Problem 4.152

A plane wave of frequency $\omega$ propagates so that a certain phase of oscillation moves along the $x, y, z$ axes with velocities $v_{1}$, $v_{2}, v_{3}$ respectively. Find the wave vector $\mathbf{k}$, assuming the unit vectors $\mathrm{e}_ {x}, \mathrm{e}_ {y}, \mathrm{e}_ {z}$ of the coordinate axes to be assigned.