École Polytechnique

About: École Polytechnique is a education organization based out in Palaiseau, France. It is known for research contribution in the topics: Laser & Plasma. The organization has 18995 authors who have published 39265 publications receiving 1225163 citations. The organization is also known as: Ecole Polytechnique & Polytechnique.

Group Invariant Scattering

Abstract: This paper constructs translation-invariant operators on L 2 .R d /, which are Lipschitz-continuous to the action of diffeomorphisms. A scattering propagator is a path-ordered product of nonlinear and noncommuting operators, each of which computes the modulus of a wavelet transform. A local integration defines a windowed scattering transform, which is proved to be Lipschitz-continuous to the action of C 2 diffeomorphisms. As the window size increases, it converges to a wavelet scattering transform that is translation invariant. Scattering coefficients also provide representations of stationary processes. Expected values depend upon high-order moments and can discriminate processes having the same power spectrum. Scattering operators are extended on L 2 .G/, where G is a compact Lie group, and are invariant under the action of G. Combining a scattering on L 2 .R d / and on L 2 .SO.d// defines a translation- and rotation-invariant scattering on L 2 .R d /. © 2012 Wiley Periodicals, Inc.

The second fermi large area telescope catalog of gamma-ray pulsars

Abstract: This catalog summarizes 117 high-confidence > 0.1 GeV gamma-ray pulsar detections using three years of data acquired by the Large Area Telescope (LAT) on the Fermi satellite. Half are neutron stars discovered using LAT data, through periodicity searches in gamma-ray and radio data around LAT unassociated source positions. The 117 pulsars are evenly divided into three groups: millisecond pulsars, young radio-loud pulsars, and young radio-quiet pulsars. We characterize the pulse profiles and energy spectra and derive luminosities when distance information exists. Spectral analysis of the off-peak phase intervals indicates probable pulsar wind nebula emission for four pulsars, and off-peak magnetospheric emission for several young and millisecond pulsars. We compare the gamma-ray properties with those in the radio, optical, and X-ray bands. We provide flux limits for pulsars with no observed gamma-ray emission, highlighting a small number of gamma-faint, radio-loud pulsars. The large, varied gamma-ray pulsar sample constrains emission models. Fermi's selection biases complement those of radio surveys, enhancing comparisons with predicted population distributions.

Plasma Expansion into a Vacuum

Abstract: The charge separation effects in the collisionless plasma expansion into a vacuum are studied in great detail. Accurate results are obtained concerning the structure of the ion front, the resultant ion energy spectrum, and more specifically the maximum ion energy. These are of crucial importance for the interpretation of recent experiments, where high-energy ion jets were produced from short pulse interaction with solid targets.

Electrochemical aspects of the generation of ramified metallic electrodeposits.

Abstract: The growth of ramified metallic deposits by electrodeposition from dilute salt solutions and in a high electric field has been considered in the geometry of a thin rectangular cell. The equations governing ion motion in the case of a dilute electrolyte have been solved numerically and analytically in a one-dimensional (1D) and a 2D approximation. It is found that ramified growth is a direct consequence of the creation of a space charge upon anion depletion in the vicinity of the cathode. The front of the ramified deposit is predicted to advance at a speed just equal to the velocity of the anions in the applied electric field. The presence of this space charge ahead of the growing front is associated with a potential drop \ensuremath{\delta}V. Resolution of the equations in the 2D case shows that the dense-parallel morphology of the deposit also results quite naturally from the existence of a space charge in the vicinity of the filament tips. The average filament spacing and sidebranch tilting angle can be directly related to the values of \ensuremath{\delta}V and of the applied electric field. The mechanism giving rise to the space charge will apply as well to any physical system involving electric conduction with two types of carriers, if one of them exhibits blocking-contact characteristics.

Sparse geometric image representations with bandelets

Abstract: This paper introduces a new class of bases, called bandelet bases, which decompose the image along multiscale vectors that are elongated in the direction of a geometric flow. This geometric flow indicates directions in which the image gray levels have regular variations. The image decomposition in a bandelet basis is implemented with a fast subband-filtering algorithm. Bandelet bases lead to optimal approximation rates for geometrically regular images. For image compression and noise removal applications, the geometric flow is optimized with fast algorithms so that the resulting bandelet basis produces minimum distortion. Comparisons are made with wavelet image compression and noise-removal algorithms.