= Constant-mass disk with viscosity proportional to radius
With $x=2\sqrt{r/(3A)}$ and $g=Ar^{3/2}\Sigma$, the half-line <heat equation> solution $g=Ct^{-1/2}e^{-x^2/(4t)}$ has $g_x(0)=0$ but $g(0)>0$. It gives $\Sigma\propto r^{-3/2}t^{-1/2}e^{-r/(3At)}$, constant disk mass and <angular momentum> proportional to $t^{1/2}$. Its radial <velocity> is $r/t$, and a central <torque> proportional to $t^{-1/2}$ supplies the increasing <angular momentum>. It is an outward-spreading, torque-driven disk, not a zero-torque accreting solution.
// Target: astrophysical-fluid-dynamics.bigb
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