“…This will be taken into consideration in the series expansion of ~. For quasi-transverse non-linear waves in an isotropic medium, the essential terms of the expansion of q5 turn out to be It was shown in [2] that quasi-transverse waves in a medium with small anisotropy (which is always the case in practice) acquire new qualitative properties. For the effects of non-linearity and anisotropy to app,ear in their interactions, the terms in the expansion of @ that account for anisotropy must be of the same order as the non-linear terms, i.e.…”