1. L. Onsager,Nuova Cimento Suppl. 6, 249 (1949).
2. R. P. Feynman, inProg. in Low Temp. Phys., Vol. 1, C. J. Gorter (ed.), (North-Holland Publ. Co., Amsterdam (1955), p. 17.
3. R. J. Donnelly,Quantized Vortices in Helium II, Cambridge University Press, Cambridge, UK (1991).
4. D. R. Tilley and J. Tilley,Superfluidity and Superconductivity, IOP Publishing Ltd, Bristol UK (1990).
5. W. E. Keller,Helium-3 and Helium-4, Plenum Press, New York (1969).