= Compact self-similar disk with density-linear viscosity
{title2=$\bar\nu=Ar^2\Sigma,\quad\Sigma=\sigma[(R/r)^a-1]_+$}
The <Keplerian viscous diffusion equation> with the displayed <kinematic viscosity> has two such nontrivial finite-support <self-similar solutions>: $a=1$, $R=Ct^{2/5}$, and $a=5/4$, $R=Ct^{1/2}$. Both have $\sigma=(15At)^{-1}$ for $A,C,t>0$. Matching powers of radius eliminates the third power only when $(5/2-2a)(2-2a)=0$. The first solution conserves disk <angular momentum>; the second conserves disk <mass> but requires an inner torque.
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