1. R. E. Peierls, Quantum Theory of Solids, Oxford (1955).
2. E. D. Belokolos, Teor. Mat. Fiz.,45, 268 (1980).
3. S. A. Brazovskii, I. E. Dzyaloshinskii, and I. M. Krichever, Zh. Eksp. Teor. Fiz.,83, 389 (1982).
4. S. B. Shastry, Phys. Rev. Lett.,50, 633 (1983).
5. V. E. Zakharov, S. V. Manakov, S. P. Novikov, and L. P. Pitaevskii, in: The Theory of Solitons: The Inverse Scattering Method [in Russian] (ed. S. P. Novikov), Nauka, Moscow (1980), p. 320.