Solution
= Solution
The three terms represent competing physical effects:
* $\Omega^2$ is the stabilizing <epicyclic motion> of a radially displaced fluid element in a <Keplerian disk>.
* $-2\pi G\Sigma|k|$ is the destabilizing self-gravity of a surface-density perturbation, obtained from the <razor-thin disk Poisson kernel>.
* $c_s^2k^2$ is the stabilizing pressure response, strongest at short wavelength.