4 1. Introduction
Josh Nichols-Barrer and Thomas Lam for helping run the course in
2004 and for useful comments. He is also very grateful to Darij Grin-
berg for his very careful reading of the text, for many useful comments
and corrections, and for suggesting Problems 2.11.6, 3.3.3, 3.8.3, 3.8.4,
4.5.2, 5.10.2 and Exercises 5.27.2, 5.27.3, 7.9.8. Finally, the authors
gratefully acknowledge the use of the Dynkin diagram pictures pre-
pared by W. Casselman.