1. E. Schrödinger, Collected Papers on Wave Mechanics (American Mathematical Society Chelsea Publishing, Providence, Rhode Island, Reprinted, 2014)
2. A. Sommerfeld, Wave Mechanics (Dutton, New York, 1929)
3. V. Fock, Z. Physik 47, 446 (1928)
4. C.G. Darwin, Cambridge Phil. Soc. 27, 86 (1930)
5. W. Pauli, Z. Physik 31, 765 (1925)