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This work was supported in part by the National Science Foundation Grant No. ASC-9005933; by the Defense Advanced Research Projects Agency under contract DAAL03-91-C-0047, administered by the Army Research Office; by the Office of Scientific Computing, U.S. Department of Energy, under Contract DE-AC05-84OR21400; and by the National Science Foundation Science and Technology Center Cooperative Agreement No. CCR-8809615.

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Department of Computer Science, University of Tennessee, Knoxville, TN 37996-1301

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Computer Science Division, University of California, Berkeley, Berkeley, CA 94720

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Department of Computer Science, University of Tennessee, Knoxville, TN 37996-1301, and Mathematical Sciences Section, Oak Ridge National Laboratory, Oak Ridge, TN 37831

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Mathematical Sciences Section, Oak Ridge National Laboratory, Oak Ridge, TN 37831