= Viscous instability of an accretion disk
The <negative-diffusion criterion for viscous disk instability> is $\partial_\Sigma(\bar\nu\Sigma)<0$. A <surface density> enhancement then transports angular momentum less effectively and grows. Freezing the <Keplerian viscous diffusion equation> locally gives growth proportional to the square of radial <wavenumber>, within the thin-disc transport range; wavelengths must still be sufficiently long compared with the <disk scale height> for that closure. The formal short-wave limit is a <backward heat equation>.
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