1. M. M. Crum,Q. J. Math. Oxford (2),6, 121 (1955).
2. G. Darboux,C. R. Acad. Sci. Paris,94, 1456 (1882).
3. S. Flügge,Practical Quantum Mechanics, Vol. 1, Springer Verlag, Berlin (1971) [Russian translation published by Mir, Moscow (1974).]
4. S. Yu. Dubov, V. M. Eleonskii, and N. E. Kulagin,Zh. Eksp. Teor. Fiz.,102, 814 (1992).
5. L. Infeld and T. E. Hull,Rev. Mod. Phys.,23, 21 (1951).