These are the revised notes of a course for graduate students and some seminar talks which I gave at the University of Rochester during Fall Term 1969/70. They would not have been written without the encouragement and the aid which I received, during all stages of the work, by friends from Rochester, Rio de Janeiro, and Bonn. I wish to thank all of them: Barbara Grabkowicz encouraged me to write these notes in English and read carefully parts of a preliminary manuscript, as did Gervasio G. Bastos, Yves A. E. Lequain, Walter Strubel, and Antonio J. Engler. Many valuable suggestions were given me by Yves A. E. Lequain, and several improvements of theorems and proofs are due to him. I am particularly grateful to Linda C. Hill for her criticism in reading the last version and for improving and smoothing many of my formulations. Last but not least I thank Wilson Goes for the excellent typing. Most of this book was elaborated when I stayed in Rio de Janeiro as a Visiting Professor at IMPA (Institute for Pure and Applied Mathematics) and as a Pesquisador-Conferencista of CNPq (National Research Council). Thanks are also due to these institu­ tions.