Weinberg this project would probably never have been finished. Particular
thanks are also due to Owen Gwilliam, for many discussions which have
greatly clarified the ideas in this book; and to Dennis Sullivan, for his en-
couragement and many helpful comments. I would also like to thank Alberto
Cattaneo, Jacques Distler, Mike Douglas, Giovanni Felder, Arthur Green-
spoon, Si Li, Pavel Mn¨ ev, Josh Shadlen, Yuan Shen, Stephan Stolz, Peter
Teichner, and A.J. Tolland. Of course, any errors are solely the responsibil-
ity of the author.