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Exact Values (exact + value)
Selected AbstractsA homogenization method for estimating the bearing capacity of soils reinforced by columnsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 10 2005B. Jellali Abstract The ultimate bearing capacity problem of a strip foundation resting on a soil reinforced by a group of regularly spaced columns is investigated in the situation when both the native soil and reinforcing material are purely cohesive. Making use of the yield design homogenization approach, it is shown that such a problem may be dealt with as a plane strain yield design problem, provided that the reinforced soil macroscopic strength condition has been previously determined. Lower and upper bound estimates for such a macroscopic criterion are obtained, thus giving evidence of the reinforced soil strong anisotropy. Performing the upper bound kinematic approach on the homogenized bearing capacity problem, by using the classical Prandtl's failure mechanism, makes it then possible to derive analytical upper bound estimates for the reinforced foundation bearing capacity, as a function of the reinforced soil parameters (volume fraction and cohesion ratio), as well as of the relative extension of the reinforced area. It is shown in particular that such an estimate is closer to the exact value of the ultimate bearing capacity, than that derived from a direct analysis which implicitly assumes that the reinforced soil is an isotropic material. Copyright © 2005 John Wiley & Sons, Ltd. [source] Approximate lower bounds of the Weinstein and Temple varietyINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2007M. G. Marmorino Abstract By using the Weinstein interval or coupling the Temple lower bound to a variational upper bound one can in principle construct an error bar about the ground-state energy of an electronic system. Unfortunately there are theoretical and calculational issues which complicate this endeavor so that at best only an upper bound to the electronic energy has been practical in systems with more than a few electrons. The calculational issue is the complexity of ,H2, which is necessary in the Temple or Weinstein approach. In this work we provide a way to approximate the ,H2, to any desired accuracy using much simpler ,H,-like information so that the lower bound calculations are more practical. The helium atom is used as a testing ground in which we obtain approximate error bars for the ground-state energy of [,2.904230, ,2.903721] hartree using the variational energy with the Temple lower bound and [,2.919098, ,2.888344] hartree for the Weinstein interval. For comparison, the slightly larger error bars using the exact value of ,H2, are: [,2.904358, ,2.903721] hartree and [,2.919765, ,2.887677] hartree, respectively. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Intraoperative magnetic resonance imaging in the surgical treatment of cerebral metastasesJOURNAL OF SURGICAL ONCOLOGY, Issue 5 2010Christian Senft MD Abstract Background and Objectives To report on the value of intraoperative magnetic resonance imaging (iMRI) in the neurosurgical treatment of cerebral metastases (CM). Methods We performed a total of 204 surgical procedures with the use of a mobile ultra-low-field iMRI-unit. Of these, there were 12 craniotomies and 2 minimal-invasive procedures for CM, and 63 craniotomies for glioblastoma (GBM). Results On intraoperative imaging, all tumors could be localized and targeted with the help of the integrated neuronavigation system. Intraoperative imaging resulted in continued tumor resection due to unexpected residual tumor tissue in 13 patients harboring GBM (20.6%), but no patient with a CM (0%). In two patients with cystic CM, iMRI helped to achieve complete collapse of cysts by means of stereotactic aspiration, relieving mass effect and allowing for adjuvant radiotherapy. All patients subsequently received adjuvant treatment according to clinical protocols. Conclusion Surgical resection represents one of several treatment modalities in metastatic brain disease. iMRI is useful for neuronavigation and resection control and as an adjunct in minimal-invasive procedures in patients with CM; however, its exact value is yet to be determined by prospective randomized trials. J. Surg. Oncol. 2010; 101:436,441. © 2010 Wiley-Liss, Inc. [source] ANALYZING MYOPIC APPROACHES FOR MULTI-AGENT COMMUNICATIONCOMPUTATIONAL INTELLIGENCE, Issue 1 2009Raphen Becker Choosing when to communicate is a fundamental problem in multi-agent systems. This problem becomes particularly challenging when communication is constrained and each agent has different partial information about the overall situation. We take a decision-theoretic approach to this problem that balances the benefits of communication against the costs. Although computing the exact value of communication is intractable, it can be estimated using a standard myopic assumption,that communication is only possible at the present time. We examine specific situations in which this assumption leads to poor performance and demonstrate an alternative approach that relaxes the assumption and improves performance. The results provide an effective method for value-driven communication policies in multi-agent systems. [source] Sum rules and exact relations for quantal Coulomb systemsCONTRIBUTIONS TO PLASMA PHYSICS, Issue 5-6 2003V.M. Adamyan Abstract A complex response function describing a reaction of a multi-particle system to a weak alternating external field is the boundary value of a Nevanlinna class function (i.e. a holomorphic function with non-negative imaginary part in the upper half-plane). Attempts of direct calculations of response functions based on standard approximations of the kinetic theory for real Coulomb condensed systems often result in considerable discrepancies with experiments and computer simulations. At the same time a relatively simple approach using only the exact values of leading asymptotic terms of the response function permits to restrict essentially a subset of Nevanlinna class functions containing this response function, and in this way to obtain sufficient data to explain and predict experimental results. Mathematical details of this approach are demonstrated on an example with the response function being the (external) dynamic electrical conductivity of cold dense hydrogen-like plasmas. In particular, the exact values of the leading terms of asymptotic expansions of the conductivity are calculated. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Analysis of OTA-C filters with weakly nonlinear transconductorsINTERNATIONAL JOURNAL OF CIRCUIT THEORY AND APPLICATIONS, Issue 7 2008Slawomir KozielArticle first published online: 11 OCT 200 Abstract An efficient approach for analysis of nonlinear distortion in OTA-C filters with weakly nonlinear transconductors is presented. The procedure is developed based on an algebraic description of a general OTA-C filter structure and, therefore, the results are valid for any filter architecture within OTA-C class. On the basis of the proposed method, explicit formulas for calculating a gain compression/expansion ratio in an arbitrary OTA-C filter are developed. The formulas are easy to implement and use in computer-aided filter design tools. For illustration purposes, several filter structures are considered. The accuracy of the method is verified by comparing the results with the exact values of gain compression/expansion ratio achieved by integrating the differential system that determines the time response of OTA-C filter. The presented approach can be generalized in order to consider other nonlinear parameters. Copyright © 2007 John Wiley & Sons, Ltd. [source] A new parameter extraction method for accurate modeling of PEM fuel cellsINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 11 2009M. T. Outeiro Abstract In this paper, a new parameter extraction method for accurate modeling of proton exchange membrane (PEM) fuel cell systems is presented. The main difficulty in obtaining an accurate PEM fuel cell dynamical model is the lack of manufacturer information about the exact values of the parameters needed for the model. In order to obtain a realistic dynamic model of the PEM system, the electrochemical considerations of the system are incorporated into the model. Although many models have been reported in the literature, the parameter extraction issue has been neglected. However, model parameters must be precisely identified in order to obtain accurate simulation results. The main contribution of the present work is the application of the simulated annealing (SA) optimization algorithm as a method for identification of PEM fuel cell model parameter identification. The major advantage of SA is its ability to avoid becoming trapped in local minimum, as well as its flexibility and robustness. The parameter extraction and performance validation are carried out by comparing experimental and simulated results. The good agreement observed confirms the usefulness of the proposed extraction approach together with adopted PEM fuel cell model as an efficient tool to help design of power fuel cell power systems. Copyright © 2009 John Wiley & Sons, Ltd. [source] Link between the kinetic- and exchange-energy functionals in the generalized gradient approximationINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2002Fabien Tran Abstract An approximate kinetic-energy functional of the generalized gradient approximation form was derived following the "conjointness conjecture" of Lee, Lee, and Parr. The functional shares the analytical form of its gradient dependency with the exchange-energy functionals of Becke and Perdew, Burke, and Ernzerhof. The two free parameters of this functional were determined using the exact values of the kinetic energy of He and Xe atoms. A set of 12 closed-shell atoms was used to test the accuracy of the proposed functional and more than 30 others taken from the literature. It is shown that the conjointness conjecture leads to a very good class of kinetic-energy functionals. Moreover, the functional developed in this work is shown to be one of the most accurate despite its simple analytical form. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 [source] Novel computer program for fast exact calculation of accessible and molecular surface areas and average surface curvatureJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2002Oleg V. Tsodikov Abstract New computer programs, SurfRace and FastSurf, perform fast calculations of the solvent accessible and molecular (solvent excluded) surface areas of macromolecules. Program SurfRace also calculates the areas of cavities inaccessible from the outside. We introduce the definition of average curvature of molecular surface and calculate average molecular surface curvatures for each atom in a structure. All surface area and curvature calculations are analytic and therefore yield exact values of these quantities. High calculation speed of this software is achieved primarily by avoiding computationally expensive mathematical procedures wherever possible and by efficient handling of surface data structures. The programs are written initially in the language C for PCs running Windows 2000/98/NT, but their code is portable to other platforms with only minor changes in input-output procedures. The algorithm is robust and does not ignore either multiplicity or degeneracy of atomic overlaps. Fast, memory-efficient and robust execution make this software attractive for applications both in computationally expensive energy minimization algorithms, such as docking or molecular dynamics simulations, and in stand-alone surface area and curvature calculations. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 600,609, 2002 [source] Inverse Problem for Composites with Imperfect Interface: Determination of Interfacial Thermal Resistance, Thermal Conductivity of Constituents, and Microstructural ParametersJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2000Ce-Wen Nan An explicit method is introduced to solve inverse problems for composites with imperfect interfaces. We apply the method to determine the thermal conductivity of constituents and the interfacial thermal resistance in SiC-particulate-reinforced aluminum-matrix composites and to estimate the whisker thermal conductivity, the interfacial thermal resistance, and the whisker alignment distribution in two types of SiC-whisker-reinforced lithium aluminosilicate glass-ceramic composites from their measured effective thermal conductivity reported in the literature. Certain bounds for these three properties of both SiC-whisker-reinforced glass-ceramic composites are obtained, and reasonable estimates for their exact values from room temperature to 500°C are made. The inverse problem is quite sensitive to noise in the measurements. We also comment on existing estimates. [source] The sequential sum problem and performance bounds on the greedy algorithm for the on-line Steiner problemNETWORKS: AN INTERNATIONAL JOURNAL, Issue 3 2005Zevi Miller Abstract This article is motivated by versions of the dynamic or "on-line" Steiner tree problem (OST) introduced by Imase and Waxman [4]. In this problem one is given an edge-weighted graph G and a sequence , = (x1,,,xn) of distinct vertices of G. The requirement is to construct for each i , n a tree Ti spanning the first i vertices of , subject to the condition that Ti,1,Ti for all i, where Ti is constructed without knowledge of the remaining vertices xj, j > i. The goal of the on-line Steiner problem is to minimize the performance ratio; that is, the maximum (over 1 , i , n) of the ratio of the weight of Ti to the weight of the minimum weight tree in G spanning the first i vertices (the latter tree is called the "Steiner tree" for these vertices). In [4] a lower bound of 1 + ˝, log2(n,1), was proved for this ratio. The authors further made the interesting conjecture that there is some on-line algorithm for the OST whose performance ratio achieves this lower bound. We show that a strong form of the greedy algorithm achieves a ratio that converges to the conjectured ˝log2(k) + O(1) as the proportion of degree 2 vertices in the instance graph grows. Our results also imply improvements in certain cases on the known upper bound ,log2(n), for the performance ratio of the greedy algorithm. Our approach is to study a related graph parameter. For each sequence , as above, define the associated cost where c(i,,) = min1 , t < idist(xi, xt). Then let Opt(n, G) be the maximum of L(,) over all such sequences , of length n. The problem of, given n and G, determining Opt(n, G) we call the Sequential Sum Problem (SSP). In this article we analyze the SSP, obtaining exact values and bounds on Opt(n, G) and relating these bounds to the greedy algorithm for the OST. For example, we calculate Opt(n, P) for the path P, and obtain a surprising characterization of all length n sequences , which realize Opt(n, P). By analyzing Opt(n, P) for the "continuous" path, we derive upper bounds on the performance ratio of the greedy algorithm for the OST in arbitrary graphs. On the other hand, generalizing the lower bound argument of [4] we show that there are instances of OST, which can significantly "fool" any on-line algorithm for OST. Specifically, given any tree T normalized to have total edge weight 1, we construct a graph G and a length k , |V(T)| sequence , of vertices of G for which the performance ratio of any on-line algorithm for the OST with input , is lower bounded by Opt(k, T). Finally, we show that the SSP for arbitrary G is NP-complete. © 2005 Wiley Periodicals, Inc. NETWORKS, Vol. 45(3), 143,164 2005 [source] Two-step mean-field renormalization group results for the large square Ising clustersPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003G. Kamieniarz Abstract A transfer matrix approach has been worked out to test the predictions of the improved three-step mean-field renormalization group approach to square Ising clusters with linear size up to L = 11. Performing the asymptotic analysis, the convergence of the finite-size critical couplings and the critical exponents towards the exact values is shown. [source] Prediction variance and G-criterion location for split-plot designsQUALITY AND RELIABILITY ENGINEERING INTERNATIONAL, Issue 4 2009Wayne R. Wesley Abstract Prediction variance properties for completely randomized designs (CRD) are fairly well covered in the response surface literature for both spherical and cuboidal designs. This paper evaluates the impact of changes in the variance ratio on the prediction properties of second-order split-plot designs (SPD). It is shown that the variance ratio not only influences the value of the G-criterion but also its location, in contrast with the G-criterion tendencies in CRD. An analytical method, rather than a heuristic optimization algorithm, is used to compute the prediction variance properties, which include the maximum, minimum and integrated variances for second-order SPD. The analytical equations are functions of the design parameters, radius and variance ratio. As a result, the exact values for these quantities are reported along with the location of the maximum prediction variance used in the G-criterion. The two design spaces of the whole plot and the subplot are studied and as a result, relative efficiency values for these distinct spaces are suggested. Copyright © 2008 John Wiley & Sons, Ltd. [source] Random rates of growth and return: introducing the expo-normal distributionAPPLIED STOCHASTIC MODELS IN BUSINESS AND INDUSTRY, Issue 1 2002Olivier de La Grandville Abstract Expected values and standard deviations of the geometric means of independent positive random variables are useful indicators of the long-term profitability of an investment, or survival of a biological population. Often these quantities cannot be evaluated in a closed form, or even if they can, there may be a choice between several probability models for the ,annual' growth factors. This paper formulates approximations for geometric means and standard deviations. It evaluates their performance and compares the best of them with the exact values for selected probability models of the annual factors. Among these is a new model for annual log-returns, called the expo-normal law. This is the law of log X, where X has a normal law conditioned on X>0. Its properties are developed in some detail. It is found that for the ranges of annual means and standard deviations typically encountered in financial applications, the longer horizon values depend little on the choice of probability model, and that, where possible, exact evaluation is computationally simpler than using approximations. Copyright © 2002 John Wiley & Sons, Ltd. [source] | |||||||||||||||||||