Past exam of the mathematical methods II course of the University of Oxford / 2021 / 11 / ii / Solution

Take constant complex amplitude $A$ and real wavenumber $k$ and angular frequency $\omega$. Substitution of the complex exponential givesThus every nonzero plane wave of the given form solves the equation precisely whenThe wavenumber specifies spatial wave phase change per unit length, with wavelength $\lambda =2\pi /|k|$. The angular frequency specifies temporal wave phase change per unit time, with period $2\pi /|\omega |$ and ordinary frequency $|\omega |/(2\pi )$. The constant $m$ in this equation has dimensions time divided by length squared; without an additional dimensional coefficient, it is not an ordinary physical mass.

For propagating oscillatory solutions assume real $m\ne 0$. If $m=0$, the equation becomes $y_{xx}=0$ and only $k=0$ among these spatial plane waves is possible; the usual dispersion relation branch and velocity discussion would degenerate.