1,694 research outputs found
Cross-spectral analysis of physiological tremor and muscle activity. I. Theory and application to unsynchronized EMG
We investigate the relationship between the extensor electromyogram (EMG) and
tremor time series in physiological hand tremor by cross-spectral analysis.
Special attention is directed to the phase spectrum and the effects of
observational noise. We calculate the theoretical phase spectrum for a second
order linear stochastic process and compare the results to measured tremor data
recorded from subjects who did not show a synchronized EMG activity in the
corresponding extensor muscle. The results show that physiological tremor is
well described by the proposed model and that the measured EMG represents a
Newtonian force by which the muscle acts on the hand.Comment: 9 pages, 6 figures, to appear in Biological Cybernetic
Electron-hole spectra created by adsorption on metals from density-functional theory
Non-adiabaticity in adsorption on metal surfaces gives rise to a number of
measurable effects, such as chemicurrents and exo-electron emission. Here we
present a quantitative theory of chemicurrents on the basis of ground-state
density-functional theory (DFT) calculations of the effective electronic
potential and the Kohn-Sham band structure. Excitation probabilities are
calculated both for electron-hole pairs and for electrons and holes separately
from first-order time-dependent perturbation theory. This is accomplished by
evaluating the matrix elements (between Kohn-Sham states) of the rate of change
of the effective electronic potential between subsequent (static) DFT
calculations. Our approach is related to the theory of electronic friction, but
allows for direct access to the excitation spectra. The method is applied to
adsorption of atomic hydrogen isotopes on the Al(111) surface. The results are
compatible with the available experimental data (for noble metal surfaces); in
particular, the observed isotope effect in H versus D adsorption is described
by the present theory. Moreover, the results are in qualitative agreement with
computationally elaborate calculations of the full dynamics within
time-dependent density-functional theory, with the notable exception of effects
due to the spin dynamics. Being a perturbational approach, the method proposed
here is simple enough to be applied to a wide class of adsorbates and surfaces,
while at the same time allowing us to extract system-specific information.Comment: 23 pages, 9 figures, accepted for publication in Phys. Rev. B,
http://prb.aps.org/, v2: some major improvements, plus correction of minor
error
Parametric, nonparametric and parametric modelling of a chaotic circuit time series
The determination of a differential equation underlying a measured time
series is a frequently arising task in nonlinear time series analysis. In the
validation of a proposed model one often faces the dilemma that it is hard to
decide whether possible discrepancies between the time series and model output
are caused by an inappropriate model or by bad estimates of parameters in a
correct type of model, or both. We propose a combination of parametric
modelling based on Bock's multiple shooting algorithm and nonparametric
modelling based on optimal transformations as a strategy to test proposed
models and if rejected suggest and test new ones. We exemplify this strategy on
an experimental time series from a chaotic circuit where we obtain an extremely
accurate reconstruction of the observed attractor.Comment: 19 pages, 8 Fig
X-ray vs. Optical Variations in the Seyfert 1 Nucleus NGC 3516: A Puzzling Disconnectedness
We present optical broadband (B and R) observations of the Seyfert 1 nucleus
NGC 3516, obtained at Wise Observatory from March 1997 to March 2002,
contemporaneously with X-ray 2-10 keV measurements with RXTE. With these data
we increase the temporal baseline of this dataset to 5 years, more than triple
to the coverage we have previously presented for this object. Analysis of the
new data does not confirm the 100-day lag of X-ray behind optical variations,
tentatively reported in our previous work. Indeed, excluding the first year's
data, which drive the previous result, there is no significant correlation at
any lag between the X-ray and optical bands. We also find no correlation at any
lag between optical flux and various X-ray hardness ratios. We conclude that
the close relation observed between the bands during the first year of our
program was either a fluke, or perhaps the result of the exceptionally bright
state of NGC 3516 in 1997, to which it has yet to return. Reviewing the results
of published joint X-ray and UV/optical Seyfert monitoring programs, we
speculate that there are at least two components or mechanisms contributing to
the X-ray continuum emission up to 10 keV: a soft component that is correlated
with UV/optical variations on timescales >1 day, and whose presence can be
detected when the source is observed at low enough energies (about 1 keV), is
unabsorbed, or is in a sufficiently bright phase; and a hard component whose
variations are uncorrelated with the UV/optical.Comment: 9 pages, AJ, in pres
Tempting long-memory - on the interpretation of DFA results
We study the inference of long-range correlations by means of Detrended Fluctuation Analysis (DFA) and argue that power-law scaling of the fluctuation function and thus long-memory may not be assumed a priori but have to be established. This requires the investigation of the local slopes. We account for the variability characteristic for stochastic processes by calculating empirical confidence regions. Comparing a long-memory with a short-memory model shows that the inference of long-range correlations from a finite amount of data by means of DFA is not specific. We remark that scaling cannot be concluded from a straight line fit to the fluctuation function in a log-log representation. Furthermore, we show that a local slope larger than α=0.5 for large scales does not necessarily imply long-memory. We also demonstrate, that it is not valid to conclude from a finite scaling region of the fluctuation function to an equivalent scaling region of the autocorrelation function. Finally, we review DFA results for the Prague temperature data set and show that long-range correlations cannot not be concluded unambiguously
Exact numerical simulation of power-law noises
Many simulations of stochastic processes require colored noises: I describe
here an exact numerical method to simulate power-law noises: the method can be
extended to more general colored noises, and is exact for all time steps, even
when they are unevenly spaced (as may often happen for astronomical data, see
e.g. N. R. Lomb, Astrophys. Space Sci. {\bf 39}, 447 (1976)). The algorithm has
a well-behaved computational complexity, it produces a nearly perfect Gaussian
noise, and its computational efficiency depends on the required degree of noise
Gaussianity.Comment: 14 postscript figures, accepted for publication on Phys. Rev.
Optimale N-bemesting zomertarwe : resultaten onderzoek 2008
In opdracht van LTO en het ministerie van LNV is PPO in 2007 onderzoek gestart naar de optimale N-bemesting van zomertarwe. Hiertoe zijn bemestingsproeven aangelegd op drie klei- en drie zandlocaties met de bedoeling de benodigde datasets te verzamelen om een eventuele aanpassing van het bestaande stikstofbemestingsadvies mogelijk te maken. In 2008 is dit onderzoek voortgezet op 2 klei- en 2 zandlocaties. In dit rapport worden de resultaten van het onderzoek in 2008 weergegeven
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