1. Kruscal, M. D. (1960).Phys. Rev.,119, 1743; Szekeres, G. (1960).Publ. Mat. Debrecen,7, 285.
2. Pimenov, R. I. (1984).Siber. Math. J.,25, 5, 119 (Russian).
3. Rosen, N. (1985).Found. Phys.,15, 4, 517.
4. Schrödinger, E. (1918).Phys. Zeitschr.,19, 4.
5. Lanczos, K. (1922).Phys. Zeitschr.,23, 24, 537.