1. Michael Reed and Barry Simon,Analysis of Operators—Methods of Modern Mathematical Physics, Vol. 4 (Academic, New York, 1978).
2. W. E. Milne,Phys. Rev.,35, 863 (1930).
3. H. Ezawa, K. Nakamura, and Y. Yamamoto,Proc. Jpn. Acad.,46, 168 (1970).
4. Makoto Saito, Private communication (1970). A part of his result is used in: H. Ezawa, “Advanced topics,” inQuantum Mechanics, H. Yukawa and T. Toyoda, eds. (Iwanami Syoten, 1972; 2nd edn. 1976). Professor M. Namiki contributed introductory chapters to this volume.
5. E. A. Coddington and N. Levinson,Theory of Ordinary Differential Equations (McGraw-Hill, New York, 1955). See Sect. 8 in Chap. 2.