1. D Pines, Elementary excitations in solids (Benjamin, New York, 1964)
2. D Pines and P Nozieres, The theory of quantum liquids (Benjamin, New York, 1966)
3. F C Khanna and H R Glyde, Can. J. Phys. 54, 648 (1976)
4. Nestor R Arista and Werner Brandt, Phys. Rev. A29, 1471 (1983)
5. H M Böhm, S Conti and M P Tosi, J. Phys.: Condens. Matter 8, 781 (1996)