2,154 research outputs found
On axiomatic definitions of non-discrete affine buildings
In this paper we prove equivalence of sets of axioms for non-discrete affine
buildings, by providing different types of metric, exchange and atlas
conditions. We apply our result to show that the definition of a Euclidean
building depends only on the topological equivalence class of the metric on the
model space. The sharpness of the axioms dealing with metric conditions is
illustrated in an appendix. There it is shown that a space X defined over a
model space with metric d is possibly a building only if the induced distance
function on X satisfies the triangle inequality.Comment: Errors corrected, results extended. (This replaces the two earlier,
separate preprints "Axioms of affine buidlings" arXiv:0909.2967v1 and "Affine
buildings II" arXiv:0909.2059v1.
Investigations of the Non-Linear LMC Cepheid Period-Luminosity Relation with Testimator and Schwarz Information Criterion Methods
In this paper, we investigate the linearity versus non-linearity of the Large
Magellanic Cloud (LMC) Cepheid period-luminosity (P-L) relation using two
statistical approaches not previously applied to this problem: the testimator
method and the Schwarz Information Criterion (SIC). The testimator method is
extended to multiple stages for the first time, shown to be unbiased and the
variance of the estimated slope can be proved to be smaller than the standard
slope estimated from linear regression theory. The Schwarz Information
Criterion (also known as the Bayesian Information Criterion) is more
conservative than the Akaike Information Criterion and tends to choose lower
order models. By using simulated data sets, we verify that these statistical
techniques can be used to detect intrinsically linear and/or non-linear P-L
relations. These methods are then applied to independent LMC Cepheid data sets
from the OGLE project and the MACHO project, respectively. Our results imply
that there is a change of slope in longer period ranges for all of the data
sets. This strongly supports previous results, obtained from independent
statistical tests, that the observed LMC P-L relation is non-linear with a
break period at/around 10 days.Comment: 9 pages, 5 figures and 3 tables, PASP accepte
Grading and metastable effects in admittance spectroscopy of CIGS-based solar cells
Cu(In, Ga)Se2-based (CIGS) solar cells have achieved efficiencies up to 20%. Despite these excellent results, the understanding of the underlying mechanisms and the influence of defects on their performance is still incomplete. The determination of the energetic position of the defects and of their density of states is important. Admittance spectroscopy is an adequate technique for this. By varying the external voltage during the measurement, the spatial position where the defect distribution is sensed can be varied. However, the application of external biases can lead to metastable effects in the absorber and therefore to defect relaxation and changes in the doping distribution. Hence, it is important to separate between the effects caused by metastable changes and the change in sensing position of the admittance spectroscopy measurement. This can be achieved by varying the applied voltage during the creation of the metastable state and the measurement itself independently or simultaneously. Admittance spectroscopy under different bias voltage conditions performed on a flexible CIGS-based solar cell are presented and assessed
PSACNN: Pulse Sequence Adaptive Fast Whole Brain Segmentation
With the advent of convolutional neural networks~(CNN), supervised learning
methods are increasingly being used for whole brain segmentation. However, a
large, manually annotated training dataset of labeled brain images required to
train such supervised methods is frequently difficult to obtain or create. In
addition, existing training datasets are generally acquired with a homogeneous
magnetic resonance imaging~(MRI) acquisition protocol. CNNs trained on such
datasets are unable to generalize on test data with different acquisition
protocols. Modern neuroimaging studies and clinical trials are necessarily
multi-center initiatives with a wide variety of acquisition protocols. Despite
stringent protocol harmonization practices, it is very difficult to standardize
the gamut of MRI imaging parameters across scanners, field strengths, receive
coils etc., that affect image contrast. In this paper we propose a CNN-based
segmentation algorithm that, in addition to being highly accurate and fast, is
also resilient to variation in the input acquisition. Our approach relies on
building approximate forward models of pulse sequences that produce a typical
test image. For a given pulse sequence, we use its forward model to generate
plausible, synthetic training examples that appear as if they were acquired in
a scanner with that pulse sequence. Sampling over a wide variety of pulse
sequences results in a wide variety of augmented training examples that help
build an image contrast invariant model. Our method trains a single CNN that
can segment input MRI images with acquisition parameters as disparate as
-weighted and -weighted contrasts with only -weighted training
data. The segmentations generated are highly accurate with state-of-the-art
results~(overall Dice overlap), with a fast run time~( 45
seconds), and consistent across a wide range of acquisition protocols.Comment: Typo in author name corrected. Greves -> Grev
Electrically Tunable Optical Nonlinearities in Graphene-Covered SiN Waveguides Characterized by Four-Wave Mixing
We present a degenerate four-wave mixing experiment on a silicon nitride
(SiN) waveguide covered with gated graphene. We observe strong dependencies on
signal-pump detuning and Fermi energy, i.e. the optical nonlinearity is
demonstrated to be electrically tunable. In the vicinity of the interband
absorption edge () a peak value of the waveguide
nonlinear parameter of 6400 mW, corresponding to a
graphene nonlinear sheet conductivity A mV is measured
The assessment and treatment of panic disorder in general practice
Panic disorder (PD) is an anxiety disorder that affects many South Africans. If undiagnosed or mismanaged, this condition may impact profoundly on an individual's functioning and quality of life. At the primary level, the assessment of panic attacks requires consideration of a number of differential diagnoses. General practitioners also play an invaluable role in the acute and long-term care and appropriate referral of individuals with PD. This article provides an overview of PD in general practice and includes a guideline for case identification and therapeutic options
R-curve evaluation of pipeline girth welds using advanced measurement techniques
A strain-based flaw assessment procedure is recommended for girth welded pipelines subjected to large deformations. To evaluate the allowable defect dimensions, the tearing resistance needs to be characterized. This paper investigates the effect of weld metal strength mismatch on the resistance curve using Single Edge Notched Tension (SENT) specimens. Several advanced measurement techniques are applied during the tests in order to obtain a continuous measurement of crack extension and to visualize the deformation fields near the crack. The resistance curves are determined using a single specimen technique. The unloading compliance method and the potential drop method result in similar predictions of ductile crack extension, yielding similar resistance curves. Next to these measurements, the full field deformations are determined using digital image correlation. The experiments indicate that the position of the applied notch in the weld has the potential to influence the strain fields
A model of driven and decaying magnetic turbulence in a cylinder
Using mean-field theory, we compute the evolution of the magnetic field in a
cylinder with outer perfectly conducting boundaries, an imposed axial magnetic
and electric field. The thus injected magnetic helicity in the system can be
redistributed by magnetic helicity fluxes down the gradient of the local
current helicity of the small-scale magnetic field. A weak reversal of the
axial magnetic field is found to be a consequence of the magnetic helicity flux
in the system. Such fluxes are known to alleviate so-called catastrophic
quenching of the {\alpha}-effect in astrophysical applications. Application to
the reversed field pinch in plasma confinement devices is discussed.Comment: 7 pages, 4 figures, submitted to Phys. Rev.
JHKs time-series observations of a few ultracool dwarfs
The M8.5 object SSSPM J0109−5101 has recently been shown to be both a periodic and a
flaring variable, based on optical observations in the extreme red. More than 16 h of monitoring
in the near-infrared (NIR) reported here failed to show any variability. Similarly, no NIR
variability could be detected in intensive monitoring of three other suspected optical variables.
This paper also reports on photometry of half a dozen targets monitored over a few weeks, and
on the comparison of intensive monitoring at different epochs. In only one case, that of the
T dwarf binary Indi Bab, is there good evidence for variability. Our results allow stringent
limits to be placed on the NIR variability levels in a large sample of ultracool dwarfs.Web of Scienc
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