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xiv Acknowledgments to the First Edition

Ruth Lawrence read the early chapters along with working the exercises. Her
contribution was very great with her appendix on the exercises and suggested im-
provements in the text. I wish to thank her for this very special addition to the book.
Free time from teaching at Haverford College during the year 1985–1986 was
made possible by a grant from the Vaughn Foundation. I wish to express my gratitude
to Mr. James Vaughn for this support, for this project as well as others, during this
difficult last period of the preparation of the manuscript.

Max-Planck-Institut f¨ ur Mathematik Dale Husem¨ oller
Bonn, Germany