Groupes plongés quasi isométriquement dans un groupe de Lie

Soit un sous-groupe discret de type fini dun groupe de Lie semi-simple G. On donne ici une condition suffisante pour que linjection de dans le groupe G admet un voisinage de représentations fidèles et discrètes.     

Sur les caractères du groupe de Suzuki

Let σ be the automorphism of $\mathrm{Sp}(4\text{,}{2}^{2n+1})$ so that the set of fixed points is the Suzuki group. We propose in this Note to calculate the irreductible characters of $\mathrm{Sp}(4\text{,}{2}^{2n+1})?〈\sigma 〉$, extension of degree two of the symplectic group, and describe two consequences: first, the principal block of the Suzuki group and of this extension are perfectly isometric, and secondly we determine the eigenvalues of Frobenius of the unipotent characters of the Suzuki group. To cite this article: O. Brunat, C. R. Acad. Sci. Paris, Ser. I 339 (2004).     

Time-resolved tracking of a sound scatterer in a complex flow: nonstationary signal analysis and applications

It is known that ultrasound techniques yield nonintrusive measurements of hydrodynamic flows. For example, the study of the echoes produced by a large number of particles insonified by pulsed wavetrains has led to a now-standard velocimetry device. In this paper, a new technique for the measurement of the velocity of individual solid particles moving in fluid flows is proposed. It relies on the ability to resolve in time the Doppler shift of the sound scattered by the continuously insonified particle. For this signal-processing problem two classes of approaches can be used: time-frequency analysis and parametric high-resolution methods. In the first class the spectrogram and reassigned spectrogram is considered, and applied to detect the motion of a small bead settling in a fluid at rest. In nonstationary flows, methods in the second class are more robust. An approximated maximum likelihood (AML) technique has been adapted, coupled with a generalized Kalman filter. This method allows for the estimation of rapidly varying frequencies; the parametric nature of the algorithm also provides an estimate of the variance of the identified frequency parameters.     

Large-scale finite-wavelength modulation within turbulent shear flows

We show that turbulent "spirals" and "spots" observed in Taylor-Couette and plane Couette flow correspond to a turbulence-intensity modulated finite-wavelength pattern which in every respect fits the phenomenology of coupled noisy Ginzburg-Landau (amplitude) equations with noise. This suggests the existence of a long-wavelength instability of the homogeneous turbulence regime.     

Self-healing slip pulses along a gel/glass interface

We present experimental evidence of self-healing shear cracks at a gel/glass interface. This system exhibits two dynamical regimes depending on the driving velocity: steady sliding at high velocity (>V(c) approximately 100--125 microm/s), characterized by a shear-thinning rheology, and periodic stick-slip dynamics at low velocity. In this last regime, slip occurs by propagation of pulses that restick via a "healing instability" occurring when the local sliding velocity reaches the macroscopic transition velocity V(c). At driving velocities close below V(c), the system exhibits complex spatiotemporal behavior.     

Multispecies pair annihilation reactions

We consider diffusion-limited reactions A(i)+A(j)--> (1< or =i2 and d> or =2, we argue that the asymptotic density decay for such mutual annihilation processes with equal rates and initial densities is the same as for single-species pair annihilation A+A-->. In d=1, however, particle segregation occurs for all q< infinity. The total density decays according to a q dependent power law, rho(t) approximately t(-alpha(q)). Within a simplified version of the model alpha(q)=(q-1)/2q can be determined exactly. Our findings are supported through Monte Carlo simulations.     

Inverse proximity effect in a strongly correlated electron system

An anomalous superconducting proximity effect between a strongly correlated electron system and a normal metal is demonstrated. The model system is a 2D ultrathin superconducting quench-condensed Pb film. Such a highly disordered film has a reduced transition temperature (T(c) = 1.7 K) due to the strong e(-)-e(-) interaction. Instead of weakening the superconductivity, an overlayer of Ag on Pb induces an increase of both the T(c) and the gap. The restoration of the electron screening brought about by the quasiparticles from the normal metal can explain this striking inverse proximity effect.     

Unprecedented zwitterion in quinonoid chemistry

The first 12 pi-electron zwitterionic structure in quinonoid chemistry is described with the N,N,O,O-molecule 6 in which the positive charge is pi-delocalized between the nitrogen atoms and the negative charge between the oxygen atoms; depending on the crystallization solvent, a 1D-tape-like H-bonded network can be generated in the solid-state.     

Laplace Transform of Temperate Holomorphic Functions

Let V be n-dimensional complex vector space. The aim of this paper is to give an elementary proof of the isomorphism O ^t _V ^{^}[n] O ^t _V*, which quantizes the Fourier–Sato transform of the conic sheaf O ^t _V of temperate holomorphic functions     

Extremal Forex Returns in Extremely Large Data Sets

Exciting information for risk and investment analysis is obtained from an exceptionally large and automatically filtered high frequency data set containing all the forex quote prices on Reuters during a ten-year period. It is shown how the high frequency data improve the efficiency of the tail risk cum loss estimates. We demonstrate theoretically and empirically that the heavy tail feature of foreign exchange rate returns implies that position limits for traders calculated under the industry standard normal model are either not prudent enough, or are overly conservative depending on the time horizon.