1. A. Kratzer, Zs. f. Phys.,3, 289, 1920.
2. R. Rydberg, Zs. f. Phys.,73, 376, 1931.
3. Y. P. Varshni, Rev. Mod. Phys.29, 664, 1957.
4. P. M. Morse, Phys. Rev.,34, 57, 1929.
5. E. Kamke, Differentialgleichungen, p. 427, Chelsea Publishing Co., New York, 1948.