1. V.I.Sheka, Fiz.Tverd.Tela 6, 3099 (1964) [Sov.Phys.Solid State 6, 2470 (1965)].
2. B.D.McCombe, Int.Conf.Appl.High.Magn.Fields, Würzburg 1974; B.D.McCombe, R.J.Wagner, Proc.11th Int.Conf.Phys.Semicond.,Warsaw 1972, p.321.
3. F.Kuchar, R.Meisels, M.Kriechbaum, ProcAth Int.Conf.Phys.Narrow Gap Semicond., Linz 1981 (Springer 1982), p.197.
4. M.Kriechbaum, R.Meisels, F.Kuchar, E.Fantner, Proc.16th Int.Conf. Phys.Semicond.,Montpellier 1982 (in print).
5. D.G.Seiler, B.D.Bajaj, A.E.Stephens, Phys.Rev. B16, 2822 (1977).