“…For example, for the wave vector k directed along the [001] direction, their contributions to the frequen cies and , according to formula (29) [34], are equal respectively to (29) The signs of t 1 and t 3 can be estimated using the Schwarz inequality. By introducing the vectors X, Y, and Z with the components (30) we find (31) For the quantity (32) the sign remains unknown. However, using the geo metrical representation, we obtain (33) and, since the geometric mean does not exceed the arithmetic mean, we have Thus, in the considered example of the frequencies Ω L and Ω T for the [001] direction, the three body forces act in the same direction as the electronphonon forces, thus decreasing the frequencies of short wavelength phonons.…”