Graph augmentation problems on a weighted graph involve determining a minimum-cost set of edges to add to a graph to satisfy a specified property, such as biconnectivity, bridge-connectivity or strong connectivity. These augmentation problems are shown to be NP-complete in the restricted case of the graph being initially connected. Approximation algorithms with favorable time complexity are presented and shown to have constant worst-case performance ratios.

Abstract. An operational atmospheric correction algorithm for Thematic Mapper (TM) imagery has been developed for both sequential and parallel computer environments considering both aerosol and molecular scattering and absorption. The aerosol optical depth is estimated from the image itself using the dark object approach on a movingwindow basis, and the surface reflectance is then retrieved by searching lookup tables that are created using a numerical radiative transfer code. The dark object pixels are identified and their surface reflectance estimated using TM channel 7 (2.1 /am Atmospheric effects include molecular and aerosol scattering and absorption by gases, such as water vapor, ozone, oxygen, and aerosols. Molecular scattering and absorption by ozone and oxygen are relatively easy to correct because their concentrations are quite stable over both time and space. The effect of water vapor absorption is significant for the TM nearinfrared (IR) channels, but there is insufficient information to allow us to estimate water vapor content from TM imagery unless other information is provided. The most disturbing and difficult component of atmospheric correction is to eliminate the effect of aerosols. Aerosol scattering increases apparent surface reflectance over dark objects and it (i.e., scattering) reduces the apparent surface reflectance over bright objects Algorithm DescriptionThe basic principle of this atmospheric correction method is based on radiative transfer theory. For a plane-parallel atmosphere bounded by a Lambertian surface, the radiance at the top of the atmosphere can be expressed as pFaT Zm -Zo + ,r(1 -sp)where L o is the upward radiance of the atmosphere for zero surface reflectance, often called path radiance, p is the surface reflectance, F a is the downward flux at the ground, T is the 17,173

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Previous schemes for sorting on general-purpose parallel machines have had to choose between poor load balancing and irregular communication or multiple rounds of all-to-all personalized communication. In this paper, we introduce a novel variation on sample sort which uses only two rounds of regular all-to-all personalized communication in a scheme that yields very good load balancing with virtually no overhead. Moreover, unlike previous variations, our algorithm efficiently handles the presence of duplicate values without the overhead of tagging each element with a unique identifier. This algorithm was implemented in Split-C and run on a variety of platforms, including the Thinking Machines CM-5, the IBM SP-2, and the Cray Research T3D. We ran our code using widely different benchmarks to examine the dependence of our algorithm on the input distribution. Our experimental results illustrate the efficiency and scalability of our algorithm across different platforms. In fact, it seems to outperform all similar algorithms known to the authors on these platforms, and its performance is invariant over the set of input distributions unlike previous efficient algorithms. Our results also compare favorably with those reported for the simpler ranking problem posed by the NAS Integer Sorting (IS) Benchmark.1998 Academic Press, Inc.

Several key studies have found that a small minority of producers, polluting at levels far exceeding group averages, generate the majority of overall exposure to industrial toxics. Frequently, such patterns go unnoticed and are understudied outside of the academic community. To our knowledge, no research to date has systematically described the scope and extent of extreme variations in industrially based exposure estimates and sought to link inequities in harm produced to inequities in exposure. In an analysis of all permitted industrial facilities across the United States, we show that there exists a class of hyper-polluters-the worst-of-the-worst-that disproportionately expose communities of color and low income populations to chemical releases. This study hopes to move beyond a traditional environmental justice research frame, bringing new computational methods and perspectives aimed at the empirical study of societal power dynamics. Our findings suggest the possibility that substantial environmental gains may be made through selective environmental enforcement, rather than sweeping initiatives.