Keplerian disk evolution with a ring source (${\Sigma }_t=\frac{3}{R}{\partial }_R[R^{1/2}{\partial }_R(\nu \Sigma R^{1/2})]+\frac{\dot{M}(t)}{2\pi R_0}\delta (R-R_0)$)

For an axisymmetric thin disc supplied at $R_0$ with the local Keplerian rotation specific angular momentum, the displayed source augments the Keplerian viscous diffusion equation. The radial velocity remains $v_R=-3(\Sigma R^{1/2}{)}^{-1}{\partial }_R(\nu \Sigma R^{1/2})$: subtracting local specific angular momentum times conservation of mass from conservation of angular momentum cancels the source term. The normalization of the Dirac delta function makes the integrated supplied mass per unit time equal to $\dot{M}(t)$.