948 research outputs found
A frequency measure robust to linear filtering
A definition of frequency (cycles per unit-time) based on an approximate
reconstruction of the phase-space trajectory of an oscillator from a signal is
introduced. It is shown to be invariant under linear filtering, and therefore
inaccessible by spectral methods. The effect of filtering on frequency in cases
where this definition does not perfectly apply is quantified.Comment: 10 pages, 2 figure
The Amplitude Mode in the Quantum Phase Model
We derive the collective low energy excitations of the quantum phase model of
interacting lattice bosons within the superfluid state using a dynamical
variational approach. We recover the well known sound (or Goldstone) mode and
derive a gapped (Higgs type) mode that was overlooked in previous studies of
the quantum phase model. This mode is relevant to ultracold atoms in a strong
optical lattice potential. We predict the signature of the gapped mode in
lattice modulation experiments and show how it evolves with increasing
interaction strength.Comment: 4 pages, 3 figure
Influence of berry growth and growth regulators on the development of grape peduncles in Vitis vinifera L.
Removal of grape berries (cv. Roter Gutedel) after setting (10 d after anthesis) had just a slight effect on the subsequent health of the peduncle. But if younger berries of flowers were removed before this stage the peduncle formed abscission layers along its length, and/or it died. Treatment of deflowered peduncles with a-naphthalene acetic acid (NAi\) prevented peduncle abscission and death; it is suggested that auxins produced by flowers have a hormonal role in maintaining normal development of the peduncle. Treatment with 2-chloroethyltrimethyl ammonium chloride (CCC), benzyladenine (BA), indole-3-acetic acid (IAA) or 3-indolebutyric acid (IBA) did not give these effects. The lack of effect of the latter two compounds (auxins) is possibly due to their catabolism.Treatment with gibberellic acid (GA) resembled NAA in that it also maintained the health of deflowered peduncles, but GA had the additional effect of causing prolonged development of metaxylem. Gibberellin produced by developing berries may also have a hormonal role in peduncle development. The occurrence of grape peduncle necrosis (stiellaehme), which appears associated with a gibberellin deficiency during setting, supports this idea
Detecting periodicity in experimental data using linear modeling techniques
Fourier spectral estimates and, to a lesser extent, the autocorrelation
function are the primary tools to detect periodicities in experimental data in
the physical and biological sciences. We propose a new method which is more
reliable than traditional techniques, and is able to make clear identification
of periodic behavior when traditional techniques do not. This technique is
based on an information theoretic reduction of linear (autoregressive) models
so that only the essential features of an autoregressive model are retained.
These models we call reduced autoregressive models (RARM). The essential
features of reduced autoregressive models include any periodicity present in
the data. We provide theoretical and numerical evidence from both experimental
and artificial data, to demonstrate that this technique will reliably detect
periodicities if and only if they are present in the data. There are strong
information theoretic arguments to support the statement that RARM detects
periodicities if they are present. Surrogate data techniques are used to ensure
the converse. Furthermore, our calculations demonstrate that RARM is more
robust, more accurate, and more sensitive, than traditional spectral
techniques.Comment: 10 pages (revtex) and 6 figures. To appear in Phys Rev E. Modified
styl
Test your surrogate data before you test for nonlinearity
The schemes for the generation of surrogate data in order to test the null
hypothesis of linear stochastic process undergoing nonlinear static transform
are investigated as to their consistency in representing the null hypothesis.
In particular, we pinpoint some important caveats of the prominent algorithm of
amplitude adjusted Fourier transform surrogates (AAFT) and compare it to the
iterated AAFT (IAAFT), which is more consistent in representing the null
hypothesis. It turns out that in many applications with real data the
inferences of nonlinearity after marginal rejection of the null hypothesis were
premature and have to be re-investigated taken into account the inaccuracies in
the AAFT algorithm, mainly concerning the mismatching of the linear
correlations. In order to deal with such inaccuracies we propose the use of
linear together with nonlinear polynomials as discriminating statistics. The
application of this setup to some well-known real data sets cautions against
the use of the AAFT algorithm.Comment: 14 pages, 15 figures, submitted to Physical Review
Using Topological Statistics to Detect Determinism in Time Series
Statistical differentiability of the measure along the reconstructed
trajectory is a good candidate to quantify determinism in time series. The
procedure is based upon a formula that explicitly shows the sensitivity of the
measure to stochasticity. Numerical results for partially surrogated time
series and series derived from several stochastic models, illustrate the
usefulness of the method proposed here. The method is shown to work also for
high--dimensional systems and experimental time seriesComment: 23 RevTeX pages, 14 eps figures. To appear in Physical Review
Calcifediol versus vitamin D3 effects on gait speed and trunk sway in young postmenopausal women: a double-blind randomized controlled trial
UNLABELLED In this double-blind RCT, 4-month treatment with calcifediol compared with vitamin D3 improved gait speed by 18 % among young postmenopausal women. Consistently, change in 25(OH)D blood levels over time were significantly correlated with improvement in gait speed in these women. No effect could be demonstrated for trunk sway. INTRODUCTION The aim of this study is to test the effect of calcifediol compared with vitamin D3 on gait speed and trunk sway. METHODS Twenty healthy postmenopausal women with an average 25(OH)D level of 13.2 ng/ml (SD = ±3.9) and a mean age of 61.5 years (SD = ±7.2) were randomized to either 20 μg of calcifediol or 20 μg (800 IU) of vitamin D3 per day in a double-blind manner. At baseline and at 4 months of follow-up, the same physiotherapist blinded to treatment allocation tested 8-m gait speed and a body sway test battery (Sway star pitch and roll angle plus velocity while walking 8 m, and standing on both legs on a hard and soft surface). All analyses adjusted for baseline measurement, age, and body mass index. RESULTS Mean 25(OH)D levels increased to 69.3 ng/ml (SD = ±9.5) in the calcifediol group and to 30.5 ng/ml (SD = ±5.0) in the vitamin D3 group (p < 0.0001). Women receiving calcifediol compared with vitamin D3 had an 18 % greater improvement in gait speed at 4-month follow-up (p = 0.046) adjusting for baseline gait speed, age, and body mass index. Also, change in gait speed was significantly correlated with change in serum 25(OH)D concentrations (r = 0.5; p = 0.04). Across three tests of trunk sway, there were no consistent differences between groups and no significant correlation between change in 25(OH)D serum concentrations and change in trunk sway. CONCLUSIONS Calcifediol improved gait speed in early postmenopausal women compared with vitamin D3 and change in 25(OH)D level was moderately correlated with improvement in gait speed. A benefit on trunk sway could not be demonstrated
SPLEND1D, a reduced one-dimensional model to investigate the physics of plasma detachment
Studying the process of divertor detachment and the associated complex
interplay of plasma dynamics and atomic physics processes is of utmost
importance for future fusion reactors. Whilst simplified analytical models
exist to interpret the general features of detachment, they are limited in
their predictive power, and complex 2D or even 3D codes are generally required
to provide a self-consistent picture of the divertor. As an intermediate step,
1D models of the Scrape-Off Layer (SOL) can be particularly insightful as the
dynamics are greatly simplified, while still self-consistently including
various source and sink terms at play, as well as additional important effects
such as flows. These codes can be used to shed light on the physics at play, to
perform fast parameter scans, or to interpret experiments. In this paper, we
introduce the SPLEND1D (Simulator of PLasma ENabling Detachment in 1D) code: a
fast and versatile 1D SOL model. We present in detail the model that is
implemented in SPLEND1D. We then employ the code to explore various elements of
detachment physics for parameters typical of the Tokamak \`a Configuration
Variable (TCV), including the atomic physics and other processes behind power
and momentum losses, and explore the various hypotheses and free parameters of
the model
Statistics of finite-time Lyapunov exponents in the Ulam map
The statistical properties of finite-time Lyapunov exponents at the Ulam
point of the logistic map are investigated. The exact analytical expression for
the autocorrelation function of one-step Lyapunov exponents is obtained,
allowing the calculation of the variance of exponents computed over time
intervals of length . The variance anomalously decays as . The
probability density of finite-time exponents noticeably deviates from the
Gaussian shape, decaying with exponential tails and presenting spikes
that narrow and accumulate close to the mean value with increasing . The
asymptotic expression for this probability distribution function is derived. It
provides an adequate smooth approximation to describe numerical histograms
built for not too small , where the finiteness of bin size trimmes the sharp
peaks.Comment: 6 pages, 4 figures, to appear in Phys. Rev.
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