Acknowledgements

I thank Professor Dr. R. Schilling and Dr. C. Bruder for discussions and I acknowledge conversations with Dr. R. Bohmer, Professor Dr. A. Aharony, Professor Dr. M. E. Fisher, Professor Dr. B. I. Halperin, and Professor Dr. R. Orbach. I am grateful to Professor Dr. T. Jøssang, Professor Dr. J. Feder, and the Center for Advanced Study for their hospitality in Oslo, and to the Deutsche Forschungsgemeinschaft for financial support.