22,817 research outputs found
Complements on disconnected reductive groups
We present various results on disconnected reductive groups, in particular
about the characteristic 0 representation theory of such groups over finite
fields.Comment: This version takes into account improvements suggested by G. Mall
Tunnel effect for semiclassical random walk
We study a semiclassical random walk with respect to a probability measure
with a finite number n_0 of wells. We show that the associated operator has
exactly n_0 exponentially close to 1 eigenvalues (in the semiclassical sense),
and that the other are O(h) away from 1. We also give an asymptotic of these
small eigenvalues. The key ingredient in our approach is a general
factorization result of pseudodifferential operators, which allows us to use
recent results on the Witten Laplacian
The space of unipotently supported class functions on a finite reductive group
We determine the Lusztig restrictions on the space of class functions with a
unipotent support on a finite reductive group. In particular we give a simple
expression for the Lusztig restrictions of the generalized Green functions and
we describe the Lusztig restrictions of the generalized Gelfand-Graev
representations. We make explicit computations for the Gelfand-Graev
representations associated to the subregular unipotent class. In the case of
SLn we show that the computations can be reduced to the case of GLn' for
various n'.Comment: 21 page
Time resolved tracking of a sound scatterer in a turbulent flow: non-stationary signal analysis and applications
It is known that ultrasound techniques yield non-intrusive measurements of
hydrodynamic flows. For example, the study of the echoes produced by a large
number of particle insonified by pulsed wavetrains has led to a now standard
velocimetry technique. In this paper, we propose to extend the method to the
continuous tracking of one single particle embedded in a complex flow. This
gives a Lagrangian measurement of the fluid motion, which is of importance in
mixing and turbulence studies. The method relies on the ability to resolve in
time the Doppler shift of the sound scattered by the continuously insonfied
particle.
For this signal processing problem two classes of approaches are used:
time-frequency analysis and parametric high resolution methods. In the first
class we consider the spectrogram and reassigned spectrogram, and we apply it
to detect the motion of a small bead settling in a fluid at rest. In more
non-stationary turbulent flows where methods in the second class are more
robust, we have adapted an Approximated Maximum Likelihood technique coupled
with a generalized Kalman filter.Comment: 16 pages 9 figure
The Power of Axisymmetric Pulsar
Stationary force-free magnetosphere of an axisymmetric pulsar is shown to
have a separatrix inclination angle of 77.3. The electromagnetic field
has an singularity inside the separatrix near the light cylinder. A
numerical simulation of the magnetosphere which crudely reproduces these
properties is presented. The numerical results are used to estimate the power
of an axisymmetric pulsar: . A need for a better
numerical simulation is pointed out.Comment: 9 page
The `bare' strange stars might not be bare
It is proposed that the `bare' strange matter stars might not be bare, and
radio pulsars might be in fact `bare' strange stars. As strange matter stars
being intensely magnetized rotate, the induced unipolar electric fields would
be large enough to construct magnetospheres. This situation is very similar to
that discussed by many authors for rotating neutron stars. Also, the strange
stars with accretion crusts in binaries could act as X-ray pulsars or X-ray
bursters. There are some advantages if radio pulsars are `bare' strange stars.Comment: 11 pages, 1 Postscript figures, LaTeX, Chin. Phys. Lett. 1998,
Vol.15, Nov.12, p.93
A Methodology to Engineer and Validate Dynamic Multi-level Multi-agent Based Simulations
This article proposes a methodology to model and simulate complex systems,
based on IRM4MLS, a generic agent-based meta-model able to deal with
multi-level systems. This methodology permits the engineering of dynamic
multi-level agent-based models, to represent complex systems over several
scales and domains of interest. Its goal is to simulate a phenomenon using
dynamically the lightest representation to save computer resources without loss
of information. This methodology is based on two mechanisms: (1) the activation
or deactivation of agents representing different domain parts of the same
phenomenon and (2) the aggregation or disaggregation of agents representing the
same phenomenon at different scales.Comment: Presented at 3th International Workshop on Multi-Agent Based
Simulation, Valencia, Spain, 5th June 201
Heavy Quark Diffusion from the Lattice
We study the diffusion of heavy quarks in the Quark Gluon Plasma using the
Langevin equations of motion and estimate the contribution of the transport
peak to the Euclidean current-current correlator. We show that the Euclidean
correlator is remarkably insensitive to the heavy quark diffusion coefficient
and give a simple physical interpretation of this result using the free
streaming Boltzmann equation. However if the diffusion coefficient is smaller
than , as favored by RHIC phenomenology, the transport
contribution should be visible in the Euclidean correlator. We outline a
procedure to isolate this contribution.Comment: 24 pages, 5 figure
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