I.N. Adamenko, K.E. Nemchenko, V.A. Slipko, and A.F.G. Wyatt, Evolution of a pulse of noninteracting quasiparticles with dispersion and initial angular width, Low Temp. Phys. 30, (2004) [Fiz. Nizk. Temp. 30, 579-589, (2004)].