77,096 research outputs found

    Diffusion in a continuum model of self-propelled particles with alignment interaction

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    In this paper, we provide the O()O(\epsilon) corrections to the hydrodynamic model derived by Degond and Motsch from a kinetic version of the model by Vicsek & coauthors describing flocking biological agents. The parameter \epsilon stands for the ratio of the microscopic to the macroscopic scales. The O()O(\epsilon) corrected model involves diffusion terms in both the mass and velocity equations as well as terms which are quadratic functions of the first order derivatives of the density and velocity. The derivation method is based on the standard Chapman-Enskog theory, but is significantly more complex than usual due to both the non-isotropy of the fluid and the lack of momentum conservation

    Regional differences in willingness to pay for organic vegetables

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    The concern about vegetable safety, together with a booming population and the rise of the middle class has made Vietnam become a potential market for organic vegetables. This paper investigates the determinants of willingness to pay (WTP) for organic vegetables in Hanoi, Vietnam with a particular attention to regional differences and the effect of risk perception. Using Contingent Valuation Method to analyze the data from a sample of 498 consumers in Hanoi, the paper shows that the perceived use values of organic vegetables, trust in organic labels, and disposable family income increased WTP for organic vegetables in both urban and rural regions.Though risk perception of conventional vegetables was high in both regions, such heightened risk perception just translated into the WTP in the rural region. In addition, the percentage of home-grown vegetables in the total vegetable consumption of the family influenced the WTP in the rural region only. Moreover, being an organic purchaser was positively related to the WTP in the urban region but not in the rural region. The paper also discusses three policy implications for Vietnam to boost the demand for organic food.fals

    L2L_2 boosting in kernel regression

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    In this paper, we investigate the theoretical and empirical properties of L2L_2 boosting with kernel regression estimates as weak learners. We show that each step of L2L_2 boosting reduces the bias of the estimate by two orders of magnitude, while it does not deteriorate the order of the variance. We illustrate the theoretical findings by some simulated examples. Also, we demonstrate that L2L_2 boosting is superior to the use of higher-order kernels, which is a well-known method of reducing the bias of the kernel estimate.Comment: Published in at http://dx.doi.org/10.3150/08-BEJ160 the Bernoulli (http://isi.cbs.nl/bernoulli/) by the International Statistical Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm

    Epidemiological time series studies of PM2.5 and daily mortality and hospital admissions: a systematic review and meta-analysis

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    Background Short-term exposure to outdoor fine particulate matter (particles with a median aerodynamic diameter <2.5 渭m (PM2.5)) air pollution has been associated with adverse health effects. Existing literature reviews have been limited in size and scope. Methods We conducted a comprehensive, systematic review and meta-analysis of 110 peer-reviewed time series studies indexed in medical databases to May 2011 to assess the evidence for associations between PM2.5 and daily mortality and hospital admissions for a range of diseases and ages. We stratified our analyses by geographical region to determine the consistency of the evidence worldwide and investigated small study bias. Results Based upon 23 estimates for all-cause mortality, a 10鈥吢礸/m3 increment in PM2.5 was associated with a 1.04% (95% CI 0.52% to 1.56%) increase in the risk of death. Worldwide, there was substantial regional variation (0.25% to 2.08%). Associations for respiratory causes of death were larger than for cardiovascular causes, 1.51% (1.01% to 2.01%) vs 0.84% (0.41% to 1.28%). Positive associations with mortality for most other causes of death and for cardiovascular and respiratory hospital admissions were also observed. We found evidence for small study bias in single-city mortality studies and in multicity studies of cardiovascular disease. Conclusions The consistency of the evidence for adverse health effects of short-term exposure to PM2.5 across a range of important health outcomes and diseases supports policy measures to control PM2.5 concentrations. However, reasons for heterogeneity in effect estimates in different regions of the world require further investigation. Small study bias should also be considered in assessing and quantifying health risks from PM2.

    Entanglement witnesses arising from Choi type positive linear maps

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    We construct optimal PPTES witnesses to detect 333\otimes 3 PPT entangled edge states of type (6,8)(6,8) constructed recently \cite{kye_osaka}. To do this, we consider positive linear maps which are variants of the Choi type map involving complex numbers, and examine several notions related to optimality for those entanglement witnesses. Through the discussion, we suggest a method to check the optimality of entanglement witnesses without the spanning property.Comment: 18 pages, 4 figures, 1 tabl

    Conformal symmetry transformations and nonlinear Maxwell equations

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    We make use of the conformal compactification of Minkowski spacetime M#M^{\#} to explore a way of describing general, nonlinear Maxwell fields with conformal symmetry. We distinguish the inverse Minkowski spacetime [M#]1[M^{\#}]^{-1} obtained via conformal inversion, so as to discuss a doubled compactified spacetime on which Maxwell fields may be defined. Identifying M#M^{\#} with the projective light cone in (4+2)(4+2)-dimensional spacetime, we write two independent conformal-invariant functionals of the 66-dimensional Maxwellian field strength tensors -- one bilinear, the other trilinear in the field strengths -- which are to enter general nonlinear constitutive equations. We also make some remarks regarding the dimensional reduction procedure as we consider its generalization from linear to general nonlinear theories.Comment: 12 pages, Based on a talk by the first author at the International Conference in Mathematics in honor of Prof. M. Norbert Hounkonnou (October 29-30, 2016, Cotonou, Benin). To be published in the Proceedings, Springer 201