744,930 research outputs found
Nonlinear time-series analysis revisited
In 1980 and 1981, two pioneering papers laid the foundation for what became
known as nonlinear time-series analysis: the analysis of observed
data---typically univariate---via dynamical systems theory. Based on the
concept of state-space reconstruction, this set of methods allows us to compute
characteristic quantities such as Lyapunov exponents and fractal dimensions, to
predict the future course of the time series, and even to reconstruct the
equations of motion in some cases. In practice, however, there are a number of
issues that restrict the power of this approach: whether the signal accurately
and thoroughly samples the dynamics, for instance, and whether it contains
noise. Moreover, the numerical algorithms that we use to instantiate these
ideas are not perfect; they involve approximations, scale parameters, and
finite-precision arithmetic, among other things. Even so, nonlinear time-series
analysis has been used to great advantage on thousands of real and synthetic
data sets from a wide variety of systems ranging from roulette wheels to lasers
to the human heart. Even in cases where the data do not meet the mathematical
or algorithmic requirements to assure full topological conjugacy, the results
of nonlinear time-series analysis can be helpful in understanding,
characterizing, and predicting dynamical systems
Comment on the choice of time in a two-component formulation of the Wheeler--DeWitt equation
The two-component formalism in quantum cosmology is revisited with a
particular emphasis on the identification of time. Its relation with the
appearance of imaginary eigenvalues is established. It is explicitly shown how
a good choice of the global time prevents this peculiarity.Comment: 8 pages; version accepted for publication in Int. J. Mod. Phys.
The Time Series Consumption Function Revisited
macroeconomics, consumption, life-cycle hypothesis, consumer spending, income
Low-temperature electron dephasing time in AuPd revisited
Ever since the first discoveries of the quantum-interference transport in
mesoscopic systems, the electron dephasing times, , in the
concentrated AuPd alloys have been extensively measured. The samples were made
from different sources with different compositions, prepared by different
deposition methods, and various geometries (1D narrow wires, 2D thin films, and
3D thickfilms) were studied. Surprisingly, the low-temperature behavior of
inferred by different groups over two decades reveals a systematic
correlation with the level of disorder of the sample. At low temperatures,
where is (nearly) independent of temperature, a scaling
is found, where
is the maximum value of measured in the experiment, is the
electron diffusion constant, and the exponent is close to or slightly
larger than 1. We address this nontrivial scaling behavior and suggest that the
most possible origin for this unusual dephasing is due to dynamical structure
defects, while other theoretical explanations may not be totally ruled out.Comment: to appear in Physica E, Proceedings for the International Seminar and
Workshop "Quantum Coherence, Noise, and Decoherence in Nanostructures", 15-26
May 2006, Dresde
Time delays for 11 gravitationally lensed quasars revisited
We test the robustness of published time delays for 11 lensed quasars by
using two techniques to measure time shifts in their light curves.
We chose to use two fundamentally different techniques to determine time
delays in gravitationally lensed quasars: a method based on fitting a numerical
model and another one derived from the minimum dispersion method introduced by
Pelt and collaborators. To analyse our sample in a homogeneous way and avoid
bias caused by the choice of the method used, we apply both methods to 11
different lensed systems for which delays have been published: JVAS B0218+357,
SBS 0909+523, RX J0911+0551, FBQS J0951+2635, HE 1104-1805, PG 1115+080, JVAS
B1422+231, SBS 1520+530, CLASS B1600+434, CLASS B1608+656, and HE 2149-2745
Time delays for three double lenses, JVAS B0218+357, HE 1104-1805, and CLASS
B1600+434, as well as the quadruply lensed quasar CLASS B1608+656 are confirmed
within the error bars. We correct the delay for SBS 1520+530. For PG 1115+080
and RX J0911+0551, the existence of a second solution on top of the published
delay is revealed. The time delays in four systems, SBS 0909+523, FBQS
J0951+2635, JVAS B1422+231, and HE 2149-2745 prove to be less reliable than
previously claimed.
If we wish to derive an estimate of H_0 based on time delays in
gravitationally lensed quasars, we need to obtain more robust light curves for
most of these systems in order to achieve a higher accuracy and robustness on
the time delays
- …