Solution (source code)

= Solution

Put $A=g(\rho_1-\rho_2)>0$. In the deep-layer limit, the <finite-depth Rayleigh-Taylor dispersion relation> gives $\omega^2=(\gamma k^3-Ak)/(\rho_1+\rho_2)$. Its sign changes at
$$
\boxed{k_c=\sqrt{\frac{g(\rho_1-\rho_2)}{\gamma}}.}
$$
The <Rayleigh-Taylor instability> band is $0<k<k_c$; $k=k_c$ is neutral and $k>k_c$ is oscillatory.