17,007 research outputs found
Sparse Distributed Learning Based on Diffusion Adaptation
This article proposes diffusion LMS strategies for distributed estimation
over adaptive networks that are able to exploit sparsity in the underlying
system model. The approach relies on convex regularization, common in
compressive sensing, to enhance the detection of sparsity via a diffusive
process over the network. The resulting algorithms endow networks with learning
abilities and allow them to learn the sparse structure from the incoming data
in real-time, and also to track variations in the sparsity of the model. We
provide convergence and mean-square performance analysis of the proposed method
and show under what conditions it outperforms the unregularized diffusion
version. We also show how to adaptively select the regularization parameter.
Simulation results illustrate the advantage of the proposed filters for sparse
data recovery.Comment: to appear in IEEE Trans. on Signal Processing, 201
-independent slow-dynamics in atomic and molecular systems
Investigating million-atom systems for very long simulation times, we
demonstrate that the collective density-density correlation time
() in simulated supercooled water and silica becomes wavevector
independent () when the probing wavelength is several times larger than
the interparticle distance. The -independence of the collective
density-density correlation functions, a feature clearly observed in
light-scattering studies of some soft-matter systems, is thus a genuine feature
of many (but not all) slow-dynamics systems, either atomic, molecular or
colloidal. Indeed, we show that when the dynamics of the density fluctuations
is due to particle-type diffusion, as in the case of the Lennard Jones binary
mixture model, the regime does not set in and the relaxation time
continues to scale as even at small .Comment: Includes the supplementary materia
Full counting statistics of weak measurement
A weak measurement consists in coupling a system to a probe in such a way
that constructive interference generates a large output. So far, only the
average output of the probe and its variance were studied. Here, the
characteristic function for the moments of the output is provided. The outputs
considered are not limited to the eigenstates of the pointer or of its
conjugate variable, so that the results apply to any observable \Hat{o} of
the probe. Furthermore, a family of well behaved complex quantities, the normal
weak values, is introduced, in terms of which the statistics of the weak
measurement can be described. It is shown that, within a good approximation,
the whole statistics of weak measurement is described by a complex parameter,
the weak value, and a real one.Comment: Expanded version: 9 pages, 3 Figs. Now the validity of the expansion
for the moments is analysed. Introduced a one-parameter family of weak
values, useful to express the correct characteristic function. More figures
added. Thanks to Referee C of PRL for asking stimulating question
RR Lyrae in XSTPS: The halo density profile in the North Galactic Cap
We present a catalog of RR Lyrae stars (RRLs) observed by the Xuyi Schmidt
Telescope Photometric Survey (XDSS). The area we consider is located in the
North Galactic Cap, covering 376.75 sq deg at RA 150 deg and Dec
27 deg down to a magnitude limit of i 19. Using the
variability information afforded by the multi-epoch nature of our XDSS data,
combined with colors from the Sloan Digital Sky Survey, we are able to identify
candidate RRLs. We find 318 candidates, derive distances to them and estimate
the detection efficiency. The majority of our candidates have more than 12
observations and for these we are able to calculate periods. These also allows
us to estimate our contamination level, which we predict is between 30% to 40%.
Finally we use the sample to probe the halo density profile in the 9-49 kpc
range and find that it can be well fitted by a double power law. We find good
agreement between this model and the models derived for the South Galactic Cap
using the Watkins et al. (2009) and Sesar et al. (2010) RRL data-sets, after
accounting for possible contamination in our data-set from Sagittarius stream
members. We consider non-spherical double power law models of the halo density
profile and again find agreement with literature data-sets, although we have
limited power to constrain the flattening due to our small survey area. Much
tighter constraints will be placed by current and future wide-area surveys,
most notably ESA's astrometric Gaia mission. Our analysis demonstrates that
surveys with a limited number of epochs can effectively be mined for RRLs. Our
complete sample is provided as accompanying online material.Comment: 14 pages, ApJ (in press
Tuning non-Markovianity by spin-dynamics control
We study the interplay between forgetful and memory-keeping evolution
enforced on a two-level system by a multi-spin environment whose elements are
coupled to local bosonic baths. Contrarily to the expectation that any
non-Markovian effect would be buried by the forgetful mechanism induced by the
spin-bath coupling, one can actually induce a full Markovian-to-non-Markovian
transition of the two-level system's dynamics, controllable by parameters such
as the mismatch between the energy of the two-level system and of the spin
environment. For a symmetric coupling, the amount of non-Markovianity
surprisingly grows with the number of decoherence channels.Comment: 7 pages, 6 figures, PRA versio
Constraining galaxy cluster temperatures and redshifts with eROSITA survey data
The nature of dark energy is imprinted in the large-scale structure of the
Universe and thus in the mass and redshift distribution of galaxy clusters. The
upcoming eROSITA mission will exploit this method of probing dark energy by
detecting roughly 100,000 clusters of galaxies in X-rays. For a precise
cosmological analysis the various galaxy cluster properties need to be measured
with high precision and accuracy. To predict these characteristics of eROSITA
galaxy clusters and to optimise optical follow-up observations, we estimate the
precision and the accuracy with which eROSITA will be able to determine galaxy
cluster temperatures and redshifts from X-ray spectra. Additionally, we present
the total number of clusters for which these two properties will be available
from the eROSITA survey directly. During its four years of all-sky surveys,
eROSITA will determine cluster temperatures with relative uncertainties of
Delta(T)/T<10% at the 68%-confidence level for clusters up to redshifts of
z~0.16 which corresponds to ~1,670 new clusters with precise properties.
Redshift information itself will become available with a precision of
Delta(z)/(1+z)<10% for clusters up to z~0.45. Additionally, we estimate how the
number of clusters with precise properties increases with a deepening of the
exposure. Furthermore, the biases in the best-fit temperatures as well as in
the estimated uncertainties are quantified and shown to be negligible in the
relevant parameter range in general. For the remaining parameter sets, we
provide correction functions and factors. The eROSITA survey will increase the
number of galaxy clusters with precise temperature measurements by a factor of
5-10. Thus the instrument presents itself as a powerful tool for the
determination of tight constraints on the cosmological parameters.Comment: accepted for publication in A&A; 17 pages, 20 figure
Drops on soft solids: Free energy and double transition of contact angles
The equilibrium shape of liquid drops on elastic substrates is determined by
minimising elastic and capillary free energies, focusing on thick
incompressible substrates. The problem is governed by three length scales: the
size of the drop , the molecular size , and the ratio of surface tension
to elastic modulus . We show that the contact angles undergo two
transitions upon changing the substrates from rigid to soft. The microscopic
wetting angles deviate from Young's law when , while the
apparent macroscopic angle only changes in the very soft limit . The elastic deformations are worked out in the simplifying case where the
solid surface energy is assumed constant. The total free energy turns out lower
on softer substrates, consistent with recent experiments
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