1,063 research outputs found
Generalized Zeta function representation of groups and 2-dimensional Topological Yang-Mills theory: The example of GL(2, F_q) and PGL(2, F_q)
We recall the relation between Zeta function representation of groups and
two-dimensional topological Yang-Mills theory through Mednikh formula. We prove
various generalisations of Mednikh formulas and define generalization of Zeta
functions representations of groups. We compute some of these functions in the
case of the finite group and We
recall the table characters of these groups for any , compute the
Frobenius-Schur indicator of their irreducible representations and give the
explicit structure of their fusion ringsComment: 27 page
Comparison of the estimation of the degree of polarization from four or two intensity images degraded by speckle noise
Active polarimetric imagery is a powerful tool for accessing the information
present in a scene. Indeed, the polarimetric images obtained can reveal
polarizing properties of the objects that are not avalaible using conventional
imaging systems. However, when coherent light is used to illuminate the scene,
the images are degraded by speckle noise. The polarization properties of a
scene are characterized by the degree of polarization. In standard polarimetric
imagery system, four intensity images are needed to estimate this degree . If
we assume the uncorrelation of the measurements, this number can be decreased
to two images using the Orthogonal State Contrast Image (OSCI). However, this
approach appears too restrictive in some cases. We thus propose in this paper a
new statistical parametric method to estimate the degree of polarization
assuming correlated measurements with only two intensity images. The estimators
obtained from four images, from the OSCI and from the proposed method, are
compared using simulated polarimetric data degraded by speckle noise
Can Lightcone Fluctuations be Probed with Cosmological Backgrounds?
Finding signatures of quantum gravity in cosmological observations is now
actively pursued both from the theoretical and the experimental side. Recent
work has concentrated on finding signatures of light-cone fluctuations in the
CMB. Because in inflationary scenarios a Gravitational Wave Background (GWB) is
always emitted much before the CMB, we can ask, in the hypothesis where this
GWB could be observed, what is the imprint of light cone fluctuations on this
GWB. We show that due to the flat nature of the GWB spectrum, the effect of
lightcone fluctuations are negligible.Comment: 10 pages, references adde
Applicability of Boussinesq approximation in a turbulent fluid with constant properties
The equations of motion describing buoyant fluids are often simplified using
a set of approximations proposed by J. Boussinesq one century ago. To resume,
they consist in assuming constant fluid properties, incompressibility and
conservation of calories during heat transport. Assuming fulfilment of the
first requirement (constant fluid properties), we derive a set of 4 criteria
for assessing the validity of the two other requirements in turbulent
Rayleigh-B\'enard convection. The first criterion simply
results from the incompressibility condition in the thermal boundary layer
( and are the thermal expansion coefficient and the
temperature difference driving the flow). The 3 other criteria are proportional
or quadratic with the density stratification or, equivalently with the
temperature difference resulting from the adiabatic gradient across the cell
. Numerical evaluations with air, water and cryogenic helium show
that most laboratory experiments are free from such Boussinesq violation as
long as the first criterion is fulfilled. In ultra high Rayleigh numbers
() experiments in He, one of the stratification criteria, scaling
with , could be violated. This criterion garanties that
pressure fluctuations have a negligible influence both on the density variation
and on the heat transfer equation through compression/expansion cycles.
Extrapolation to higher suggests that strong violation of Boussinesq
approximation could occur in atmospheric convection.Comment: Submitted to Phys.Fluids (oct 2007
Universal Vertex-IRF Transformation for Quantum Affine Algebras
We construct a universal Vertex-IRF transformation between Vertex type
universal solution and Face type universal solution of the quantum dynamical
Yang-Baxter equation. This universal Vertex-IRF transformation satisfies the
generalized coBoundary equation and is an extension of our previous work to the
quantum affine case. This solution has a simple Gauss
decomposition which is constructed using Sevostyanov's characters of twisted
quantum Borel algebras. We show that the evaluation of this universal solution
in the evaluation representation of gives the standard
Baxter's transformation between the 8-Vertex model and the IRF height model.Comment: 58 page
Energy cascade and the four-fifths law in superfluid turbulence
The 4/5-law of turbulence, which characterizes the energy cascade from large
to small-sized eddies at high Reynolds numbers in classical fluids, is verified
experimentally in a superfluid 4He wind tunnel, operated down to 1.56 K and up
to R_lambda ~ 1640. The result is corroborated by high-resolution simulations
of Landau-Tisza's two-fluid model down to 1.15 K, corresponding to a residual
normal fluid concentration below 3 % but with a lower Reynolds number of order
R_lambda ~ 100. Although the K\'arm\'an-Howarth equation (including a viscous
term) is not valid \emph{a priori} in a superfluid, it is found that it
provides an empirical description of the deviation from the ideal 4/5-law at
small scales and allows us to identify an effective viscosity for the
superfluid, whose value matches the kinematic viscosity of the normal fluid
regardless of its concentration.Comment: 6 pages, 7 figure
- …