1. L. D. Landau, Collected Works, Vol. 1 [in Russian], Nauka, Moscow (1969), p. 128.
2. I. A. Akhiezer and A. E. Borovik, ?Theory of spin waves of finite amplitude,? Zh. Eksp. Teor. Fiz.,52, 508 (1967); I. A. Akhiezer and A. E. Borodik, ?Nonlinear spin waves in ferromagnets and antiferromagnets,? Zh. Eksp. Teor. Fiz.,52, 1332 (1967); B. A. Ivanov, A. E. Kovalev, and A. M. Kosevich, Fiz. Nizk. Temp.,3, 906 (1977).
3. A. E. Borovik and K. V. Maslov, Fiz. Nizk. Temp.,4, 94 (1978).
4. L. A. Takhtajan, Phys. Lett. A,64, 235 (1977).
5. V. A. Marchenko, Sturm-Liouville Operators and Their Applications [in Russian], Naukova Dumka, Kiev (1978).