1. P. Nozieres, in Quantum Fluids, vol. 1, ed. by D.E. Brewer (North-Holland, Amsterdam, 1966); G. Baym, in Mathematical Methods in Solid State and Superfluid Theory, vol. 121, ed. by R.C. Clark, G.H. Derrick (Oliver and Boyd, Edinburgh, 1969)
2. S. Koh, Phys. Rev. B. 74, 054501 (2006)
3. G.B. Hess, W.M. Fairbank, Phys. Rev. 19, 216 (1967)
4. A.F. Andreev, I.M. Lifshitz, Zh. Eksp. Teor. Fiz. 56, 2057 (1969); Sov. Phys. JETP. 29, 1107 (1969)
5. A.J. Leggett, Phys. Rev. Lett. 25, 1543 (1970)