= Gas-pressure branch of a mixed-pressure alpha disk
{title2=$\bar\nu\propto r\Sigma^{2/3}$}
For small positive radiation-to-gas ratio, constant <opacity>, fixed alpha and molecular constants, eliminate $\lambda$ between $\lambda\propto H\Omega\Sigma$ and $\lambda\Sigma\propto H^7\Omega^6$. This gives $H^6\propto\Sigma^2\Omega^{-5}$ and $\bar\nu\propto\Sigma^{2/3}\Omega^{-2/3}$. <Keplerian rotation> supplies the displayed radial power. Since $\bar\nu\Sigma\propto\Sigma^{5/3}$, its derivative with respect to <surface density> is positive and local viscous perturbations diffuse rather than amplify.
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