...algorithm.

4#4Mathematics Department and 5#5Laboratory for Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139. This research was supported by the Defense Advanced Research Projects Agency under Contracts N00014-87-K-825 and N00014-89-J-1988, the Air Force under Contract OSR-89-0271, and the Army under Contract DAAL-03-86-K-0171. Tom Leighton was supported by an NSF Presidential Young Investigator Award with matching funds provided by IBM and AT & T. A preliminary version of this paper appeared in the Proceedings of the 30th Annual Symposium on Foundations of Computer Science, pages 384-389, October, 1989. Second author's current address: NEC Research Institute, Princeton NJ 08540

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...number

Throughout this paper, 8#8 denotes 9#9.

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