1. Letter from E. Schrödinger to A. Sommerfeld, January 23, 1926.
2. R. Courant and D. Hilbert,Methoden der mathematischen Physik, Springer-Verlag, Berlin (1924).
3. E. Schrödinger, Ref. 256, p. 515.
4. E. Schrödinger, Ref. 256, p. 518, footnote 1; English translation, p. 33, footnote 1.
5. R. Courant and D. Hilbert, Ref. 313..