931 research outputs found
Determinants of short-period heart rate variability in the general population
Decreased heart rate variability (HRV) is associated with a worse prognosis in a variety of diseases and disorders. We evaluated the determinants of short-period HRV in a random sample of 149 middle-aged men and 137 women from the general population. Spectral analysis was used to compute low-frequency (LF), high-frequency (HF) and total-frequency power. HRV showed a strong inverse association with age and heart rate in both sexes with a more pronounced effect of heart rate on HRV in women. Age and heart rate-adjusted LF was significantly higher in men and HF higher in women. Significant negative correlations of BMI, triglycerides, insulin and positive correlations of HDL cholesterol with LF and total power occurred only in men. In multivariate analyses, heart rate and age persisted as prominent independent predictors of HRV. In addition, BMI was strongly negatively associated with LF in men but not in women, We conclude that the more pronounced vagal influence in cardiac regulation in middle-aged women and the gender-different influence of heart rate and metabolic factors on HRV may help to explain the lower susceptibility of women for cardiac arrhythmias. Copyright (C) 2001 S. Karger AG, Basel
Semiclassical Approach to Parametric Spectral Correlation with Spin 1/2
The spectral correlation of a chaotic system with spin 1/2 is universally
described by the GSE (Gaussian Symplectic Ensemble) of random matrices in the
semiclassical limit. In semiclassical theory, the spectral form factor is
expressed in terms of the periodic orbits and the spin state is simulated by
the uniform distribution on a sphere. In this paper, instead of the uniform
distribution, we introduce Brownian motion on a sphere to yield the parametric
motion of the energy levels. As a result, the small time expansion of the form
factor is obtained and found to be in agreement with the prediction of
parametric random matrices in the transition within the GSE universality class.
Moreover, by starting the Brownian motion from a point distribution on the
sphere, we gradually increase the effect of the spin and calculate the form
factor describing the transition from the GOE (Gaussian Orthogonal Ensemble)
class to the GSE class.Comment: 25 pages, 2 figure
Partner orbits and action differences on compact factors of the hyperbolic plane. Part I: Sieber-Richter pairs
Physicists have argued that periodic orbit bunching leads to universal
spectral fluctuations for chaotic quantum systems. To establish a more detailed
mathematical understanding of this fact, it is first necessary to look more
closely at the classical side of the problem and determine orbit pairs
consisting of orbits which have similar actions. In this paper we specialize to
the geodesic flow on compact factors of the hyperbolic plane as a classical
chaotic system. We prove the existence of a periodic partner orbit for a given
periodic orbit which has a small-angle self-crossing in configuration space
which is a `2-encounter'; such configurations are called `Sieber-Richter pairs'
in the physics literature. Furthermore, we derive an estimate for the action
difference of the partners. In the second part of this paper [13], an inductive
argument is provided to deal with higher-order encounters.Comment: to appear on Nonlinearit
Laughlin states on the Poincare half-plane and its quantum group symmetry
We find the Laughlin states of the electrons on the Poincare half-plane in
different representations. In each case we show that there exist a quantum
group symmetry such that the Laughlin states are a representation of
it. We calculate the corresponding filling factor by using the plasma analogy
of the FQHE.Comment: 9 pages,Late
Intermediate statistics for a system with symplectic symmetry: the Dirac rose graph
We study the spectral statistics of the Dirac operator on a rose-shaped
graph---a graph with a single vertex and all bonds connected at both ends to
the vertex. We formulate a secular equation that generically determines the
eigenvalues of the Dirac rose graph, which is seen to generalise the secular
equation for a star graph with Neumann boundary conditions. We derive
approximations to the spectral pair correlation function at large and small
values of spectral spacings, in the limit as the number of bonds approaches
infinity, and compare these predictions with results of numerical calculations.
Our results represent the first example of intermediate statistics from the
symplectic symmetry class.Comment: 26 pages, references adde
NGC 2419, M92, and the Age Gradient in the Galactic Halo
The WFPC2 camera on HST has been used to obtain deep main sequence photometry
of the low-metallicity ([Fe/H]=-2.14), outer-halo globular cluster NGC 2419. A
differential fit of the NGC 2419 CMD to that of the similarly metal-poor \
standard cluster M92 shows that they have virtually identical principal
sequences and thus the same age to well within 1 Gyr. Since other
low-metallicity clusters throughout the Milky Way halo have this same age to
within the 1-Gyr precision of the differential age technique, we conclude that
the earliest star (or globular cluster) formation began at essentially the same
time everywhere in the Galactic halo throughout a region now almost 200 kpc in
diameter. Thus for the metal-poorest clusters in the halo there is no
detectable age gradient with Galactocentric distance. To estimate the absolute
age of NGC 2419 and M92, we fit newly computed isochrones transformed through
model-atmosphere calculations to the (M_V,V-I) plane, with assumed distance
scales that represent the range currently debated in the literature.
Unconstrained isochrone fits give M_V(RR) = 0.55 \pm 0.06 and a resulting age
of 14 to 15 Gyr. Incorporating the full effects of helium diffusion would
further reduce this estimate by about 1 Gyr. A distance scale as bright as
M_V(RR) = 0.15 for [Fe/H] = -2, as has recently been reported, would leave
several serious problems which have no obvious solution in the context of
current stellar models.Comment: 32 pages, aastex, 9 postscript figures; accepted for publication in
AJ, September 1997. Also available by e-mail from [email protected]
Semiclassical Approach to Chaotic Quantum Transport
We describe a semiclassical method to calculate universal transport
properties of chaotic cavities. While the energy-averaged conductance turns out
governed by pairs of entrance-to-exit trajectories, the conductance variance,
shot noise and other related quantities require trajectory quadruplets; simple
diagrammatic rules allow to find the contributions of these pairs and
quadruplets. Both pure symmetry classes and the crossover due to an external
magnetic field are considered.Comment: 33 pages, 11 figures (appendices B-D not included in journal version
Semiclassical approximations for Hamiltonians with operator-valued symbols
We consider the semiclassical limit of quantum systems with a Hamiltonian
given by the Weyl quantization of an operator valued symbol. Systems composed
of slow and fast degrees of freedom are of this form. Typically a small
dimensionless parameter controls the separation of time
scales and the limit corresponds to an adiabatic limit, in
which the slow and fast degrees of freedom decouple. At the same time
is the semiclassical limit for the slow degrees of freedom.
In this paper we show that the -dependent classical flow for the
slow degrees of freedom first discovered by Littlejohn and Flynn, coming from
an \epsi-dependent classical Hamilton function and an -dependent
symplectic form, has a concrete mathematical and physical meaning: Based on
this flow we prove a formula for equilibrium expectations, an Egorov theorem
and transport of Wigner functions, thereby approximating properties of the
quantum system up to errors of order . In the context of Bloch
electrons formal use of this classical system has triggered considerable
progress in solid state physics. Hence we discuss in some detail the
application of the general results to the Hofstadter model, which describes a
two-dimensional gas of non-interacting electrons in a constant magnetic field
in the tight-binding approximation.Comment: Final version to appear in Commun. Math. Phys. Results have been
strengthened with only minor changes to the proofs. A section on the
Hofstadter model as an application of the general theory was added and the
previous section on other applications was remove
Cosmic String Network Evolution in arbitrary Friedmann-Lemaitre models
We use the velocity-dependent one-scale model by Martins & Shellard to
investigate the evolution of a GUT long cosmic string network in arbitrary
Friedmann-Lemaitre models. Four representative models are used to show that in
general there is no scaling solution. The implications for structure formation
are briefly discussed.Comment: 8 pages, 4 postscript figures included, submitted to Phys. Rev.
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