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排序方式: 共有2849条查询结果,搜索用时 78 毫秒
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J. B. Lewis E. L. Baldeschwieler L. D. Tabern E. F. Shelberg D. Hillis S. Cromer I. F. Bingham S. Givaudon M. Aubert M. R. Moutte A. Andant P. Lambert J. Lecomte J. Tausz H. Wolf B. Gordon W. Dachnow I. Perrier M. Lobunetz M. O. Chaikin C. Engler D. Holde 《Analytical and bioanalytical chemistry》1940,119(7-8):307-310
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Theorists often characterize modeling as a cyclic problem-solving process. One builds the model, assesses its validity with regard to the underlying problem situation, and revises accordingly. The process halts when, in the opinion of the modeler, the model generates a valid solution to the underlying problem. Recent research suggests that students, like experts, employ cyclic modeling processes. Extensive observations of university and high school students’ modeling efforts, however, suggest the use of linear rather than cyclic modeling strategies. That is, novice modelers often fail to look back or revise their initial models. This paper offers empirical evidence on behalf of the linear modeling theory and identifies five factors that promote the use of linear modeling strategies: students’ conceptions of models and the modeling process, the perceived objectives of the modeling activity, constraints on time and resources, statistical misconceptions, and an overall lack of interest. The paper concludes with several promising instructional strategies (strategies that address students’ difficulties and promote reflective modeling behavior), as well as suggestions for future research. 相似文献
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The N-carboethoxy precursor to (±)-tecomanine has been prepared in 11 steps from 2-methyl-1-buten-3-yne. The key step, Pauson-Khand cyclization of a methylated 5-aza-6-nonen-1-yne succeeds, but only in low yield, a consequence of the dialkyl substitution about the azaenyne framework. Nevertheless, the overall sequence to that point is one of the more efficient to be described. 相似文献
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Muscarine, oxazole, thiazole, imidazole and peptide alkaloids and other miscellaneous alkaloids. 总被引:1,自引:0,他引:1
J R Lewis 《Natural product reports》1992,9(1):81-101
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An optimization approach for planning daily drayage operations 总被引:1,自引:0,他引:1
Yetkin Ileri Mokhtar Bazaraa Ted Gifford George Nemhauser Joel Sokol Erick Wikum 《Central European Journal of Operations Research》2006,14(2):141-156
Daily drayage operations involve moving loaded or empty equipment between customer locations and rail ramps. Our goal is to
minimize the cost of daily drayage operations in a region on a given day. Drayage orders are generally pickup and delivery
requests with time windows. The repositioning of empty equipment may also be required in order to facilitate loaded movements.
The drayage orders are satisfied by a heterogeneous fleet of drivers. Driver routes must satisfy various operational constraints.
We present an optimization methodology for finding cost-effective schedules for regional daily drayage operations. The core
of the formulation is a set partitioning model whose columns represent routes. Routes are added to the formulation by column
generation. We present numerical results for real-world data which demonstrate that our methodology produces low cost solutions
in a reasonably short time. 相似文献