5,142 research outputs found
On the entropy in II1 von Neumann algebras
Let α be an automorphism of a finite von Neumann algebra and let H(α) be its Connes-Størmer's entropy. We show that H(α) = 0 if α is the induced automorphism on the crossed product of a Lebesgue space by a pure point spectrum transformation. We also show that H is not continuous in α and we compute H(α) for some
Lithium in field Am and normal A-F-type stars
Preliminary abundances of lithium and a few other elements have been obtained
for 31 field Am stars with good Hipparcos parallaxes, as well as for 36 normal
A and F stars. Radial and projected rotational velocities were determined as
well. We examine the Li abundance as a function of the stellar parameters: for
normal stars, it is clearly bimodal for Teff < 7500 K, while Am-Fm stars are
all somewhat Li-deficient in this range. The most Li-deficient stars - either
Am or normal - tend to be at least slightly evolved, but the reverse is not
true.Comment: 4 pages, 2 figures, poster presented at the conference "Element
stratification in stars, 40 years of atomic diffusion", eds. G. Alecian, O.
Richard and S. Vauclair, EAS Publication Series, in pres
Mathematical justification of the hydrostatic approximation in the primitive equations of geophysical fluid dynamics
Geophysical fluids all exhibit a common feature: their aspect ratio (depth to horizontal width) is very small. This leads to an asymptotic model widely used in meteorology, oceanography, and limnology, namely the hydrostatic approximation of the time-dependent incompressible Navier–Stokes equations. It relies on the hypothesis that pressure increases linearly in the vertical direction. In the following, we prove a convergence and existence theorem for this model by means of anisotropic estimates and a new time-compactness criterium.Fonds Franco-Espagnol D.R.E.I.FMinisterio de Educación y Cienci
Stabilized Schemes for the Hydrostatic Stokes Equations
Some new stable finite element (FE) schemes are presented for the hydrostatic Stokes
system or primitive equations of the ocean. It is known that the stability of the mixed formulation ap-
proximation for primitive equations requires the well-known Ladyzhenskaya–Babuˇska–Brezzi condi-
tion related to the Stokes problem and an extra inf-sup condition relating the pressure and the vertical
velocity.
The main goal of this paper is to avoid this extra condition by adding a residual stabilizing term to the
vertical momentum equation. Then, the stability for Stokes-stable FE combinations is extended to
the primitive equations and some error estimates are provided using Taylor–Hood P2 –P1 or miniele-
ment (P1 +bubble)–P1 FE approximations, showing the optimal convergence rate in the P2 –P1 case.
These results are also extended to the anisotropic (nonhydrostatic) problem. On the other hand,
by adding another residual term to the continuity equation, a better approximation of the vertical
derivative of pressure is obtained. In this case, stability and error estimates including this better
approximation are deduced, where optimal convergence rate is deduced in the (P 1 +bubble)–P1 case.
Finally, some numerical experiments are presented supporting previous results
A comparative study of SAR data compression schemes
The amount of data collected from spaceborne remote sensing has substantially increased in the last years. During same time period, the ability to store or transmit data has not increased as quickly. At this time, there is a growing interest in developing compression schemes that could provide both higher compression ratios and lower encoding/decoding errors. In the case of the spaceborne Synthetic Aperture Radar (SAR) earth observation system developed by the French Space Agency (CNES), the volume of data to be processed will exceed both the on-board storage capacities and the telecommunication link. The objective of this paper is twofold: to present various compression schemes adapted to SAR data; and to define a set of evaluation criteria and compare the algorithms on SAR data. In this paper, we review two classical methods of SAR data compression and propose novel approaches based on Fourier Transforms and spectrum coding
Parameter estimation for chirp signals with deterministic time-varying amplitude
We consider the problem of estimating the parameters of chirp signals with deterministic time-varying amplitude . A method which
is computationally simpler than the Maximum Likelihood estimator is proposed . It invokes the extended invariance principle to
split the minimization problem and to decouple estimation of the phase parameters from that of the amplitude parameters . In a
first step, using a less detailed model for the signal, a simple scheme for estimating the phase parameters is presented . Then,
amplitude parameters are obtained from least-squares minimization techniques . The overall procedure provides asymptotically
efficient estimates . Numerical simulations attest to the validity of the theoretical analysis .Nous traitons dans cet article de l'estimation de signaux chirp dont l'amplitude, déterministe, varie dans le temps. Nous proposons une alternative à l'estimateur du Maximum de Vraisemblance qui est plus simple d'un point de vue calculatoire. Pour ceci, nous utilisons le principe d'invariance étendu qui permet de scinder le problème de minimisation et de découpler l'estimation des paramètres de phase de celle des paramètres d'amplitude. Dans un premier temps, en utilisant un modèle moins détaillé pour le signal, c'est-à-dire en considérant que tous les échantillons de l'amplitude sont à estimer, on obtient de manière simple les paramètres de phase. Les paramètres d'amplitude sont ensuite estimés par une technique des moindres carrés. La procédure permet d'obtenir des estimateurs asymptotiquement efficaces. Des simulations numériques viennent valider l'étude théorique
Photon pair production at flavour factories with per mille accuracy
We present a high-precision QED calculation, with 0.1% theoretical accuracy,
of two photon production in annihilation, as required by more and
more accurate luminosity monitoring at flavour factories. The accuracy of the
approach, which is based on the matching of exact next-to-leading order
corrections with a QED Parton Shower algorithm, is demonstrated through a
detailed analysis of the impact of the various sources of radiative corrections
to the experimentally relevant observables. The calculation is implemented in
the latest version of the event generator BabaYaga, available for precision
simulations of photon pair production at colliders of moderately high
energies.Comment: 11 pages, 5 figures, 1 tabl
Absence of lattice strain anomalies at the electronic topological transition in zinc at high pressure
High pressure structural distortions of the hexagonal close packed (hcp)
element zinc have been a subject of controversy. Earlier experimental results
and theory showed a large anomaly in lattice strain with compression in zinc at
about 10 GPa which was explained theoretically by a change in Fermi surface
topology. Later hydrostatic experiments showed no such anomaly, resulting in a
discrepancy between theory and experiment. We have computed the compression and
lattice strain of hcp zinc over a wide range of compressions using the
linearized augmented plane wave (LAPW) method paying special attention to
k-point convergence. We find that the behavior of the lattice strain is
strongly dependent on k-point sampling, and with large k-point sets the
previously computed anomaly in lattice parameters under compression disappears,
in agreement with recent experiments.Comment: 9 pages, 6 figures, Phys. Rev. B (in press
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