1. V. G. Skobov and A. S. Chernov, Fiz. Tverd. Tela (St. Petersburg) 45(10), 1770 (2003) [Phys. Solid State 45 (10), 1860 (2003)].
2. V. G. Skobov and A. S. Chernov, Fiz. Tverd. Tela (St. Petersburg) 48(1), 21 (2006) [Phys. Solid State 48 (1), 20 (2006)].
3. G. A. Vugal’ter and V.Ya. Demikhovskiĭ, Zh. Éksp. Teor. Fiz. 70(4), 1419 (1976) [Sov. Phys. JETP 43 739 (1976)].