1. G. I. Mironov, Fiz. Tverd. Tela (St. Petersburg) 39(9), 1594 (1997) [Phys. Solid State 39 (9), 1420 (1997)].
2. G. I. Mironov, Fiz. Tverd. Tela (St. Petersburg) 41(6), 951 (1999) [Phys. Solid State 41 (9), 864 (1999)].
3. J. Hubbard, Proc. R. Soc. A 276(1365), 238 (1963).
4. G. I. Mironov, Fiz. Tverd. Tela (St. Petersburg) 44(2), 209 (2002) [Phys. Solid State 44 (2), 216 (2002)].
5. V. J. Emery, Phys. Rev. Lett. 58(26), 2794 (1987).