1. Kubo, R.: J. Phys. Soc. Jpn. 12, 570 (1957)
2. Smith, D.Y., Shiles, E.: Phys. Rev. B 17, 4689 (1978)
3. This was stated clearly in R.A. Ferrell, R.E. Glover: Phys. Rev. 109, 1398 (1958), with important precursors in R.E. Glover, and M. Tinkham, Phys. Rev. 108, 243 (1957) and in M. Tinkham, Phys. Rev. 104, 845 (1956)
4. Hirsch, J.E.: Physica C 199, 305 (1992)
5. Hirsch, J.E.: Physica C 201, 347 (1992)