1. R.A. Cowley and A.D.B. Woods: Can.J.Phys. 49, 177 (1971).
2. A. Miller, D. Pines and P. Nozieres: Phys. Rev. 127, 1452 (1962)
3. T.J. Greytak and J. Yan: Phys. Rev. Lett. 22, 987 (1969)
4. N.M. Blagoveshchenskii, E.B. Dokukin, Zh. A. Kozlov and V.A. Parfenov: JETP Lett. 30,12(1979), ibid 31, 4 (1980)
5. K. Ohbayashi and M. Udagawa: Proc. Int. Conf. on Low Temp. Phys - LT 17, ed by U.Eckern, A.Schmidt, W.Weber and H.Wuhi(North-Holland, Amsterdam 1984 ) p. 1201