3,262 research outputs found
What can be inferred from surrogate data testing?
Surrogate data testing for linearity is frequently applied to confirm the
results of nonlinear time series analysis. We argue that this, in general, is
not possible.Comment: 1 pag
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
Confluence reduction for Markov automata
Markov automata are a novel formalism for specifying systems exhibiting nondeterminism, probabilistic choices and Markovian rates. Recently, the process algebra MAPA was introduced to efficiently model such systems. As always, the state space explosion threatens the analysability of the models generated by such specifications. We therefore introduce confluence reduction for Markov automata, a powerful reduction technique to keep these models small. We define the notion of confluence directly on Markov automata, and discuss how to syntactically detect confluence on the MAPA language as well. That way, Markov automata generated by MAPA specifications can be reduced on-the-fly while preserving divergence-sensitive branching bisimulation. Three case studies demonstrate the significance of our approach, with reductions in analysis time up to an order of magnitude
Increasing insightful thinking in analytic geometry
Elsewhere in this issue Ferdinand Verhulst described the discussion of the interaction of anal-
ysis and geometry in the 19th century. In modern times such discussions come up again and again. As of 2014, synthetic geometry will not be part of the Dutch `vwo ¿ mathematics B¿
programme anymore. Instead, the focus will be more on analytic geometry. Mark Timmer and Nellie Verhoef explored possibilities to connect the two disciplines in order to have students look at analytical exercises from a more synthetic point of vie
Selective low concentration ammonia sensing in a microfluidic lab-on-a-chip
In the medical community, there is a considerable interest in a diagnostic breath analyzer for ammonia that is selectively enough to measure in exhaled air and small enough for the small volumes available in such an application. An indirect measurement system for low gaseous ammonia concentrations has been miniaturized and integrated on a chip in order to reach this goal. The detection limit of the system was calculated to be 1.1 parts per billion (ppb). The response time was determined to be 1.6 min with a gas How of 50 ml/min. The required gas volume for one measurement is therefore sufficiently small, although sampling assistance is required for breath analysis. The selectivity of the system is sufficient to measure ammonia concentrations in the low-ppb range. The system is even sufficiently selective to be used in environments that contain elevated carbon dioxide levels, like exhaled air. The lower ammonia concentration expected in diagnostic breath analysis applications, 50 ppb, was demonstrated to be detectable
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