Variation du nombre de Lefschetz dans une famille d'automorphismes analytiques

Sans résumé     

Abrupt increase in thermal diffusion belowT c in YBa2Cu3O7−δ

We report the variation of the thermal diffusivity in the high temperature superconductor YBa₂Cu₃O_7−δ below the transition temperature measured using photo-acoustic technique α is found to increase sharply belowT _c. Qualitative explanations are given.     

Efficient room-temperature operation of Cr3+-sensitized,flashlamp-pumped, 2µm lasers

Utilizing the results of Cr³⁺ → Tm³⁺ transfer efficiency studies, we have demonstrated that yttrium aluminium garnet (YAG) is the preferred host for room-temperature, flashlamp-pumped solid-state lasers operating in the 2.0 µm spectral range. We report data on two different sensitizer-activator combinations in YAG and yttrium scandium gallium garnet (YSGG) laser materials: one is doped with Cr:Tm:Ho and operates on the Ho³⁺⁵I₇ →⁵I₈ transition at 2.097 µm; the other is doped only with Cr:Tm, which lases on the Tm³⁺³F₄ →³H₆ transition at 2.014 µm. We have achieved a slope efficiency of 5.1% with the Cr:Tm:Ho:YAG laser, which is the highest slope efficiency yet reported for a room-temperature, flashlamp-pumped, 2 µm solid-state laser. We have measured thresholds as low as 38 J and output energies >1.5 J for that system. We also report the first room-temperature operation of an efficient flashlamp-pumped Cr:Tm:YAG laser at 2.014 µm. Thresholds as low as 43 J, output energies exceeding 2 J, and slope efficiencies as high as 4.5% have been achieved. This is an order of magnitude higher than the efficiency previously reported for a 2.01 µm Cr:Tm:YAG laser operated at cryogenic temperatures. These two efficient 2 µm laser systems (Cr:Tm:Ho:YAG and Cr:Tm:YAG) are discussed in terms of their potential for Q-switched operation.     

Multiple feedback at a single-server station

A single server facility is equipped to perform a collection of operations. The service rendered to a customer is a branching process of operations. While the performance of an operation may not be interrupted before its completion, once completed, the required follow-up work may be delayed, at a cost per unit time of waiting that depends on the type and load of work being delayed. Under some probabilistic assumptions on the nature of the required service and on the stream of customers, the problem is to find service schedules that minimize expected costs. The authors generalize results of Bruno [2], Chazan, Konheim and B. Weiss [4], Harrison [8], Klimov [10], Konheim [11], and Meilijson and G. Weiss [13], using a dynamic programming approach.     

The expected value of some functions of the convex hull of a random set of points sampled inR d

This paper presents formulas and asymptotic expansions for the expected number of vertices and the expected volume of the convex hull of a sample ofn points taken from the uniform distribution on ad-dimensional ball. It is shown that the expected number of vertices is asymptotically proportional ton ^{(d−1)/(d+1)}, which generalizes Rényi and Sulanke’s asymptotic raten ^(1/3) ford=2 and agrees with Raynaud’s asymptotic raten ^{(d−1)/(d+1)} for the expected number of facets, as it should be, by Bárány’s result that the expected number ofs-dimensional faces has order of magnitude independent ofs. Our formulas agree with the ones Efron obtained ford=2 and 3 under more general distributions. An application is given to the estimation of the probability content of an unknown convex subset ofR ^d .     

Anodic oxidation of porous silicon bilayers

This paper focuses on the study of the effect of anodic oxidation in porous silicon bilayers composed of two porous layers of different porosities. The order of the two types of layers has been alternated, and the thicknesses and refractive indices have been optically characterized by Fourier transform infrared spectroscopy. The results show that the refractive index of anodic oxidized porous silicon is reduced significantly with respect to just formed porous silicon. It is also observed that the quality of the oxidation is related to the porosity of the inner porous layer of the silicon bilayer structure. This effect is interpreted in terms of quantum size effects.     

Influence of various types of elastic foundation on the divergence and flutter of Ziegler's model structure

Ziegler's model structure (consisting of two rigid massless bars connected by a frictionless joint, with two rotational springs under follower force) is utilized in order to illustrate the influence of various types of elastic foundation on the buckling loads. In particular, continuous rotary or translational (Winkler) foundations are studied, whereas in the case of discrete foundation two translational springs are introduced in addition to the rotary springs.On leave from the Technion, Haifa, Israel.     

Riccati differential inequalities and dichotomies for linear systems

A general notion of dichotomy for linear differential systems is investigated. It is well known that a system x′=A(t)x in which the matrix A(t) is bounded and diagonally dominant by rows or columns has an invariant splitting of its solution space into two subspaces each uniformly asymptotically stable, one for increasing time and the other for decreasing time. Similar results are obtained here where the concept of diagonal dominance is weakened using Riccati inequalities.