These lecture notes are based on a course that I gave at the Swiss Federal Institute of Technology during the Summer Semester 1969 at the invitation of Professors K. Pierz, K. Hepp and R. Jost. I am extremely grateful to them for this opportunity they gave me to teach on my favourite subject. Dr. R. Sohrader has been kind enough to aooept to take notes and reoonstruot a ooherent version of what I said. If the reader is dissatisfied with, the presentation of the material oontained in the leoture notes I am to blame because I think that Dr. Sohrader did a very good job in disentangling my sineous progression and restating in a more ocrreot mathematical language what I said. Dr. Sohrader and I are also very muoh indebted to Dr. C. Chandler and Prof. K. Hepp who oritioally read the manusoript and eliminated the most obvious grammatical mistakes. However, they should not be held responsible for the general Un-English appearenoe of leoture notes originating from a oourse given in Prenoh to a German speaking audienoe. Many thanks are also due to Miss Hintermann who had the task to type these notes.