1. Yu. A. Izyumov and M. V. Medvedev, JETP, 21, 381?388 (1965); V. V. Gann and L. G. Zazunov, Fiz. Tverd. Tela, 15, 3535?3569 (1973); Y.-L. Wang and H. Callen, Phys. Rev., 160, 358?363 (1967); T. Oguchi and I. Ono, J. Phys. Soc. Japan, 26, 32?42 (1969); T. Wolfram and J. Callaway, Phys. Rev., 130, 2207?2217 (1963); I. Ono and Y. Endo, Phys. Rev. Lett. A, 41, 440?442 (1972).
2. S. M. Tashpulatov, Theor. Math. Phys., 126, 403?408 (2001).
3. E. Schrödinger, Proc. Roy. Irish. Acad. A, 48, 39 (1941).
4. M. Reed and B. Simon, Methods of Modern Mathematical Physics, Vol. 1, Functional Analysis, Acad. Press, New York (1972).
5. V. V. Val?kov, S. G. Ovchinnikov, and O. P. Petrakovskii, Fiz. Tverd. Tela, 30, 3044?3047 (1988).