337 research outputs found
On the three-dimensional temporal spectrum of stretched vortices
The three-dimensional stability problem of a stretched stationary vortex is
addressed in this letter. More specifically, we prove that the discrete part of
the temporal spectrum is only associated with two-dimensional perturbations.Comment: 4 pages, RevTeX, submitted to PR
Building block libraries and structural considerations in the self-assembly of polyoxometalate and polyoxothiometalate systems
Inorganic metal-oxide clusters form a class of compounds that are unique in their topological and electronic versatility and are becoming increasingly more important in a variety of applications. Namely, Polyoxometalates (POMs) have shown an unmatched range of physical properties and the ability to form structures that can bridge several length scales. The formation of these molecular clusters is often ambiguous and is governed by self-assembly processes that limit our ability to rationally design such molecules. However, recent years have shown that by considering new building block principles the design and discovery of novel complex clusters is aiding our understanding of this process. Now with current progress in thiometalate chemistry, specifically polyoxothiometalates (POTM), the field of inorganic molecular clusters has further diversified allowing for the targeted development of molecules with specific functionality. This chapter discusses the main differences between POM and POTM systems and how this affects synthetic methodologies and reactivities. We will illustrate how careful structural considerations can lead to the generation of novel building blocks and further deepen our understanding of complex systems
A critical evaluation of network and pathway based classifiers for outcome prediction in breast cancer
Recently, several classifiers that combine primary tumor data, like gene
expression data, and secondary data sources, such as protein-protein
interaction networks, have been proposed for predicting outcome in breast
cancer. In these approaches, new composite features are typically constructed
by aggregating the expression levels of several genes. The secondary data
sources are employed to guide this aggregation. Although many studies claim
that these approaches improve classification performance over single gene
classifiers, the gain in performance is difficult to assess. This stems mainly
from the fact that different breast cancer data sets and validation procedures
are employed to assess the performance. Here we address these issues by
employing a large cohort of six breast cancer data sets as benchmark set and by
performing an unbiased evaluation of the classification accuracies of the
different approaches. Contrary to previous claims, we find that composite
feature classifiers do not outperform simple single gene classifiers. We
investigate the effect of (1) the number of selected features; (2) the specific
gene set from which features are selected; (3) the size of the training set and
(4) the heterogeneity of the data set on the performance of composite feature
and single gene classifiers. Strikingly, we find that randomization of
secondary data sources, which destroys all biological information in these
sources, does not result in a deterioration in performance of composite feature
classifiers. Finally, we show that when a proper correction for gene set size
is performed, the stability of single gene sets is similar to the stability of
composite feature sets. Based on these results there is currently no reason to
prefer prognostic classifiers based on composite features over single gene
classifiers for predicting outcome in breast cancer
Characterization and correction of stray light in TROPOMI-SWIR
The shortwave infrared (SWIR) spectrometer module of the Tropospheric
Monitoring Instrument (TROPOMI), on board the ESA Copernicus Sentinel-5
Precursor satellite, is used to measure atmospheric CO and methane columns.
For this purpose, calibrated radiance measurements are needed that are
minimally contaminated by instrumental stray light. Therefore, a method has
been developed and applied in an on-ground calibration campaign to
characterize stray light in detail using a monochromatic quasi-point light
source. The dynamic range of the signal was extended to more than 7Â orders of magnitude by performing measurements with different exposure times,
saturating detector pixels at the longer exposure times. Analysis of the
stray light indicates about 4.4 % of the detected light is correctable stray
light. An algorithm was then devised and implemented in the operational data
processor to correct in-flight SWIR observations in near-real time, based on
Van Cittert deconvolution. The stray light is approximated by a far-field
kernel independent of position and wavelength and an additional kernel
representing the main reflection. Applying this correction significantly
reduces the stray-light signal, for example in a simulated dark forest scene
close to bright clouds by a factor of about 10. Simulations indicate that
this reduces the stray-light error sufficiently for accurate gas-column
retrievals. In addition, the instrument contains five SWIR diode lasers that
enable long-term, in-flight monitoring of the stray-light distribution.</p
Determination of the TROPOMI-SWIR instrument spectral response function
The Tropospheric Monitoring Instrument (TROPOMI) is the single instrument on
board the ESA Copernicus Sentinel-5 Precursor satellite. TROPOMI is a
nadir-viewing imaging spectrometer with bands in the ultraviolet and visible,
the near infrared and the shortwave infrared (SWIR). An accurate instrument
spectral response function (ISRF) is required in the SWIR band where
absorption lines of CO, methane and water vapor overlap. In this paper, we
report on the determination of the TROPOMI-SWIR ISRF during an extensive
on-ground calibration campaign. Measurements are taken with a monochromatic
light source scanning the whole detector, using the spectrometer itself to
determine the light intensity and wavelength. The accuracy of the resulting
ISRF calibration key data is well within the requirement for trace-gas
retrievals. Long-term in-flight monitoring of SWIR ISRF is achieved using
five on-board diode lasers.</p
Evaluation dans différentes conditions environnementales de la résistance des variétés de carotte à Alternaria dauci: prise en compte du stade de développement de la plante et de l’agressivité de la souche
Evaluation dans différentes conditions environnementales de la résistance des variétés de carotte à Alternaria dauci : prise en compte du stade de développement de la plante et de l'agressivité de la souch
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