Journal of Computer and System Sciences MANAGING EDITOR E. K. Blum Department of Mathematics University of Southern California Los Angeles, California 90089 EDITORS J. Hartmanis Christos Papadimitriou Ithaca, New York Berkeley, California Michael Rabin John Hopcroft Jerusalem, Israel Ithaca, New York Hartley Rogers, Jr. R. M. Karp Cambridge, Massachusetts Berkeley, California Dana Scott Pittsburgh, Pennsylvania D. E. Knuth J. D. Ullman Stanford, California Stanford, California Raymond E. Miller L. A. Zadeh College Park, Maryland Berkeley, California ASSOCIATE EDITORS Alfred V. Aho Michael T. Goodrich Pavel Pevzner Murray Hill, New Jersey Baltimore, Maryland La Jolla, California Michael Benedikt Georg Gottlob Greg Plaxton Chicago, Illinois Vienna, Austria Austin, Texas Manfred Broy Fan Chung Graham W. Raskind München, Germany La Jolla, California Los Angeles, California Todd Brun Mingdei Huang John Savage Los Angeles, California Los Angeles, California Providence, Rhode Island Arthur W. Burks R. E. Kalman Ann Arbor, Michigan Zurich, Switzerland Jack Schwartz New York, New York Jin-Yi Cai A. Karlin Madison, Wisconsin Seattle, Washington Alan Selman Buffalo, New York Richard Chang Sanjeev Khanna Baltimore, Maryland Philadelphia, Pennsylvania Ron Shamir Chandra Chekuri Scott Kirkpatrick Tel Aviv Ramat, Israel Urbana, Illinois Yorktown Heights, New York Larry Snyder Edith Cohen S. Rao Kosaraju Seattle, Washington Florham Park, New Jersey Baltimore, Maryland Eduardo D. Sontag Sanjoy Dasgupta Elias Koutsoupias New Brunswick, New Jersey San Diego, California Athens, Greece Aravind Srinivasan Hartmut Ehrig Richard Ladner College Park, Maryland Berlin, Germany Seattle, Washington Shang-Hua Teng Ronald Fagin Ming Li Urbana, Illinois San Jose, California Waterloo, Ontario, Canada Moshe Vardi Michael Fellows Wolfgang Maass Houston, Texas Newcastle, Australia Graz, Austria Umesh Vazirani Faith Fich Rajeev Motwani Berkeley, California Toronto, Canada Stanford, California Harold N. Gabow Andrew M. Odlyzko Xin Wang Boulder, Colorado Murray Hill, New Jersey Los Angeles, California William Gasarch Ian Parberry Mihalis Yannakakis College Park, Maryland Denton, Texas Murray Hill, New Jersey Sally Goldman Ramamohan Paturi Chee Yap St. Louis, Missouri La Jolla, California New York, New York.