1. L. Landau and I. Pomeranchuk, Phys. Zs. Sowjetunion 10, 649 (1936).
2. A. A. Abrikosov, Fundamentals of the Theory of Metals (North-Holland, New, York, 1988).
3. R. W. Keyes, J. Phys. Chem. Solids 6, 1 (1958).
4. V. F. Gantmakher and I. B. Levinson, Sov. Phys. JETP 47, 133 (1978).
5. J. Appel and A. W. Overhauser, Phys. Rev. B 18, 758 (1978).