1. F. LondonSuperfluids, VolII, Dover Publications N.Y. 1954, p.192 Eq. 1 is a slight variation of London's original expression.
2. J. Wilks,The Properties of Liquid and Solid Helium, Clarendon Press, Oxford, 1967
3. I.M. Khalatnikov and V.N. Zharkov, Zh. Eksp. Teor. Fiz.32, 1108 (1957) [Sov. Phys. JETP5 905 919570]
4. I.M. Khalatnikov,Introduction to the Theory of Superfluidity (Benjamin N.Y. 1965) p.149
5. A. Singsaas and G. Ahlers, Phys. Rev.B29, 4951 (1984) and refs therein.