1. Landau, L. D. J. Phys. U.S.S.R. 5, 71 (1941); 11, 91 (1947).
2. Feynman, R. P. in Progress in Low Temperature Physics Vol. 1 (ed. Gorter, C. J.) Ch. 2 (North-Holland, Amsterdam, 1955).
3. Jones, C. A. & Roberts, P. H. J. Phys. A15, 2599 (1982).
4. Sears, V. F., Svensson, E. C., Mattel, P. & Woods, A. D. B. Phys. Rev. Lett. 49, 279 (1982).
5. Svensson, E. C. in Elementary Excitations in Quantum Fluids (eds Ohbayashi, K. & Watabe, M.) 59–95 (Springer, Berlin, 1989).