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I.N. Adamenko, Yu.A. Kitsenko, K.E. Nemchenko, V.A. Slipko, and A.F.G. Wyatt, Creation of high-energy phonons by four-phonon processes in anisotropic phonon systems of He II, Low Temp. Phys. 33, (2007) [Fiz. Nizk. Temp. 33, 523-537, (2007)] DOI: https://doi.org/10.1063/1.2737546.