Solution
= Solution
Put $F=\bar\nu\Sigma$. In <steady state>, $r^{1/2}\partial_r(r^{1/2}F)$ is constant, so
$$
\boxed{F=A+B r^{-1/2}.}
$$
The inward <mass accretion rate> and outward <viscous torque in an accretion disk> are
$$
\dot M=-2\pi r\Sigma v_r=6\pi r^{1/2}\partial_r(r^{1/2}F)=3\pi A,\qquad
\mathcal G=3\pi(GM_*)^{1/2}r^{1/2}F.
$$