1. This technique was first applied to a Schördinger-like equation byH. Jeffreys:Proc. Math. Soc. London,23, 428 (1923) and to quantum mechanics byL. Brillouin:Compt. Rend.,183, 24 (1926);G. Wentzel (ref. (3)) andH. A. Kramers:Zeits. f. Phys.,39, 828 (1926). For a mathematical discussion of the argument seeA. Erdelyi:Asymptotic Expansions (New York, 1956), andF. Tricomi:Equazioni differenziali, 3rd ed. (Torino, 1960). In particular the paper we discuss in the following is byR. E. Langer:Trans. Amer. Math. Soc.,67, 461 (1949).
2. For a complete discussion of this method seeE. C. Kemble:The Fundamental Principles of Quantum Mechanics (New York, 1958).
3. G. Wentzel:Zeits. f. Phys.,38, 518 (1926).
4. L. Dunham:Phys. Rev.,41, 713 (1932).
5. E. C. Kemble:Phys. Rev.,48, 549 (1935).