1. L. D. Landau, Zh. Eksp. Teor. Fiz. 11: 592 (1941)
2. L. D. Landau, J. Phys. Moscow 5: 718 (1941).
3. L. D. Landau, J. Phys. Moscow 11: 91 (1947).
4. N. Bogoliubov, J. Phys. USSR 11:23 (1947); for further references and discussion, see references 4 and 5.
5. D. Pines, “The Many Body Problem,” W. P. Benjamin, New York (1962).