1. K. R. Atkins,Phys. Rev. 116, 1339 (1959).
2. (a) K. W. Schwarz,Adv. Chem. Phys. 33, 1 (1975); (b) A. L. Fetter, inThe Physics of Liquid and Solid Helium, Part I, K. H. Bennemann and J. B. Ketterson, eds. (Wiley, New York, 1976), Chapter 3.
3. G. Akinci and J. A. Northby,Phys. Rev. Lett. 42, 573 (1979).
4. F. P. Boyle and A. J. Dahm,J. Low Temp. Phys. 23, 477 (1976).
5. A. P. Volodin and M. S. Khaikin,JETP Lett. 30, 572 (1979).