1. K. R. Atkins and Y. Narahara,Phys. Rev. 138A, 437 (1965).
2. A. A. Sobyanin,Zh. Eksperim. i Teor. Fiz. 61, 433 (1971) [English transl.:Soviet Phys.?JETP 34, 229 (1972)].
3. V. L. Ginzburg and L. P. Pitaevskii,Zh. Eksperim. i Teor. Fiz. 52, 729 (1967) [English transl.:Soviet Phys.?JETP 25, 479 (1967)].
4. P. C. Hohenberg, private communication;J. Low Temp. Phys. 13, 433 (1973).
5. J. Eckardt, D. O. Edwards, F. M. Gasparini, and S. Y. Shen, inLow Temperature Physics LT-13 (Plenum, New York, 1973); D. O. Edwards, inLow Temperature Physics LT-13 (Plenum, New York, 1973).