The article presented here reflects a scientific work of more than ten years. During this too long period I enjoyed useful discussions with many researchers and scientists. I offer my apologies for being unable to thank every single one of them here explicitly. Valuable hints of Dr. A.Klodt, Prof. Z.Lipecki, and Prof. B.Peleg improved the work. The help­ ful conversations with and the hospitality of Prof. T.E.Armstrong and Prof. M.K.Richter are gratefully acknowledged. The instructions in macro­ economics by Prof. G.Gabisch were very helpful for my principal reasoning. I am indebted to my teacher and supervisor Prof. D.Pallaschke, who moti­ vated and supported always our research.