This is a preliminary version of the text. Some chapters are not yet written, or are incomplete. Other chapters have not been checked, and contain typographical and mathematical errors. I ask the reader to be patient with the manuscript, and to send me comments and corrections, both substantive and typographical, no matter how trivial. My e-mail address is: email@example.com.
I wish to thank Imre Z. Ruzsa for providing me with preprints of his
papers on Freiman's theorem, Endre Szemer\'edi for describing to me his
joint result with Antal Balog, and Gregory Freiman for valuable discussions
about his work.
Paper available at: ftp://dimacs.rutgers.edu/pub/dimacs/TechnicalReports/TechReports/1993/93-33.ps