by Vivek S. Borkar, Sanjoy K. Mitter.Caption title. "August 1988." On the title page, the bracketed word in the title appears as the Greek mathematical symbol representing [phi], the numeral four preceded by a double underscore appears as superscript, and the numeral two preceded by a double underscore appears as subscript.Includes bibliographical references.The research of the second author was supported in part by the U.S. Army Research Office, contract DAAL03-86-K-0171 The research of the second author was supported in part by the Air Force Office of Scientific Research under contract AFOSR-85-0227