1. Bohr, Niels: Drei Aufsätze über Spektren und Atombau; Vorwort zur 1. Aufl., Vieweg 1922
2. The first detailed paper on this subject was published by H. P. Dürr, W. Heisenberg, H. Mitter, S. Schlieder u. K. Yamazaki, Z. Naturforschung 14a, 441 (1959). For later literature see W. Heisenberg, Introduction to the Unified Field Theory of Elementary Particles, Interscience publ. 1966
3. Dürr, H. P.: Z. Naturforschung 16a, 327 (1961)
4. First discussed by P. A. M. Dirac, Proc. Roy. Soc. A 180, 1 (1942)
5. Heisenberg, W.: Introduction to the Unified Field Theory of Elementary Particles, p. 131. Interscience publ. 1966