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排序方式: 共有423条查询结果,搜索用时 15 毫秒
1.
Surrogate modeling approximation using a mixture of experts based on EM joint estimation 总被引:1,自引:1,他引:0
Dimitri Bettebghor Nathalie Bartoli Stéphane Grihon Joseph Morlier Manuel Samuelides 《Structural and Multidisciplinary Optimization》2011,43(2):243-259
An automatic method to combine several local surrogate models is presented. This method is intended to build accurate and
smooth approximation of discontinuous functions that are to be used in structural optimization problems. It strongly relies
on the Expectation−Maximization (EM) algorithm for Gaussian mixture models (GMM). To the end of regression, the inputs are
clustered together with their output values by means of parameter estimation of the joint distribution. A local expert is
then built (linear, quadratic, artificial neural network, moving least squares) on each cluster. Lastly, the local experts
are combined using the Gaussian mixture model parameters found by the EM algorithm to obtain a global model. This method is
tested over both mathematical test cases and an engineering optimization problem from aeronautics and is found to improve
the accuracy of the approximation. 相似文献
2.
Approximate policy iteration: a survey and some new methods 总被引:2,自引:0,他引:2
Dimitri P. BERTSEKAS 《控制理论与应用(英文版)》2011,9(3):310-335
We consider the classical policy iteration method of dynamic programming(DP),where approximations and simulation are used to deal with the curse of dimensionality.We survey a number of issues:convergence and rate of convergence of approximate policy evaluation methods,singularity and susceptibility to simulation noise of policy evaluation,exploration issues,constrained and enhanced policy iteration,policy oscillation and chattering,and optimistic and distributed policy iteration.Our discussion of policy evaluation is couched in general terms and aims to unify the available methods in the light of recent research developments and to compare the two main policy evaluation approaches:projected equations and temporal differences(TD),and aggregation.In the context of these approaches,we survey two different types of simulation-based algorithms:matrix inversion methods,such as least-squares temporal difference(LSTD),and iterative methods,such as least-squares policy evaluation(LSPE) and TD(λ),and their scaled variants.We discuss a recent method,based on regression and regularization,which recti?es the unreliability of LSTD for nearly singular projected Bellman equations.An iterative version of this method belongs to the LSPE class of methods and provides the connecting link between LSTD and LSPE.Our discussion of policy improvement focuses on the role of policy oscillation and its effect on performance guarantees.We illustrate that policy evaluation when done by the projected equation/TD approach may lead to policy oscillation,but when done by aggregation it does not.This implies better error bounds and more regular performance for aggregation,at the expense of some loss of generality in cost function representation capability.Hard aggregation provides the connecting link between projected equation/TD-based and aggregation-based policy evaluation,and is characterized by favorable error bounds. 相似文献
3.
Xiaoying Wu Dimitri Theodoratos Calisto Zuzarte 《The VLDB Journal The International Journal on Very Large Data Bases》2010,19(5):661-686
The streaming evaluation is a popular way of evaluating queries on XML documents. Besides its many advantages, it is also
the only option for a number of important XML applications. Unfortunately, existing algorithms focus almost exclusively on
tree-pattern queries (TPQs). Requirements for flexible querying of XML data have motivated recently the introduction of query
languages that are more general and flexible than TPQs. These languages are not supported by existing algorithms. In this
paper, we consider a partial tree-pattern query (PTPQ) language which generalizes and strictly contains TPQs. PTPQs can express
a fragment of XPath which comprises reverse axes and the node identity equality (is) operator, in addition to forward axes, wildcards and predicates. They constitute an important subclass of XPath, which is
very useful in practice. Unfortunately, previous streaming algorithms for TPQs cannot be applied to PTPQs. PTPQs can be represented
as dags enhanced with constraints. We explore this representation to design an original polynomial time streaming algorithm
for PTPQs. Our algorithm aggressively filters incoming data that is irrelevant to the query and wisely avoids processing redundant
query matches (i.e., matches of the query dag that do not contribute to new solutions). Our algorithm is the first one to
support the streaming evaluation of such a broad fragment of XPath. We provide an analysis of it, and conduct an extensive
experimental evaluation of its performance and scalability. Compared to the only known streaming algorithm that supports TPQs
extended with reverse axes, our algorithm performs better by orders of magnitude while consuming a much smaller fraction of
memory space. Current streaming applications have stringent requirements on query response time and memory consumption because
of the large (possibly unbounded) size of data they handle. In order to keep memory usage and CPU consumption low for the
PTPQ streaming evaluation, we design another streaming algorithm called Eager PSX for PTPQs. Its key feature is that it applies an eager evaluation strategy to quickly determine when node matches should
be returned as solutions to the user and also to proactively detect redundant matches. We theoretically analyze Eager PSX, and experimentally test its time and space performance and scalability. We compare it with PSX. Our results show that Eager PSX not only achieves better space performance without compromising time performance, but it also greatly improves query response
time for both simple and complex queries, in many cases, by orders of magnitude. 相似文献
4.
Xiaoying Wu Dimitri Theodoratos Stefanos Souldatos Theodore Dalamagas Timos Sellis 《World Wide Web》2010,13(4):441-474
Finding the occurrences of structural patterns in XML data is a key operation in XML query processing. Existing algorithms
for this operation focus almost exclusively on path patterns or tree patterns. Current applications of XML require querying
of data whose structure is complex or is not fully known to the user, or integrating XML data sources with different structures.
These applications have motivated recently the introduction of query languages that allow a partial specification of path
patterns in a query. In this paper, we consider partial path queries, a generalization of path pattern queries, and we focus
on their efficient evaluation under the indexed streaming evaluation model. Our approach explicitly deals with repeated labels
(that is, multiple occurrences of the same label in a query). We show that partial path queries can be represented as rooted
dags for which a topological ordering of the nodes exists. We present three algorithms for the efficient evaluation of these
queries. The first one exploits a structural summary of data to generate a set of path patterns that together are equivalent
to a partial path query. To evaluate these path patterns, we extend a previous algorithm for path-pattern queries so that
it can work on path patterns with repeated labels. The second one extracts a spanning tree from the query dag, uses a stack-based
algorithm to find the matches of the root-to-leaf paths in the tree, and merge-joins the matches to compute the answer. Finally,
the third one exploits multiple pointers of stack entries and a topological ordering of the query dag to apply a stack-based
holistic technique. We analyze our algorithms and perform extensive experimental evaluations. Our experimental results show
that the holistic algorithm outperforms the other ones. Our approaches are the first ones to efficiently evaluate this class
of queries in the indexed streaming model. 相似文献
5.
Dimitri Papukashvili Nino Rcheulishvili Cong Liu Yang Ji Yunjiao He Peng George Wang 《International journal of molecular sciences》2022,23(21)
Messenger RNA (mRNA) technology has already been successfully tested preclinically and there are ongoing clinical trials for protein replacement purposes; however, more effort has been put into the development of prevention strategies against infectious diseases. Apparently, mRNA vaccine approval against coronavirus disease 2019 (COVID-19) is a landmark for opening new opportunities for managing diverse health disorders based on this approach. Indeed, apart from infectious diseases, it has also been widely tested in numerous directions including cancer prevention and the treatment of inherited disorders. Interestingly, self-amplifying RNA (saRNA)-based technology is believed to display more developed RNA therapy compared with conventional mRNA technique in terms of its lower dosage requirements, relatively fewer side effects, and possessing long-lasting effects. Nevertheless, some challenges still exist that need to be overcome in order to achieve saRNA-based drug approval in clinics. Hence, the current review discusses the feasibility of saRNA utility for protein replacement therapy on various health disorders including rare hereditary diseases and also provides a detailed overview of saRNA advantages, its molecular structure, mechanism of action, and relevant delivery platforms. 相似文献
6.
Rodrigo Quezada-Lzaro Yessica Vzquez-Cobix Rocío Fonseca-Lin Porfirio Nava Daniel Dimitri Hernndez-Cueto Carlos Cedillo-Pelez Yolanda Lpez-Vidal Sara Huerta-Yepez M. Guadalupe Ortega-Pierres 《International journal of molecular sciences》2022,23(21)
In giardiasis, diarrhoea, dehydration, malabsorption, weight loss and/or chronic inflammation are indicative of epithelial barrier dysfunction. However, the pathogenesis of giardiasis is still enigmatic in many aspects. Here, we show evidence that a cysteine protease of Giardia duodenalis called giardipain-1, contributes to the pathogenesis of giardiasis induced by trophozoites of the WB strain. In an experimental system, we demonstrate that purified giardipain-1 induces apoptosis and extrusion of epithelial cells at the tips of the villi in infected jirds (Meriones unguiculatus). Moreover, jird infection with trophozoites expressing giardipain-1 resulted in intestinal epithelial damage, cellular infiltration, crypt hyperplasia, goblet cell hypertrophy and oedema. Pathological alterations were more pronounced when jirds were infected intragastrically with Giardia trophozoites that stably overexpress giardipain-1. Furthermore, Giardia colonization in jirds results in a chronic inflammation that could relate to the dysbiosis triggered by the protist. Taken together, these results reveal that giardipain-1 plays a key role in the pathogenesis of giardiasis. 相似文献
7.
Reactive phosphorus removal from aquaculture and poultry productions systems using polymeric hydrogels 总被引:2,自引:0,他引:2
This work reports on the features of a sorption processes for the ultimate removal and recovery of reactive phosphorus from aquaculture and poultry production wastewater effluents. The sorbent used was a cross-linked polyamine (PAA-HCl) polymeric hydrogel. The PAA-HCl hydrogels were prepared by chemically cross-linking aqueous solutions of linear PAA-HCl chains with epichlorohydrin (EPI). The phosphorus binding capacity of the gels was measured in standard aqueous solutions as a function of ionic strength. Equilibrium PO4(3-), loadings of 100 mg anion/g gel were obtained. The regeneration ability of the gels was demonstrated by release of the bound phosphorus anions upon washing with 1-2 M NaOH solution, providing opportunities to recover and reuse the gel over multiple cycles. The ionic polyamine gels have been demonstrated to be appropriate materials for treating poultry and aquaculture wastewater effluents. Upon treatment phosphorus anion concentrations were reduced to levels suitable for discharge into natural surface waters. 相似文献
8.
Biological adhesion is a critical mechanical function of complex organisms. At the scale of cell–cell contacts, adhesion is remarkably tunable to enable both cohesion and malleability during development, homeostasis and disease. It is physically supported by transient and laterally mobile molecular bonds embedded in fluid membranes. Thus, unlike specific adhesion at solid–solid or solid–fluid interfaces, peeling at fluid–fluid interfaces can proceed by breaking bonds, by moving bonds or by a combination of both. How the additional degree of freedom provided by bond mobility changes the mechanics of peeling is not understood. To address this, we develop a theoretical model coupling diffusion, reactions and mechanics. Mobility and reaction rates determine distinct peeling regimes. In a diffusion-dominated Stefan-like regime, bond motion establishes self-stabilizing dynamics that increase the effective fracture energy. In a reaction-dominated regime, peeling proceeds by travelling fronts where marginal diffusion and unbinding control peeling speed. In a mixed reaction–diffusion regime, strengthening by bond motion competes with weakening by bond breaking in a force-dependent manner, defining the strength of the adhesion patch. In turn, patch strength depends on molecular properties such as bond stiffness, force sensitivity or crowding. We thus establish the physical rules enabling tunable cohesion in cellular tissues and in engineered biomimetic systems. 相似文献
9.
Maarten Houbraken Sofie Demeyer Dimitri Staessens Pieter Audenaert Didier Colle Mario Pickavet 《Natural computing》2013,12(2):277-289
Physarum polycephalum, a true slime mould, is a primitive, unicellular organism that creates networks to transport nutrients while foraging. The design of these natural networks proved to be advanced, e.g. the slime mould was able to find the shortest path through a maze. The underlying principles of this design have been mathematically modelled in literature. As in real life the slime mould can design fault tolerant networks, its principles can be applied to the design of man-made networks. In this paper, an existing model and algorithm are adapted and extended with stimulation and migration mechanisms which encourage formation of alternative paths, optimize edge positioning and allow for automated design. The extended model can then be used to better design fault tolerant networks. The extended algorithm is applied to several national and international network configurations. Results show that the extensions allow the model to capture the fault tolerance requirements more accurately. The resulting extended algorithm overcomes weaknesses in geometric graph design and can be used to design fault tolerant networks such as telecommunication networks with varying fault tolerance requirements. 相似文献
10.
The neuroimaging community heavily relies on statistical inference to explain measured brain activity given the experimental paradigm. Undeniably, this method has led to many results, but it is limited by the richness of the generative models that are deployed, typically in a mass-univariate way. Such an approach is suboptimal given the high-dimensional and complex spatiotemporal correlation structure of neuroimaging data.Over the recent years, techniques from pattern recognition have brought new insights into where and how information is stored in the brain by prediction of the stimulus or state from the data. Pattern recognition is intrinsically multivariate and the underlying models are data-driven. Moreover, the predictive setting is more powerful for many applications, including clinical diagnosis and brain–computer interfacing. This special issue features a number of papers that identify and tackle remaining challenges in this field. The specific problems at hand constitute opportunities for future research in pattern recognition and neurosciences. 相似文献