Showing papers by "Eindhoven University of Technology published in 1990"

Force fields for silicas and aluminophosphates based on ab initio calculations.

Abstract: We address the problem of finding interatomic force fields for silicas from ab initio calculations on small clusters. It is shown that the force field cannot be determined from cluster data alone; incorporation of bulk-system information into the force field remains essential. Bearing this in mind, we derive a force field based on both microscopic (ab initio) and macroscopic (experimental) data. This force field combines accuracy with transferability to other polymorphs. The possibility of parametrizing other elements is also demonstrated.

Approximation algorithms for scheduling unrelated parallel machines

Abstract: We consider the following scheduling problem. There arem parallel machines andn independent jobs. Each job is to be assigned to one of the machines. The processing of jobj on machinei requires timep ij . The objective is to find a schedule that minimizes the makespan. Our main result is a polynomial algorithm which constructs a schedule that is guaranteed to be no longer than twice the optimum. We also present a polynomial approximation scheme for the case that the number of machines is fixed. Both approximation results are corollaries of a theorem about the relationship of a class of integer programming problems and their linear programming relaxations. In particular, we give a polynomial method to round the fractional extreme points of the linear program to integral points that nearly satisfy the constraints. In contrast to our main result, we prove that no polynomial algorithm can achieve a worst-case ratio less than 3/2 unlessP = NP. We finally obtain a complexity classification for all special cases with a fixed number of processing times.

Shear thickening (dilatancy) in concentrated dispersions

Abstract: A new criterion is introduced for the onset of shear thickening in a concentrated dispersion. The criterion follows from the assumption that shear thickening occurs when the shear forces overrule the interparticle forces. A force balance at small interparticle distances is used to predict the dependence of this critical shear rate on the volume fraction. It is deduced that the critical shear rate is proportional to the interparticle distance, has a linear dependence on the magnitude of the stabilizing force, an inverse linear dependence on the dispersion medium viscosity, and an inverse linear dependence on the particle radius. The model is confirmed experimentally with viscosity measurements on various electrostatically stabilized dispersions. The validity of the model is also checked with data obtained from the literature. Experiments indicate that polydisperse dispersions exhibit pronounced shear thickening but with a less dramatic increase in viscosity than monodisperse dispersions.

Stated preference and choice models applied to recreation research : a review

Abstract: This article discusses the use and usefulness of stated preference and choice models in recreation/leisure research. Stated preference and choice models require one to design decision experiments to study recreational and leisure decisions made in hypothetical or simulated markets. Historically, such experiments were uncommon in recreation and leisure research; therefore we pay particular attention to comparisons of the stated preference modeling approaches with modeling approaches based on observations of choices made in real markets, such as the analysis of discrete choices using conditional and nested multinomial logit models. The conceptual and theoretical bases of stated preference and choice models are discussed; and procedures for developing such models, including different design strategies, are outlined. Potential uses of these models in recreation research are illustrated with reference to several recent empirical applications.

Global Convergence of Genetic Algorithms: A Markov Chain Analysis

Abstract: In this paper we are trying to make a step towards a concise theory of genetic algorithms (GAs) and simulated annealing (SA). First, we set up an abstract stochastic algorithm for treating combinatorial optimization problems. This algorithm generalizes and unifies genetic algorithms and simulated annealing, such that any GA or SA algorithm at hand is an instance of our abstract algorithm. Secondly, we define the evolution belonging to the abstract algorithm as a Markov chain and find conditions implying that the evolution finds an optimum with probability 1. The results obtained can be applied when designing the components of a genetic algorithm.

Genetic Local Search Algorithms for the Travelling Salesman Problem

Abstract: We briefly review previous attempts to generate near-optimal solutions of the Traveling Salesman Problem by applying Genetic Algorithms. Following the lines of Johnson [1990] we discuss some possibilities for speeding up classical Local Search algorithms by casting them into a genetic frame. In an experimental study two such approaches, viz. Genetic Local Search with 2-Opt neighbourhoods and Lin-Kernighan neighbourhoods, respectively, are compared with the corresponding classical multi-start Local Search algorithms, as well as with Simulated Annealing and Threshold Accepting, using 2-Opt neighbourhoods. As to be expected a genetic organization of Local Search algorithms can considerably improve upon performance though the genetic components alone can hardly counterbalance a poor choice of the neighbourhoods.

Gate sizing in MOS digital circuits with linear programming

Abstract: In this paper a solution is presented to tune the delay of a circuit composed of cells to a prescribed value, while minimizing power consumption. The tuning is performed by adapting the load drive capabilities of the cells. This optimization problem is mapped onto a linear program, which is then solved by the Simplex algorithm. This approach guarantees to find the global optimum, and has proven feasible for circuits of up to several thousand cells. The method can be used with any convex delay model. Results show that circuits can be speeded up by a factor of 2 at a cost of only 10 to 30% of extra power.

1/f noise in MODFETs at low drain bias

Abstract: The 1/f noise in normally-on MODFETs biased at low drain voltages is investigated. The experimentally observed relative noise in the drain current S/sub I//I/sup 2/ versus the effective gate voltage V/sub G/=V/sub GS/-V/sub off/ shows three regions which are explained. The observed dependencies are S/sub I//I/sup 2/ varies as V/sub G//sup m/ with the exponents m=-1, -3, 0 with increasing values of V/sub G/. The model explains m=-1 as the region where the resistance and the 1/f noise stem from the 2-D electron gas under the gate electrode; the region with m=0 at large V/sub G/ or V/sub GS/ equivalent to 0 is due to the dominant contribution of the series resistance. In the region at intermediate V/sub G/, m=-3, the 1/f noise stems from the channel under the gate electrode, and the drain-source resistance is already dominated by the series resistance. >

Multiprocessor scheduling with communication delays

Abstract: This paper adresses certain types of scheduling problems that arise when a parallel computation is to be executed on a multiprocessor. We define a model that allows for communication delays between precedence-related tasks, and propose a classification of various submodels. We also review complexity results and optimization and approximation algorithms that have been presented in the literature.

Programming: The Derivation Of Algorithms

Abstract: * Predicate Calculus * The Guarded Command Language * Quantifications * General Programming Techniques * Designing Efficient Programs * Searching * Segment Problems * Slope Search * Mixed Problems * Array Manipulations * Sorting * Auxiliary Arrays

Platinum distribution in intraperitoneal tumors after intraperitoneal cisplatin treatment

Abstract: The spatial distribution of platinum (Pt) in the kidney was studied by an autoradiographic technique, in which cisplatin (CDDP) was replaced by 195mPt-labeled CDDP, and by proton-induced X-ray emission (PIXE). Although both studies demonstrated comparable spatial distribution patterns, PIXE had the advantage that Pt concentrations could be determined quantitatively, in contrast to the relative information obtained by the autoradiographic technique. Using PIXE, the distribution of Pt in i.p. tumors was studied after i.p. administration of CDDP. The highest Pt concentrations were always found on the periphery of tumors, indicating that the periphery was exposed to a higher drug concentration than the center. Dose was correlated to the concentration of CDDP at both the center and the periphery (r = 0.99). The Pt concentration in the periphery was usually higher by a factor of 2-3 after i.p. administration than after i.v. treatment, whereas in the center of the tumor no concentration difference could be detected. The penetration depth of CDDP lay between 1 and 2 mm and was calculated from the differences in Pt concentration after i.p. and i.v. treatment. This indicates that the effective advantage of i.p. chemotherapy with CDDP in cases of cancers limited to the peritoneal cavity is accentuated at the periphery of the tumor.

Stress-absorbing elements in dental implants

Abstract: By means of finite element analysis, calculations, were made of the stress-distribution in bone around implants with and without stress-absorbing elements. A freestanding implant and an implant, it was concluded the variation in the E-modulus of the stress-absorbing element had no effect on the stresses in bone. Changing the shape of the stress-absorbing element had little effect on the stresses in cortical bone. For the implant connected with a natural tooth, it was concluded that a more uniform stress was obtained around the implant with a low E-modulus of the stress-absorbing element. It was also concluded that the bone surrounding the natural tooth showed a decrease in the height of the peak stresses.

Nonideal quantum measurements

Abstract: A partial ordering in the class of observables (∼ positive operator-valued measures, introduced by Davies and by Ludwig) is explored. The ordering is interpreted as a form of nonideality, and it allows one to compare ideal and nonideal versions of the same observable. Optimality is defined as maximality in the sense of the ordering. The framework gives a generalization of the usual (implicit) definition of self-adjoint operators as optimal observables (von Neumann), but it can, in contrast to this latter definition, be justified operationally. The nonideality notion is compared to other quantum estimation theoretic methods. Measures for the amount of nonideality are derived from information theory.

Analysis of the asymmetric shortest queue problem

Abstract: In this paper we study a system consisting of two parallel servers with different service rates. Jobs arrive according to a Poisson stream and generate an exponentially distributed workload. On arrival a job joins the shortest queue and in case both queues have equal lengths, he joins the first queue with probability q and the second one with probability 1- q, where q is an arbitrary number between 0 and 1. In a previous paper we showed for the symmetric problem, that is for equal service rates and q = ½, that the equilibrium distribution of the lengths of the two queues can be exactly represented by an infinite sum of product form solutions by using an elementary compensation procedure. The main purpose of the present paper is to prove a similar product form result for the asymmetric problem by using a generalization of the compensation procedure. Furthermore, it is shown that the product form representation leads to a numerically efficient algorithm. Essentially, the method exploits the convergence properties of the series of product forms. Because of the fast convergence an efficient method is obtained with upper and lower bounds for the exact solution. For states further away from the origin the convergence is faster. This aspect is also exploited in the paper. Keywords: bounds, difference equation, product form, queues in parallel, stationary queue length distribution, shortest queue problem.

Unconditionally Secure Digital Signatures

Abstract: All known digital signature schemes can be forged by anyone having enough computing power. For a finite set of participants, we can overcome this weakness.We present a polynomial time protocol in which a participant can convince (with an exponentially small error probability) any other participant that his signature is valid. Moreover, such a convinced participant can convince any other participant of the signature's validity, without interaction with the original signer.An extension allows, in most cases, a participant who receives a signature from any source to convince each other participant of its validity. If a participant cannot use the signature to convince others, he knows so when he receives it.

An analytical theory of multi-echelon production/distribution systems

Abstract: In this paper, we study inventory control problems arising in multi-echelon production/distribution chains. In these chains, material is delivered by outside suppliers, proceeds through a number of manufacturing stages, and is distributed finally among a number of local warehouses in order to meet market demand. Each stage requires a fixed leadtime; furthermore, we assume a stochastic, stationary end-item demand process. The problem to balance inventory levels and service degrees can be modelled and analyzed by defining appropriate cost functions. Under an average cost criterion, we study the three most important structures arising in multi-echelon systems: assembly systems, serial systems and distribution systems. For all three systems, it is possible to prove exact decomposition results which reduce complex multi-dimensional control problems to simple one-dimensional problems. In addition, we establish the optimality of base-stock control policies.