scale multiphysics approach and parameter evaluation
Acknowledgments This research was primarily based on research grant‐aided by the Irish Department of Communications, Energy and Natural Resources under the National Geoscience Programme 2007–2013. It also benefited from complementary funding from the Scottish Alliance for Geoscience, Environment and Society (SAGES). We acknowledge the contribution in data acquisition of the MSc students in Environmental Engineering at Queen's University Belfast, the landowner for access to the inland fields and the Department of Geography, Archaeology and Paleoecology at QUB for provision of the tidal model of Belfast Lough. The data used are listed in the references, tables, and figures and are available from the corresponding author upon demand. We acknowledge the constructive comments by the Associate Editor and three reviewers, which helped in improving the final manuscript.Peer reviewedPublisher PD
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