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Computational Problem (computational + problem)
Selected AbstractsConvergence of control performance by unfalsification of models,levels of confidenceINTERNATIONAL JOURNAL OF ADAPTIVE CONTROL AND SIGNAL PROCESSING, Issue 5 2001S. M. Veres Abstract A general framework is introduced for iterative/adaptive controller design schemes by model unfalsification. An important feature of the schemes is their convergence near to the best possible controller given a set of model and controller structures. The problem of stability assured controller tuning is examined through unfalsified Riemannian bands of the Nyquist plot. Instability tolerant H, and l1 -norm-based controller tuning schemes are introduced. Computational problems are discussed and a simulation is used to illustrate the new scheme. Copyright © 2001 John Wiley & Sons, Ltd. [source] Semi-analytical far field model for three-dimensional finite-element analysisINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 11 2004James P. Doherty Abstract A challenging computational problem arises when a discrete structure (e.g. foundation) interacts with an unbounded medium (e.g. deep soil deposit), particularly if general loading conditions and non-linear material behaviour is assumed. In this paper, a novel method for dealing with such a problem is formulated by combining conventional three-dimensional finite-elements with the recently developed scaled boundary finite-element method. The scaled boundary finite-element method is a semi-analytical technique based on finite-elements that obtains a symmetric stiffness matrix with respect to degrees of freedom on a discretized boundary. The method is particularly well suited to modelling unbounded domains as analytical solutions are found in a radial co-ordinate direction, but, unlike the boundary-element method, no complex fundamental solution is required. A technique for coupling the stiffness matrix of bounded three-dimensional finite-element domain with the stiffness matrix of the unbounded scaled boundary finite-element domain, which uses a Fourier series to model the variation of displacement in the circumferential direction of the cylindrical co-ordinate system, is described. The accuracy and computational efficiency of the new formulation is demonstrated through the linear elastic analysis of rigid circular and square footings. Copyright © 2004 John Wiley & Sons, Ltd. [source] Collaboration Online: The Example of Distributed ComputingJOURNAL OF COMPUTER-MEDIATED COMMUNICATION, Issue 4 2005Anne Holohan Distributed Computing is a new form of online collaboration; such projects divide a large computational problem into small tasks that are sent out over the Internet to be completed on personal computers. Millions of people all over the world participate voluntarily in such projects, providing computing resources that would otherwise cost millions of dollars. However, Distributed Computing only works if many people participate. The technical challenge is to slice a problem into thousands of tiny pieces that can be solved independently, and then to reassemble the solutions. The social problem is how to find all those widely dispersed computers and persuade their owners to participate. This article examines what makes a collaborative Distributed Computing project successful. We report on data from a quantitative survey and a qualitative study of participants on several online forums, and discuss and analyze Distributed Computing using Arquilla and Ronfeldt's (2001) five-level network organization framework. [source] Low-Energy-Barrier Proton Transfer Induced by Electron Attachment to the Guanine,,,Cytosine Base PairCHEMPHYSCHEM, Issue 4 2010Anna Szyperska Abstract The photoelectron spectrum of the anion of the guanine,,,cytosine base apair (GC)., is recorded for the first time. The observed variation in the spectral peak-height ratios with the source conditions suggests the presence of two or more anionic isomers. Two maxima of the broad bands in the photoelectron spectrum were measured at about 1.9 and about 2.6 eV. These values are very well reproduced by the vertical detachment energies of the B3LYP/6-31++G(d,p) calculated low-energy anionic structures, which are 1) the Watson,Crick base-pair anion with proton transferred from N1 of guanine to N3 of cytosine, 2) its analogue in which the proton is transferred from N9 of guanine to N7 of guanine, and 3) the global minimum geometry, which is formed from the latter anion by rotation of guanine about the axis approximately defined by C2 of guanine and C4 of cytosine. Furthermore, a minor difference in the stabilities of the two lowest energy anions explains the experimentally observed source (temperature) dependence of the PES spectrum. A rational procedure, based on the chemistry involved in the formation of anionic dimers, which enables the low-energy anions populated in the photoelectron spectrum to be identified is proposed. In contrast to the alternative combinatorial approach, which in the studied case would lead to carrying out quantum chemical calculations for 2000,2500 structures, the procedure described here reduces the computational problem to only 15 geometries. [source] Non parametric VaR Techniques.ECONOMIC NOTES, Issue 2 2001Myths, Realities VaR (value-at-risk) estimates are currently based on two main techniques: the variance-covariance approach or simulation. Statistical and computational problems affect the reliability of these techniques. We illustrate a new technique , filtered historical simulation (FHS) , designed to remedy some of the shortcomings of the simulation approach. We compare the estimates it produces with traditional bootstrapping estimates. (J.E.L.: G19). [source] Conceptual problems in laypersons' understanding of individualized cancer risk: a qualitative studyHEALTH EXPECTATIONS, Issue 1 2009Paul K. J. Han MD MA MPH Abstract Objective, To explore laypersons' understanding of individualized cancer risk estimates, and to identify conceptual problems that may limit this understanding. Background, Risk prediction models are increasingly used to provide people with information about their individual risk of cancer and other diseases. However, laypersons may have difficulty understanding individualized risk information, because of conceptual as well as computational problems. Design, A qualitative study was conducted using focus groups. Semi-structured interviews explored participants' understandings of the concept of risk, and their interpretations of a hypothetical individualized colorectal cancer risk estimate. Setting and participants, Eight focus groups were conducted with 48 adults aged 50,74 years residing in two major US metropolitan areas. Participants had high school or greater education, some familiarity with information technology, and no personal or family history of cancer. Results, Several important conceptual problems were identified. Most participants thought of risk not as a neutral statistical concept, but as signifying danger and emotional threat, and viewed cancer risk in terms of concrete risk factors rather than mathematical probabilities. Participants had difficulty acknowledging uncertainty implicit to the concept of risk, and judging the numerical significance of individualized risk estimates. The most challenging conceptual problems related to conflict between subjective and objective understandings of risk, and difficulties translating aggregate-level objective risk estimates to the individual level. Conclusions, Several conceptual problems limit laypersons' understanding of individualized cancer risk information. These problems have implications for future research on health numeracy, and for the application of risk prediction models in clinical and public health settings. [source] High dimensional multivariate mixed models for binary questionnaire dataJOURNAL OF THE ROYAL STATISTICAL SOCIETY: SERIES C (APPLIED STATISTICS), Issue 4 2006Steffen Fieuws Summary., Questionnaires that are used to measure the effect of an intervention often consist of different sets of items, each set possibly measuring another concept. Mixed models with set-specific random effects are a flexible tool to model the different sets of items jointly. However, computational problems typically arise as the number of sets increases. This is especially true when the random-effects distribution cannot be integrated out analytically, as with mixed models for binary data. A pairwise modelling strategy, in which all possible bivariate mixed models are fitted and where inference follows from pseudolikelihood theory, has been proposed as a solution. This approach has been applied to assess the effect of physical activity on psychocognitive functioning, the latter measured by a battery of questionnaires. [source] | ||||||||||