Global existence for Maxwell-Bloch systems

Maxwell-Bloch equations describe the propagation of an electromagnetic wave through a quantum medium. For any number of quantum levels, in space dimension 3, we show the global existence of weak (L²) solutions to the initial-value problem. In the case of smoother electromagnetic fields (with curl in L²), the solution is unique. For smooth data (H^s, s?2), the solutions remain smooth for all times.     

Tunable middle infrared radiation with HgGa2S4 crystal

Optical properties of orange phased HgGa₂S₄ crystal is investigated. Generation of tunable middle infrared radiation by second harmonic of tunable CO₂ laser radiation has been demonstrated. Second harmonic conversion efficiency with respect to other infrared crystal has also been discussed.     

Propagation and its failure in a lattice delayed differential equation with global interaction

We study the existence, uniqueness, global asymptotic stability and propagation failure of traveling wave fronts in a lattice delayed differential equation with global interaction for a single species population with two age classes and a fixed maturation period living in a spatially unbounded environment. In the bistable case, under realistic assumptions on the birth function, we prove that the equation admits a strictly monotone increasing traveling wave front. Moreover, if the wave speed does not vanish, then the wave front is unique (up to a translation) and globally asymptotic stable with phase shift. Of particular interest is the phenomenon of “propagation failure” or “pinning” (that is, wave speed c = 0), we also give some criteria for pinning in this paper.     

H.26L中整数变换的研究及改进

针对DCT变换存在浮点运算、运算量大等问题,H.26L提出了整数变换,它能够很好地消除浮点运算,避免逆变换不匹配,但存在精度不高的问题。在基于提升结构(liftingstructure)的无乘法二进制DCT(binDCT)的基础上,对参数进行选择优化,并将浮点DCT、整数变换和binDCT在结构和性能上进行比较分析,选择一组binDCT参数,使binDCT既能避免浮点运算又能降低运算量,且更有利于压缩。     

Optimal shape of a strongest inverted column

By using Pontryagin's maximum principle we determine the shape of the strongest column positioned in a constant gravity field, simply supported at the lower end and clamped at upper end (with the possibility of axial sliding). It is shown that the cross-sectional area function is determined from the solution of a nonlinear boundary value problem. A variational principle for this boundary value problem is formulated and two first integrals are constructed. These integrals lead to an a priori estimate of the value of one the missing initial condition and to the reduction of the order of the system. The optimal shape of a column is determined by numerical integration.     

Localization and completion of nilpotent groups of automorphisms

We study nilpotent subgroups of automorphism groups in the category of groups and the homotopy category of spaces. We establish localization and completion theorems for nilpotent groups of automorphisms of nilpotent groups. We then apply these algebraic theorems to prove analogous results for certain groups of self-homotopy equivalences of spaces.     

H.26L中熵编码原理分析及改进

分析了H.26L中熵编码的原理和特点,比较了两种不同熵编码方法的优缺点、性能差异和发展方向,提出了一种基于量化参数QP的熵编码改进方法。实验证明这种方法能在信道传输率变化到一定程度时,动态调整熵编码的方法,使H.26L熵编码器在复杂度和编码效率两方面作出合理的权衡。     

A note on pseudo-symplectic Runge-Kutta methods

Aubry and Chartier introduced (1998) the concept of pseudo-symplecticness in order to construct explicit Runge-Kutta methods, which mimic symplectic ones. Of particular interest are methods of order (p, 2p), i.e., of orderp and pseudo-symplecticness order 2p, for which the growth of the global error remains linear. The aim of this note is to show that the lower bound for the minimal number of stages can be achieved forp=4 andp=5.     

Fredholm theory and finite section method for band-dominated operators

The topics of this paper are Fredholm properties and the applicability of the finite section method for band operators onl ^p -spaces as well as for their norm limits which we call band-dominated operators. The derived criteria will be established in terms of the limit operators of the given band-dominated operator. After presenting the general theory, we present its specifications to concrete classes of band-dominated operators.To the memory of Professor Mark KreinSupported by the DFG grant 436 RUS/17/148/95Supported by a DFG Heisenberg grant     

REMPI as a tool for studies of OH radicals in catalytic reactions

2 +? O₂→H₂O on polycrystalline Pt foils has been studied by detection of desorbing OH radicals using the Resonance Enhanced Multiphoton Ionization, REMPI, technique. The measurements were performed at catalyst temperatures of 1000–1400 K and a total pressure below 10^-4 mbar. The studies of OH desorption by REMPI were achieved using a two-photon excitation D²Σ^-–X²Π (1–0), followed by one-photon ionization. The ions were detected in a time-of-flight mass spectrometer, TOF-MS, in order to avoid interference from non-resonantly ionized molecules. By applying TOF-MS, a simultaneous non-resonant ionization and detection of H₂, O₂ and H₂O was achieved. Recorded REMPI spectra were compared with spectra simulated using known molecular constants. The kinetics of the reaction derived from the measurements were compared with what was obtained in earlier LIF detection of OH, performed at higher total reactant pressure using the A–X transition. REMPI TOF-MS is shown to be a complement to LIF for reaction studies below 5×10^-4 mbar total pressure, where LIF is too insensitive for quantification. The reaction kinetics was found to be in agreement with a theoretic model and previous LIF studies. Received: 8 March 1996/Revised version: 4 October 1996