This book is the product of research which I undertook for my doc- toral thesis. The project was started whilst I was at the Free Univer- sity of Amsterdam, and the State University of Groningen gave me ample opportunity to complete the work. At both universities I was lucky enough to find kind colleagues who were willing to perform my teaching tasks, enabling me to spend much of my time some of on my research. I should like to thank Wietze Boomsma, Kees van den Hoeven and Jan Oosterhaven for their kind help. I was also most encouraged to discover several students at both institutions who were interested in the research topic. This meant that some research could be performed with their co-operation, which proved most stimulating. Harry ter Braak and Henk van Metelen were especially enthusiastic helpers. During the initial stage of research, Fons Bertens did a great deal of meticulous work, with never a com- plaint. In the final stages, Arend Stemerding helped me greatly. The completed manuscript was read by Nol Merkies and Peter Nijkamp, who had some helpful comments on the contents. Pro- fessor Nijkamp succeeded in stimulating me during the research by his interest in the project and subsequent edifying discussions. Several persons helped to type the manuscript, but Yvonne van Tuyl took the lion's share, typing a perfect final copy in record time.