Multiphonon boundary of the excitation spectrum in He2

The disintegration of phonons into elementary excitations in a superfluid He while the decay is possible only into a great number of phonons at the same time according to the energy conservation law (the case of the absolute zero of temperature) is studied. The formfactors behaviour near the sonic line w = up is found. The general calculation formulae for asymptotics of the imaginary part of the Green function at high frequencies and pulses have been obtained. While deducing the solution the saddle-point method for the continual integral representing the Green function is used. The method proposed is suitable for the case of the crystal phonon spectrum