These notes are a transcript of lectures delivered by Oyvind Gron during the spring of 1997 at the University of Oslo. The present version of this document is an extended and corrected version of a set of Lecture Notes which were typesetted by S. Bard, Andreas O. Jaunsen, A Frode Hansen and Ragnvald J. Irgens using LT X2 . Svend E. Hjelmeland has made E many useful suggestions which have improved the text. I would also like to thank Jon Magne Leinaas and Sigbjorn Hervik for contributing with problems, and Gorm Krogh Johnsen for help with nishing the manuscript. I also want to thank prof. Finn...