# Dictionary:WKBJ solutions

${\displaystyle {\frac {d^{2}\phi }{dx^{2}}}+\omega ^{2}s^{2}\phi =0}$,
where ${\displaystyle \omega }$ is large and positive and s2 is a monotonically increasing function of x. The WKBJ approximation is a high-frequency approximation that assumes that the properties of the medium are constant over several wavelengths. See Aki and Richards (1980, 416-418).