The results of this paper were mainly obtained during
my activity as a research fellow at INCREST-Bucharest, Romania, and
were announced in [34]. The writing up, as well as several improvements
in the proof, were done at the University of Warwick, England.
I would like to thank Ciprian Foia~, as well as my INCREST
colleagues Grigore Arsene, Zoia Ceau~escu, Mihai Pimsner, Sorin Popa,
Dan Voiculescu, and especially ~erban Str~til~ for their support.
I received further support from David Evans, Klaus Schmidt and
Christopher Zeeman at Warwick.
Benjamin Weiss was very kind to send me a description of the
results in [36] prior to their publication.
I am grateful to Vaughan Jones for useful discussions and for the
generosity with which he helped make the results in this paper known.
My wife Ileana generously helped me carry on my work.
Deborah Craig typed and edited the manuscript with remarkable skill
and accuracy.