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Robert McKelvey 《Natural Resource Modeling》1986,1(1):171-189
Fishing vessels, harvesting geographically dispersed and heterogeneous multispecies aggregates, are presumed to target on the most profitable pools. But, this behavior may lead to depletion of biologically valuable species, and consequent early closure by the management agency of the entire fishery. In this article, a simple model is analyzed in order to understand the dynamics of such targeting behavior. Further analysis explores the theoretical extent to which targeting might be altered through economic rationalization measures, landings taxes or quota markets, to achieve a more nearly optimal pattern of exploitation. 相似文献
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Characterization of nanostructured hollow polymer spheres with small-angle neutron scattering (SANS)
Hollow polymer spheres synthesized from a vesicle-directed polymerization can be dried and redispersed in water using a variety of nonionic ethoxylated alcohol surfactants as stabilizers. The final dispersions consist of both polymer shells and surfactant micelles, which remain together in colloidal suspension for at least several months. Small-angle neutron scattering (SANS) is used to measure the polymer shell thickness (63 A) and core radius (560 A) of the surfactant-stabilized hollow polymer spheres in the presence of surfactant micelles. Characterization by SANS provides information about the surfactant bilayer and polymer shell thicknesses which were previously unattainable. 相似文献
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This paper develops a model, with assumptions similar to those of the linear model, for use when the observed dependent variable is ordinal. This model is an extension of the dichotomous probit model, and assumes that the ordinal nature of the observed dependent variable is due to methodological limitations in collecting the data, which force the researcher to lump together and identify various portions of an (otherwise) interval level variable. The model assumes a linear eflect of each independent variable as well as a series of break points between categories for the dependent variable. Maximum likelihood estimators are found for these parameters, along with their asymptotic sampling distributions, and an analogue of R 2 (the coefficient of determination in regression analysis) is defined to measure goodness of fit. The use of the model is illustrated with an analysis of Congressional voting on the 1965 Medicare Bill. 相似文献
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Liondas CA; Chrissoulidis DP 《The Quarterly Journal of Mechanics and Applied Mathematics》2007,60(3):275-287
This paper is intended to clarify a misunderstanding concerningthe source singularity of the electric Green's tensor for aperfectly conducting semi-infinite cone of circular cross-section.Tai's series expansion of the Green's tensor is known to lacka singular term at the source region. Jones has reconstructedthe solution to this problem and has pointed out the differencebetween his result and that of Tai. The aim of our paper isto demonstrate that, although Jones's closed-form solution iscorrect, there is a mistake in his comparison with Tai's seriessolution. We conclude that one of the two additional singularterms that Jones claims as missing from Tai's formula must beomitted. Besides, we compare Jones's closed-form solution withSmyshlyaev's solution to the very same problem. We concludethat the magnetic field expressions given by Jones and Smyshlyaevcoincide, but a singular term is missing from Smyshlyaev's expressionfor the electric field. 相似文献
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Goodman TNT; Micchelli CA; Rodriguez G; Seatzu S 《IMA Journal of Numerical Analysis》1998,18(3):331-354
It is shown that, under certain conditions, orthonormalizingthe positive integer shifts of an exponentially decaying functionon the half line by the Gram-Schmidt process leads to a limitingprofile given by orthonormalizing all their integer shifts onthe whole line. These results derive from properties of Choleskyfactorization of bi-infinite and semi-infinite matrices. Anexample is provided by the negative exponential function andconjectures are given, supported by numerical evidence, forthe Gaussian and Lorentz function. 相似文献