1. Kemble and Hill, Rev. Mod. Phys. 1, 157 (1929)
2. and 2, 1 (1930).
3. Condon and Morse, Quantum Mechanics, New York, McGraw-Hill, 1929.
4. Sommerfeld, Wave Mechanics, New York, E.P. Dutton, 1930.
5. Frenkel, Einführung in die Wellenmechanik, Berlin, J. Springer, 1929.