This book is a slightly revised version of my doctoral thesis which I wrote during my time as an assistant at the Faculty of Economics of the University of Magdeburg. I am grateful that I had the opportunity to write my the sis in the stimulating atmosphere of this young and lively faculty. lowe a great amount of gratitude to my supervisor Prof. G. Schwodiauer who con stantly encouraged my work and helped to improve it in many discussions. I also would like to thank Prof. K-H. Paque and Prof. P. Flaschel who, as members of my doctoral committee, commented on various details of this study in a...