1. Ruch,E.: Theoret. Chim. Acta (Bert.)38, 167 (1975)
2. Ruch,E., Schiönhofer,A.: Theoret. Chim. Acta (Bert.)19, 225 (1970)
3. von Neumann,J.: Mathematische Grundlagen der Quantenmechanik, pp. 202?12. Berlin: SpringerVerlag 1932; English translation: Mathematical foundations of quantum mechanics, trans. by R.T. Beyer, pp. 379?398. Princeton: Princeton University Press 1955
4. Hardy,G.H., Littlewood,J.E., Pólya,G.: Inequalities, 2nd Ed., p. 49. Cambridge: Cambridge University Press 1952.
5. Uhlmann,A.: Wiss.Z., Karl-Marx-Univ. Leipzig, Math-Naturwiss. R.21, 421 (1972);22, 139 (1973)