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  • 1
    Electronic Resource
    Electronic Resource
    Oxford UK : Blackwell Science Ltd
    Alimentary pharmacology & therapeutics 15 (2001), S. 0 
    ISSN: 1365-2036
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: To evaluate the sensitivity and specificity of a new 13C urea breath test, Oridion BreathID, for the diagnosis of Helicobacter pylori.〈section xml:id="abs1-2"〉〈title type="main"〉Methods:A total of 97 consecutive symptomatic patients referred for upper endoscopy were included in the ‘pre-therapy’ part of the study. After endoscopy the patients were analysed for H. pylori by Oridion BreathID. BreathID continuously sampled the subject’s breath for 20 min, and displayed the results on the BreathID screen in real time. Results of the BreathID were compared with the ‘gold standard’ (rapid urease test and histology). We also prospectively tested the validity of BreathID in comparison to isotope ratio mass spectrometry, in 40 patients referred to monitor the efficacy of H. pylori eradication treatment.〈section xml:id="abs1-3"〉〈title type="main"〉Results:Complete agreement was observed between the ‘gold standard’ and the Breath ID test in 96.9% (94 out of 97) of the patients. The sensitivity and specificity of BreathID were 97.8% and 96.1%, respectively. The correlation between BreathID and isotope ratio mass spectrometry breath test was 100%.〈section xml:id="abs1-4"〉〈title type="main"〉Conclusions:The Oridion BreathID has comparable sensitivity and specificity to the claims of the currently available urea breath tests. Furthermore, BreathID has the potential advantages of ease of use with minimal medical staff requirement, and real time rapid results (20 min maximum) which may make the BreathID preferable to other urea breath test assays.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Journal of intelligent manufacturing 8 (1997), S. 385-403 
    ISSN: 1572-8145
    Keywords: Supply management ; stochastic optimization ; discrete event systems ; continuous flow models ; gradient descent
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract This paper is concerned with inventory control in assembly systems for minimizing production costs. The system manufactures multiple products assembled from various components, and it operates according to a cyclic schedule. At the start of each cycle time, two decisions are made: the product volumes to be assembled during the current cycle, and the component-stock levels to be ordered. For a given decision, there is an associated cost incurred by backlogging of the finished products on one hand, and the component inventory holding cost, on the other hand. The objective here is to balance the two costs so as to minimize their sum. One complicating factor stems from uncertainties in both product demand levels and components yield times. These uncertainties can be modelled by probabilistic means, and hence the cost minimization problem becomes a stochastic problem. This problem can be quite difficult due to the nonlinearity of the equations involved, the mix of integer and continuous parameters, and their large number in moderate-size problems. Our approach in this paper is to first define certain control parameters and thus reduce the number of the variables involved in the optimization problem, and then solve the latter problem by using sophisticated optimization techniques in conjunction with heuristic modelling. We will demonstrate, by numerical means, the resolution of fairly difficult problems and thus establish the viability of the proposed numerical techniques.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1572-9338
    Keywords: Smoothed perturbation analysis ; serial queueing networks ; occupancy-related functions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics , Economics
    Notes: Abstract We present unbiased Smoothed Perturbation Analysis (SPA) estimators for the derivatives of occupancy-related performance functions in serial networks ofG/G/1 queues with respect to parameters of the distributions of service times at the queues. The sample functions for these performance measures are piecewise constant, and established Infinitesimal Perturbation Analysis (IPA) methods typically fail to provide unbiased estimators in this case. The performance measures considered in this paper are: the average network occupancy as seen by an arrival, the average occupancy of a specific queue as seen by an arrival to it, the probability that a customer is blocked at a specific queue, and the probability that a customer leaves a queue idle. The SPA estimators derived are quite simple and flexible, and they lend themselves to straightforward analysis. Unlike most of the established SPA algorithms, ours are not based on the comparison of hazard rates, and the proofs of their unbiasedness do not require the boundedness of such hazard rates.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Journal of optimization theory and applications 56 (1988), S. 285-311 
    ISSN: 1573-2878
    Keywords: Constraint-dropping schemes ; outer-approximation methods ; stochastic algorithms ; optimality functions ; continua of constraints
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract Optimization algorithms for solving mathematical programming problems involving continua of inequalities are presented. The algorithms use an outer-approximation method, by which they attempt to approximate, at each point, the maxima of sets of inequality constraints. They do so by performing random experiments, resulting in a finite number of points, over which the maximum is taken. They use constraint-dropping schemes, by which they eliminate points from the constraint set at hand, which are felt to be irrelevant. At each point that the algorithms construct, they evaluate a measure of optimality, which indicates the distance of the point from the set of solutions of the optimization problem. They use this measure to determine the number of random experiments performed. Thus, the number of such experiments tends to be small initially, when the points at hand are far from optimal, and they tend to increase when an optimal point is approached.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Journal of optimization theory and applications 64 (1990), S. 217-217 
    ISSN: 1573-2878
    Keywords: Stochastic approximations ; Robbins-Monro algorithms
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract We point out that the main result in Ref. 1 is not new.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Journal of optimization theory and applications 64 (1990), S. 615-640 
    ISSN: 1573-2878
    Keywords: Stochastic approximation ; nondifferentiable optimization ; minimax problems
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract A stochastic approximation algorithm for minimax optimization problems is analyzed. At each iterate, it performs one random experiment, based on which it computes a direction vector. It is shown that, under suitable conditions, it a.s. converges to the set of points satisfying necessary optimality conditions. The algorithm and its analysis bring together ideas from stochastic approximation and nondifferentiable optimization.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Journal of optimization theory and applications 64 (1990), S. 399-417 
    ISSN: 1573-2878
    Keywords: Stochastic optimization ; method of steepest descent ; method of feasible directions ; optimality functions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract Stochastic algorithms for optimization problems, where function evaluations are done by Monte Carlo simulations, are presented. At each iteratex i, they draw a predetermined numbern(i) of sample points from an underlying probability space; based on these sample points, they compute a feasible-descent direction, an Armijo stepsize, and the next iteratex i+1. For an appropriate optimality function σ, corresponding to an optimality condition, it is shown that, ifn(i) → ∞, then σ(x i) → 0, whereJ is a set of integers whose upper density is zero. First, convergence is shown for a general algorithm prototype: then, a steepest-descent algorithm for unconstrained problems and a feasible-direction algorithm for problems with inequality constraints are developed. A numerical example is supplied.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    Journal of optimization theory and applications 61 (1989), S. 473-485 
    ISSN: 1573-2878
    Keywords: Stochastic optimization ; gradient methods ; stochastic approximations ; supermartingales
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract A stochastic algorithm for finding stationary points of real-valued functions defined on a Euclidean space is analyzed. It is based on the Robbins-Monro stochastic approximation procedure. Gradient evaluations are done by means of Monte Carlo simulations. At each iteratex i , one sample point is drawn from an underlying probability space, based on which the gradient is approximated. The descent direction is against the approximation of the gradient, and the stepsize is 1/i. It is shown that, under broad conditions, w.p.1 if the sequence of iteratesx 1,x 2,...generated by the algorithm is bounded, then all of its accumulation points are stationary.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Journal of optimization theory and applications 68 (1991), S. 181-202 
    ISSN: 1573-2878
    Keywords: Perturbation analysis ; discrete-event dynamical systems
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract The subject of discrete-event dynamical systems has taken on a new direction with the advent of perturbation analysis (PA), an efficient method for estimating the gradients of a steady-state performance measure, by analyzing data obtained from a single-simulation experiment in the time domain. A crucial issue is whether PA gives strongly consistent estimates, namely, whether average time-domain-based gradients converge, over infinite horizon, to the steady-state gradients. In this paper, we investigate this issue for a queue with a finite buffer capacity and a loss policy. The performance measure in question is the average amount of lost customers, as a function of the buffer's capacity, which is assumed to be continuous in our work. It is shown that PA gives strongly consistent estimates. The analysis uses a new technique, based on busy period-dependent inequalities. This technique may have possible extensions to analyses of consistency of PA for more general queueing systems.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Journal of optimization theory and applications 59 (1988), S. 307-323 
    ISSN: 1573-2878
    Keywords: Steepest-descent algorithm ; Armijo stepsize ; adaptive precision schemes ; stochastic algorithms
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract A stochastic steepest-descent algorithm for function minimization under noisy observations is presented. Function evaluation is done by performing a number of random experiments on a suitable probability space. The number of experiments performed at a point generated by the algorithm reflects a balance between the conflicting requirements of accuracy and computational complexity. The algorithm uses an adaptive precision scheme to determine the number of random experiments at a point; this number tends to increase whenever a stationary point is approached and to decrease otherwise. Two rules are used to determine the number of random experiments at a point; one, in the inner loop of the algorithm, uses the magnitude of the observed gradient of the function to be minimized; and the other, in the outer-loop, uses a measure of accumulated errors in function evaluations at past points generated by the algorithm. Once a stochastic approximation of the function to be minimized is obtained at a point, the algorithm proceeds to generate the next point by using the steepest-descent deterministic methods of Armijo and Polak (Refs. 3, 4). Convergence of the algorithm to stationary points is demonstrated under suitable assumptions.
    Type of Medium: Electronic Resource
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