627,476 research outputs found
Odd order cases of the logarithmically averaged Chowla conjecture
A famous conjecture of Chowla states that the Liouville function
has negligible correlations with its shifts. Recently, the authors established
a weak form of the logarithmically averaged Elliott conjecture on correlations
of multiplicative functions, which in turn implied all the odd order cases of
the logarithmically averaged Chowla conjecture. In this note, we give a new and
shorter proof of the odd order cases of the logarithmically averaged Chowla
conjecture. In particular, this proof avoids all mention of ergodic theory,
which had an important role in the previous proof.Comment: 15 pages, no figures, submitted, J. Numb. Thy. Bordeau
Multichannel sparse recovery of complex-valued signals using Huber's criterion
In this paper, we generalize Huber's criterion to multichannel sparse
recovery problem of complex-valued measurements where the objective is to find
good recovery of jointly sparse unknown signal vectors from the given multiple
measurement vectors which are different linear combinations of the same known
elementary vectors. This requires careful characterization of robust
complex-valued loss functions as well as Huber's criterion function for the
multivariate sparse regression problem. We devise a greedy algorithm based on
simultaneous normalized iterative hard thresholding (SNIHT) algorithm. Unlike
the conventional SNIHT method, our algorithm, referred to as HUB-SNIHT, is
robust under heavy-tailed non-Gaussian noise conditions, yet has a negligible
performance loss compared to SNIHT under Gaussian noise. Usefulness of the
method is illustrated in source localization application with sensor arrays.Comment: To appear in CoSeRa'15 (Pisa, Italy, June 16-19, 2015). arXiv admin
note: text overlap with arXiv:1502.0244
Cooling and Heating Functions of Photoionized Gas
Cooling and heating functions of cosmic gas are a crucial ingredient for any
study of gas dynamics and thermodynamics in the interstellar and intergalactic
medium. As such, they have been studied extensively in the past under the
assumption of collisional ionization equilibrium. However, for a wide range of
applications, the local radiation field introduces a non-negligible, often
dominant, modification to the cooling and heating functions. In the most
general case, these modifications cannot be described in simple terms, and
would require a detailed calculation with a large set of chemical species using
a radiative transfer code (the well-known code Cloudy, for example). We show,
however, that for a sufficiently general variation in the spectral shape and
intensity of the incident radiation field, the cooling and heating functions
can be approximated as depending only on several photoionization rates, which
can be thought of as representative samples of the overall radiation field.
This dependence is easy to tabulate and implement in cosmological or
galactic-scale simulations, thus economically accounting for an important but
rarely-included factor in the evolution of cosmic gas. We also show a few
examples where the radiation environment has a large effect, the most
spectacular of which is a quasar that suppresses gas cooling in its host halo
without any mechanical or non-radiative thermal feedback.Comment: replaced with the accepted version; note that the revised version
differs substantially from the original draf
Central dark matter trends in early-type galaxies from strong lensing, dynamics and stellar populations
We analyze the correlations between central dark matter (DM) content of
early-type galaxies and their sizes and ages, using a sample of
intermediate-redshift (z ~ 0.2) gravitational lenses from the SLACS survey, and
by comparing them to a larger sample of z ~ 0 galaxies. We decompose the
deprojected galaxy masses into DM and stellar components using combinations of
strong lensing, stellar dynamics, and stellar populations modeling. For a given
stellar mass, we find that for galaxies with larger sizes, the DM fraction
increases and the mean DM density decreases, consistently with the cuspy halos
expected in cosmological formation scenarios. The DM fraction also decreases
with stellar age, which can be partially explained by the inverse correlation
between size and age. The residual trend may point to systematic dependencies
on formation epoch of halo contraction or stellar initial mass functions. These
results are in agreement with recent findings based on local galaxies by
Napolitano, Romanowsky & Tortora (2010) and suggest negligible evidence of
galaxy evolution over the last ~ 2.5 Gyr other than passive stellar aging.Comment: 5 pages, 3 figures, accepted for publication on ApJL. Version
including further updates and a complementary note added in proo
Application of MARSplines Method for Failure Rate Prediction
In this paper MARSplines method was presented to model failure rate of water pipes in years 2015-2016 in the selected Polish city. The output parameters were chosen as three dependent variables - three values of failure rate of water mains, distribution pipes and house connections. Diameter, season, material and kind of the conduit were selected as independent variables. At the beginning of modelling 21 basis (splines) function were assumed. On a final note two functions were selected (after reduction of negligible functions). The model consists of three factors: β0, β1 and β2. The penalty for adding basis function was assumed at the level of 2. The correlation was equalled to 0.44. Relatively huge discrepancies between real and predicted values of failure rate of water mains and house connections were observed. In the future investigations concerning this problem the three separated models for each kind of conduit should be created. The calculations using MARSplines method were carried out in the program Statistica 13.1
Top quark mass determination from the energy peaks of b-jets and B-hadrons at NLO QCD
We analyze the energy spectra of b-jets and B-hadrons resulting from
the production and decay of top quarks within the SM at the LHC at the NLO QCD.
For both hadrons and jets, we calculate the correlation of the peak of the
spectrum with the top quark mass, considering the "energy-peak" as an
observable to determine the top quark mass. Such a method is motivated by our
previous work where we argued that this approach can have reduced sensitivity
to the details of the production mechanism of the top quark, whether it is
higher-order QCD effects or new physics contributions. As part of the NLO
improvement over the original proposal, we assess the residual sensitivity of
the extracted top quark mass to perturbative effects both in top quark
production and decay. For a 1% jet energy scale uncertainty (and assuming
negligible statistical error), the top quark mass can then be extracted using
the energy-peak of b-jets with an error +- (1.2 (exp) + 0.6(th)) GeV. We note
that recently the CMS collaboration reported a top quark mass measurement based
on the original proposal (with b-jets) so that our result contributes to a
precise evaluation of the associated theory uncertainty. In view of the
dominant jet energy scale uncertainty in the measurement using b-jets, we also
investigate the extraction of the top quark mass from the energy-peak of the
corresponding B-hadrons which, in principle, can be measured without this
uncertainty. The calculation of the B-hadron energy spectrum is carried out
using fragmentation functions at NLO. The dependence on the fragmentation scale
turns out to be the largest theoretical uncertainty in this extraction of top
quark mass. Future improvement of the treatment of bottom quark hadronization
can reduce this uncertainty, rendering methods based on the B-hadron
energy-peak competitive for the top quark mass measurement.Comment: 5 figures, 12 page
Odd order cases of the logarithmically averaged chowla conjecture
A famous conjecture of Chowla states that the Liouville function λ(n) has negligible correlations with its shifts. Recently, the authors established a weak form of the logarithmically averaged Elliott conjecture on correlations of multiplicative functions, which in turn implied all the odd order cases of the logarithmically averaged Chowla conjecture. In this note, we give a new proof of the odd order cases of the logarithmically averaged Chowla conjecture. In particular, this proof avoids all mention of ergodic theory, which had an important role in the previous proof. </p
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