Showing papers by "University of Paderborn published in 1996"

Explicit and exact solutions to a Kolmogorov-Petrovskii-Piskunov equation

Abstract: Some explicit traveling wave solutions to a Kolmogorov-Petrovskii-Piskunov equation are presented through two ansatze. By a Cole-Hopf transformation, this Kolmogorov-Petrov-skii-Piskunov equation is also written as a bilinear equation and two solutions to describe nonlinear interaction of traveling waves are further generated. Backlund transformations of the linear form and some special cases are considered.

Observation of Atomlike Nitrogen in Nitrogen-Implanted Solid C60.

Abstract: A new paramagnetic defect in solid ${\mathrm{C}}_{60}$ was produced by nitrogen implantation in solid ${\mathrm{C}}_{60}$. The hyperfine splitting and the isotope effect unambiguously show that the paramagnetic center contains one nitrogen nucleus. The hyperfine interaction is isotropic, its value is comparable to that of the free nitrogen atom, and the spin of the electron system is $S\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3/2$, as in atomic nitrogen. The complex responsible for this center is soluble in toluene and C${\mathrm{S}}_{2}$ and is stable. We suggest that the complex consists of nitrogen inside ${\mathrm{C}}_{60}$.

Integrable theory of the perturbation equations

Abstract: An integrable theory is developed for the perturbation equations engendered from small disturbances of solutions. It includes various integrable properties of the perturbation equations, such as hereditary recursion operators, master symmetries, linear representations (Lax and zero curvature representations) and Hamiltonian structures, and provides us with a method of generating hereditary operators, Hamiltonian operators and symplectic operators starting from the known ones. The resulting perturbation equations give rise to a sort of integrable coupling of soliton equations. Two examples (MKdV hierarchy and KP equation) are carefully carried out.

Dissemination of Information in Interconnection Networks (Broadcasting & Gossiping)

Abstract: Considerable attention in recent theoretical computer science is devoted to parallel computing. Here, we would like to present a special part of this large topic, namely, the part devoted to an abstract study of the dissemination of information in interconnection networks. The importance of this research area lies in the fact that the ability of a network to effectively disseminate information is an important qualitative measure for the suitabilty of the network for parallel computing. This follows simply from the observation that the communication among processes working in parallel is one of the main parts of the whole parallel computation. So, the effectivity of information exchange among processors essentially influences the effectivity of the whole computation process.

Physics of solids under strong compression

Abstract: Progress in high pressure physics is reviewed with special emphasis on recent developments in experimental techniques, pressure calibration, equations of state for simple substances and structural systematics of the elements. Short sections are also devoted to hydrogen under strong compression and general questions concerning new electronic ground states. This review was received in February 1995

Optimal PWM based on real-time solution of harmonic elimination equations

Abstract: Pulse-width modulation of DC/AC power converters (PWM) based on the elimination of low-order harmonics necessitates solving systems of nonlinear equations. Conventional implementations of this technique based on storing off-line calculated solutions have the common problem that the system flexibility is very limited, especially for applications that require both amplitude and frequency control. A new implementation scheme based on real-time solution of nonlinear harmonic elimination equations using a digital signal processor DSP56001 is reported in this paper. With this digital signal processor (DSP), optimal pulse patterns having 15 switching angles in each quarter fundamental period can be determined within 2.15 ms. Details of the system hardware and software are described. New theoretical results concerning the solvability of harmonic elimination equations are also presented.

The Bi-Hamiltonian Structure of the Perturbation Equations of KdV Hierarchy

Abstract: The bi-Hamiltonian structure is established for the perturbation equations of KdV hierarchy and thus the perturbation equations themselves provide also examples among typical soliton equations. Besides, a more general bi-Hamiltonian integrable hierarchy is proposed and a remark is given for a generalization of the resulting perturbation equations to $1+2$ dimensions.

Epitaxial growth and optical transitions of cubic GaN films

Abstract: Single-phase cubic GaN layers are grown by plasma-assisted molecular-beam epitaxy. The temperature dependence of the surface reconstruction is elaborated. The structural stability of the cubic growth in dependence of the growth stoichiometry is studied by RHEED measurements and numerical simulations of the experimental RHEED patterns. Growth oscillations on cubic GaN are recorded at higher substrate temperatures and nearly stoichiometric adatom coverage. Photoluminescence reveals the dominant optical transitions of cubic GaN and, by applying an external magnetic field, their characteristic $g$ factors are determined.

Efficient PRAM simulation on a distributed memory machine

Abstract: We present algorithms for the randomized simulation of a shared memory machine (PRAM) on a Distributed Memory Machine (DMM). In a PRAM, memory conflicts occur only through concurrent access to the same cell, whereas the memory of a DMM is divided into modules, one for each processor, and concurrent accesses to the same module create a conflict. Thedelay of a simulation is the time needed to simulate a parallel memory access of the PRAM. Any general simulation of anm processor PRAM on ann processor DMM will necessarily have delay at leastm/n. A randomized simulation is calledtime-processor optimal if the delay isO(m/n) with high probability. Using a novel simulation scheme based on hashing we obtain a time-processor optimal simulation with delayO(log log(n) log*(n)). The best previous simulations use a simpler scheme based on hashing and have much larger delay: Θ (log(n)/log log(n)) for the simulation of an n processor PRAM on ann processor DMM, and Θ(log(n)) in the case where the simulation is time-processor optimal. Our simulations use several (two or three) hash functions to distribute the shared memory among the memory modules of the PRAM. The stochastic processes modeling the behavior of our algorithms and their analyses based on powerful classes of universal hash functions may be of independent interest.

Remark on uniqueness of weak solutions to the navier-stokes equations

Abstract: (N-S) du — Au + u • Vu + Vp = / in χ e Ω, 0 < t < Τ, at div u = 0 in χ e Ω, 0 < t < Τ, u = 0 on 3Ω, w|t=o a, where u = u(x,t) = («'(χ, i), · · •, u(x, t)) and ρ = p(x,t) denote the unknown velocity vector and pressure of the fluid at the point (x,t) € Ω χ (0,T), while α = a(x) = (a(x), • ••, a(x)) is the given initial velocity vector field; / = f(x, t) = ( f l { x , t), • • •, f(x, t)) is a given external force. The purpose of the present paper is to improve the criterion on uniqueness of weak solutions to (N-S) in the class ¿°°(0,Γ; £(Ω)). We know that for every a G Ζ£(Ω) and every / e ¿'(Ο,T; L(ù)), there exists at least one weak solution u of (N-S) satisfying the energy inequality. Here we mean by the weak solution a function u in L°°(0, Τ; Ζ^(Ω)) Π L 2 ( 0 , T ; H ^ m satisfying (N-S) in the sense of distributions (Definition of Section 1). For the energy inequality, see (1.1) below.

Ti:Er:LiNbO/sub 3/ waveguide laser of optimized efficiency

Abstract: The development of a Fabry-Perot-type Ti,Er:LiNbO/sub 3/ waveguide laser of optimized CW output power up to 63 mW (/spl lambda//sub s/=1561 nm) at a pump power level of 210 mW (/spl lambda//sub p/=1480 nm) and a slope efficiency of up to 37% is reported. The theoretical model for the waveguide laser is presented and applied to determine the optimum resonator configuration using waveguide parameters obtained from a detailed characterization of the laser sample. With pulsed pumping, waveguide laser pulses of up to 6.2 W peak power were observed. Apart from residual relaxation oscillations, the laser emission proved to be shot-noise limited.

Møller's energy-momentum complex — Once again

Abstract: The 35-year-old problem of Moller's energy-momentum four-vector when transformed by a global Lorentz transformation is discussed again. It is argued that Moller's result is adequate from the viewpoint of general relativity and hence does not defeat his energy-momentum complex.

Transition-Metal Catalyzed Oxidations. 7. Zirconium-Catalyzed Oxidation of Primary and Secondary Alcohols with Hydroperoxides

Abstract: A new procedure for the oxidation (dehydrogenation) of primary and secondary alcohols employing Zr(O-t-Bu)4 or Zr(O-n-Pr)4/tert-butyl hydroperoxide/3 A molecular sieves is presented. Secondary alcoholsif not severely sterically hinderedare usually converted quantitatively to the corresponding ketones. Esters or acids can be byproducts in the reaction of primary alcohols. However, the aldehydes are obtained in good yield by lowering the reaction temperature, decreasing the amount of TBHP or replacing TBHP by cumene hydroperoxide (CHP), and/or exchanging the catalyst Zr(O-t-Bu)4 by Zr(O-n-Pr)4 or silica gel-supported Zr(OR)x. A remarkable selectivity of equatorial alcohol groups (e.g., 11 and 13) is observed in contrast to chromium(VI)-based oxidations. Strongly chelating substrates such as furfuryl alcohol (18) or 1,2-diol 25 that prevent hydride transfer in the six-membered transition state A are not converted.

Universal algorithms for store-and-forward and wormhole routing

Abstract: In this paper we present routing algorithms that are tmiversal in the sense that they route messages along arbitrary (simple) paths in arbitrary networks. The algorithms are analyzed in terms of the number of messages being routed, the maximum number of messages that must cross any edge in the network (edge congestion), the maximum number of edges that a message must cross (dilation), the bufler size, and the bandwidth of the links. We present two main results, both of which have applications to ttnivexsal storeand-forwwd routing and universal wormhole routing. Our results yield significant performance improvements over all previously known universal routing algorithms for a wide range of parameters, and they even improve many time bounds for standard networks. In addition, we present adaptations of our main results for routing along shortest paths in arbitrary networks, and for routing in leveled networks, node-symmetric networks, edge-symmetric networks, expanders, butterflies, and meshes.

Mathematical vs. Experimental Stress Analysis of Inhomogeneities in Solids

Abstract: The intent of this paper is to apply the discrete Fourier transform to the computation of eigenstress and eigenstrain fields around heterogeneities in composite materials. To this end the discrete Fourier transform is first briefly reviewed and then used to solve the basic equations of linear elasticity as pertinent to eigenstrained bodies under external loads. The results of this procedure are then used to discuss a few typical geometries such as an hexagonal two-dimensional array of thermally as well as elastically mismatched fibers in a composite matrix and a spherical Zirconia inclusion after a phase transformation. The resulting stress-strain fields are finally compared to experimental observations of eigenstress fields. The experimental techniques considered include photoelastic analyses as well as electron diffusion contrast techniques. It will be shown that the discrete Fourier transform as applied to eigenstress problems is capable of simulating the outcome of such experiments.

All-optical switching in lithium niobate directional couplers with cascaded nonlinearity

Abstract: We report on intensity-dependent switching in lithium niobate directional couplers. Large nonlinear phase shifts that are due to cascading detune the coupling between the coupler branches, which makes all-optical switching possible. Depending on the input intensity, the output could be switched between the cross and the bar coupler branches with a switching ratio of 1:5 and a throughput of 80%.

p- and n-type cubic GaN epilayers on GaAs

Abstract: Temperature-dependent Hall-effect measurements are performed on cubic GaN layers grown by plasmaassisted molecular-beam epitaxy on ~100! GaAs substrates. We find that under N-rich conditions, cubic GaN films are p-type with hole concentrations of '10 cm and mobilities of about 350 cm/V s at room temperature. The acceptors have an activation energy of EA50.44560.015 eV. Ga-rich growth conditions result in n-type conductivity with electron concentrations of about 10 cm and room-temperature mobilities of mn'100cm /Vs. Since for n-type samples a strong influence of the underlaying semi-insulating GaAs substrate is observed, a two-layer model is used to evaluate the Hall data, yielding a shallow donor with an activation energy of ED50.1660.07 eV and a deeper donor with EDD50.6060.10 eV. @S0163-1829~96!53840-3#

Modelling and control of the longitudinal and lateral dynamics of a series hybrid vehicle

Abstract: This paper presents models and control structures for series hybrid drive trains. For the longitudinal dynamics of the series hybrid drive train, models of different components are introduced. On the basis of these models, control structures for the different subsystems are presented. Selected simulations show the potential of these control structures. As for the lateral dynamics, the potential to influence the vehicle dynamics by electric single-wheel motors is also investigated. For this purpose, an extended single-track model and approaches to the robust control of vehicle dynamics are used. Side-wind tests are presented as well as a hardware-in-the-loop testbed for the series hybrid drive train.

Introduction to High-Performance Scientific Computing

Abstract: An overview of scientific computing: introduction, large-scale scientific problems, the scientific computing environment, workstations, supercomputers, further reading. Part 1 Background: a review of selected topics from numerical analysis - notation, error, floating-point numbers, Taylor's series, linear algebra, differential equations, fourier series IEEE arithmetic short reference - single precision, double precision, rounding, infinity, NaN, and zero, of things not said, further reading UNIX, vi, and ftp - a quick review - UNIX short reference, vi short reference, ftp short reference elements of UNIX make - introduction, an example of using make, some advantages of make, the makefile, further examples, dynamic macros, user-defined macros, additional features, other examples, a makefile for C, creating your own makefile, futher information, a makefile for fortran modules, a makefile for C modules elements of fortran - introduction, overview, definitions and basic rules, description of statements, reading and writing, examples. Part 2 Tools: elements of matlab - what is MATLAB?, getting started, some examples, short outline of the language, built-in functions, MATLAB scripts and user-defined functions, input/output, graphics, that's it! elements of IDL - getting started, exploring the basic concepts, plotting, programming in IDL, input/output, using IDL efficiently, summary elements of AVS - basic concepts, AVS graphical programming - the Network editor, the geometry viewer, AVS applications, further reading. Part 3 Scientific visualization: scientific visualization - definitions and goals of scientific visualization, history of scientific visualization, example of scientific visualization, concepts of scientific visualization, visual cues, characterization of scientific data, visualization techniques, annotations, interactivity, interpretation goals to pursue with visualization, quantitative versus qualitative data interpretation. Part 4 Architectures: computer performance - introduction and background, computer performance, benchmarks, the effect of optimizing compilers, other architectural factors, vector and parallel computers, summary. (Part contents).

Temperature- and pressure-induced structural transitions in rare-earth-deficient (R = Y, Sm, Gd, Tb) Laves phases

Abstract: Electrical resistivity, thermal expansion, and temperature-dependent x-ray diffraction measurements on compounds give mutually consistent evidence for structural phase transitions at 740 K, 550 K, 600 K, and 450 K respectively for R = Y, Sm, Gd, Tb; 0 < x < 005 Arguments are given as to why most of the rare-earth - nickel compounds with the 1:2 ratio do not crystallize in the simple cubic Laves phase (C15 type) but show a superstructure of the cubic Laves phase at room temperature and at ambient pressure This superstructure with the space group and a doubled cell parameter is characterized by ordered vacancies on the R sites It is shown that the observed structural instabilities result in transitions to the cubic Laves phase (space group ), however with disordered vacancies at high temperatures High-pressure x-ray powder diffraction experiments show that the phase transition in shifts down to room temperature for a pressure of 27 GPa

Field weakening of induction motors in a very wide region with regard to parameter uncertainties

Abstract: In this paper, a new approach to induction machine control is presented that ensures the production of maximum torque over the permitted speed range. In the first part of the paper the steady state torque capability of an induction motor with saturation air gap inductance is analysed, when voltage as well as current are limited. This maximum torque can be achieved with different control schemes. Nevertheless, the schemes differ in view of robustness and the requirements to reach the optimum. In this paper, a scheme is presented that achieves the optimum operating point in the whole field weakening region only by means of the measured values of voltage and current. This method guarantees a robust adjustment with regard to the parameter uncertainties and the variable DC link voltage.

Binary non-linearization of Lax pairs of Kaup-Newell soliton hierarchy

Abstract: Kaup-Newell soliton hierarchy is derived from a kind of Lax pairs different from the original ones. A binary non-linearization procedure corresponding to the Bargmann symmetry constraint is carried out for those Lax pairs. The proposed Lax pairs together with adjoint Lax pairs are constrained as a hierarchy of commutative, finite-dimensional integrable Hamiltonian systems in the Liouville sense, which also provides us with new examples of finite-dimensional integrable Hamiltonian systems. A sort of involutive solutions to the Kaup-Newell hierarchy are exhibited through the obtained finite-dimensional integrable systems and the general involutive system engendered by binary non-linearization is reduced to a specific involutive system generated by mono-non-linearization.

Optimized space vector modulation and regular-sampled PWM: a reexamination

Abstract: The relationship between space vector modulation and regular-sampled PWM techniques for hard-switching PWM inverters and rectifiers are reexamined in this paper. Space vector modulation with optimized apportioning of null vector time that results in minimum current THD is investigated and is compared with previously known methods. The conventional optimal regular-sampled PWM method and its relation to the optimized space vector modulation are discussed. Optimal discontinuous modulation for minimizing both the switching losses and the current THD is also studied.

A lower bound for randomized algebraic decision trees

Abstract: We extend the lower bounds on the depth of algebraic decision trees to the case of randomized algebraic decision trees (with two-sided error) for languages being finite unions of hyperplanes and the intersections of halfspaces. As an application, among other things, we derive, for the first time, $\Omega(n^2)$ randomized lower bound for the {\em knapsack problem} (which was previously only known for deterministic algebraic decision trees).