1. L. D. Landau, Collected Works (Nauka, Moscow, 1969), Vol. 2, pp. 119–125 [in Russian].
2. V. I. Marchenko and A. Ya. Parshin, Pis’ma Zh. Éksp. Teor. Fiz. 83, 485 (2006) [JETP Lett. 83, 416 (2006)].
3. K. O. Keshishev and O. A. Andreeva, in Excitations in Two-Dimensional and Three-Dimensional Quantum Fluids, Ed. by A. F. G. Wyatt and H. J. Lauter (Plenum, New York, 1991).
4. K. O. Keshishev, A. Ya. Parshin, and A. V. Babkin, Zh. Éksp. Teor. Fiz. 80, 716 (1981) [Sov. Phys. JETP 53, 362 (1981)].
5. O. A. Andreeva and K. O. Keshishev, Phys. Scr. 39, 352 (1991).