Between 1960 and 1980 more than 1000 articles have been published within the field covered by this book. About half of them are referred to here. It has been my aim to quote older papers which are historically or scientifically significant, as well as the most recent work. It is hoped that these references will make it possible to trace other work which has been omitted, inadvertently or because of lack of space.
I am grateful to many colleagues who have read parts of the manuscript. My special thanks go to H. Citrynell who has read it all. I am also most grateful to my children Eva and Mats and my wife Siv who, without any serious complaints, have let me spend endless evenings and weekends typing and retyping the manuscript, checking formulas and references etc. It is the more admirable as Siv has been completing her thesis at the same time.