1. Bohr, Niels: Drei Aufs�tze �ber Spektren und Atombau; Vorwort zur 1. Aufl., Vieweg 1922
2. The first detailed paper on this subject was published by H. P. D�rr, W. Heisenberg, H. Mitter, S. Schlieder u. K. Yamazaki, Z. Naturforschung14a, 441 (1959). For later literature see W. Heisenberg, Introduction to the Unified Field Theory of Elementary Particles, Interscience publ 1966
3. D�rr, H. P.: Z. Naturforschung16a, 327 (1961)
4. First discussed by P. A. M. Dirac, Proc. Roy. Soc. A180, 1 (1942)
5. Heisenberg, W.: Introduction to the Unified Field Theory of Elementary Particles, p. 131. Interscience publ. 1966