1. Prandl, W.: Solid State Commun.10, 529 (1972)
2. Plumier, R.: Solid State Commun.10, 5 (1972)
3. Shender, E.: Zh. Eksp. Teor. Fiz.83, 326 (1982) [Sov. Phys. JETP56, 178, 1982] note that a factor $$\sqrt 2 $$ is missing in (11)
4. Brueckel, Th., Prandl, W.: Z. Phys. B?Condensed Matter (accepted for publication)
5. Prandl, W.: Habilitationsschrift LMU München 1973