1. Schrödinger, E., Sitzungsbr. Preuss. Akad. Wiss. Phys.-Math. Kl. 24, 418 (1930).
2. Dirac, P.A.M., The Principles of Quantum Mechanics, 4th edn., Sec. 69, Clarendon Press, Oxford, 1958.
3. Guertin, R.F. and Guth, E., Phys. Rev. D7, 1057 (1973).
4. Barut, A.O. and Bracken, A.J., in K.B.Wolf (ed.), Group Theoretical Methods in Physics, Lecture Notes in Physics, Vol. 135, p. 206, Springer-Verlag, New York, 1980; Phys. Rev. D23, 2454 (1981), Austr. J. Phys. 35, 353 (1982); Phys. Rev. D24, 3333 (1981).
5. Barut, A.O. and Bracken, A.J., submitted for publication.