Second cohomology groups and finite covers

For D an infinite set, k>1 and W the set of k-sets from D, there is a natural closed permutation group G_k which is a non-split extension of \mathbb{Z}_2^W by \Sym(D). We classify the closed subgroups of G_k which project onto \Sym(D)$. The question arises in model theory as a problem about finite covers, but here we formulate and solve it in algebraic terms.Comment: Typos corrected; change of title to 'Second cohomology groups and finite covers of infinite symmetric groups' in published versio

Systems issues in airborne Doppler radar/LIDAR certification

This report outlines: (1) thesis; (2) ground clutter challenge; (3) moving scatters (birds, bugs); (4) range ambiguities; and rain attenuation

The Federal Aviation Administration/Massachusetts Institute of Technology (FAA/MIT) Lincoln Laboratory Doppler weather radar program

The program focuses on providing real-time information on hazardous aviation weather to end users such as air traffic control and pilots. Existing systems will soon be replaced by a Next Generation Weather Radar (NEXRAD), which will be concerned with detecting such hazards as heavy rain and hail, turbulence, low-altitude wind shear, and mesocyclones and tornadoes. Other systems in process are the Central Weather Processor (CWP), and the terminal Doppler weather radar (TDWR). Weather measurements near Memphis are central to ongoing work, especially in the area of microbursts and wind shear

Using styles for more effective learning in multicultural and e-learning environments

Purpose – This Special Issue contains selected papers from the thirteenth annual European Learning Styles Information Network (ELSIN) conference held in Ghent, Belgium in June 2008. One of the key aims of ELSIN is to promote understanding of individual learning and cognitive differences through the dissemination of international multidisciplinary research about learning and cognitive styles and strategies of learning and thinking. Design/methodology/approach – Three papers within this special issue consider how style differences can inform the development of e-learning opportunities to enhance the learning of all (Vigentini; Kyprianidou, Demetriadis, Pombortsis and Karatasios; Zhu, Valcke and Schellens). The influence of culture on learning is also raised in the paper of Zhu and colleagues and those of Sulimma and Eaves which both focus more directly on cultural influences on style, learning and teaching. Findings – A number of key themes permeate the studies included in this Special Edition such as: the nature of styles; the intrinsic difficulty of isolating style variables from other variables impacting on performance; inherent difficulties in choosing the most appropriate style measures; the potential of e-learning to attend to individual learning differences; the role of culture in informing attitudes and access to learning; the development of constructivist learning environments to support learning through an understanding of individual differences; and most importantly how one can apply such insights about individual differences to inform and enhance instruction. Originality/value – The papers in this Special Issue contribute to enhanced knowledge about the value of style differences to design constructive learning environments in multicultural and e-learning contexts

Tight Clothing: How the MFA Affects Asian Apparel Exports

International trade in apparel and textiles is regulated by a system of bilateral tariffs and quotas known as the Multifiber Arrangement or MFA. Using a time series of detailed product-level data from the United States on the quotas and tariffs that comprise the MFA, we analyze how the MFA affects the sources and prices of US apparel imports, with a particular focus on the effects on East Asian exporters during the 1990s. We show that while a large fraction of US apparel is imported under binding quotas, there are many quotas that remain unfilled. We also show that binding quotas substantially raise import prices, suggesting both quality upgrading and rent capture by exporters. In contrast, tariffs reduce import prices. Lastly, we argue that the substantial shift of US apparel imports away from Asia in favor of Mexico and the Caribbean during the 1990s is only partly due to discriminatory trade policy: the other reason is an increasing demand for timely delivery that gives a competitive advantage to nearby exporters.