The work of the first author was supported in part by NSF Grant DMS-9001857. The
work of the second author was supported in part by NSF Grant DMS-8704097. The
third author acknowledges the support of CNP
, Brazil, and the warm hospitality of
the Department of Mathematics at Yale University. The first author would also like to
acknowledge the support of the Institute for Advanced Study, Princeton, NJ, where part
of this work was completed.