Undoubtedly a number of typos and other errors are present in
the ﬁnal version, for which I alone bear responsibility. I am maintain-
ing a website for the book. It can be reached from the AMS webpage
for the book, whose URL is given on the copyright page and on the
back cover of the book. The website will contain lists of typographical
errors and their corrections.
I thank David Cox, Joachim Koch, and Michael Stone for assis-
tance with ﬁgures, and I also thank Randy McCarthy for providing
A number of people involved with PCMI deserve thanks for their
assistance: William Barker, Herb Clemens, Catherine Giesbrecht,
Roger Howe, and David Morrison.
I also thank the AMS staﬀ who have assisted me directly during
this project: Barbara Beeton, Arlene O’Sean, and especially Edward
Dunne, as well as the rest of the team working behind the scenes to
make this book a reality.
Finally, I want to thank my family for their support and patience
during the times when this project made me less available for them
than any of us would have liked.
November 2005 Sheldon Katz