1. There is a number of excellent review articles the reader can refer to for further information and references: Leggett, A.J.: Rev. Mod. Phys.47, 331 (1975)
2. Wheatly, J.C.: Rev. Mod. Phys.47, 415 (1975)
3. Anderson, P.W., Brinkman, W.F.: In: The physics of liquid and solid helium. Bennemann, K.H., Ketterson, J.B. (eds.). New York: Wiley 1977
4. Lee, D.M., Richardson, R.C.: ibid
5. Brinkman, W.F., Cross, M.C.: In: Progress in low temperature physics. Brewer, D.F. (eds.), Vol. VII A. Amsterdam: North Holland 1978