If one has a simple variable coefficient process like y'(t) = r(t)*y(t), is there a way to control it to a set point by feedback hitting the variable coefficients in r(t)? I am interested in feedback control of population processes. y'(t) = r*y(t) is simple proportional growth with a constant growth term. I assume one would control this process by "mortality" -- reducing y(t) - u(t) when necessary to keep it under the setpoint. (A delay ODE is the next step...) I have not seen any reference to adjusting variable coefficients in the controls books I have looked at. I wonder if it is possible to somehow make r(t) a "real" ODE variable so that the conventional models work? I don't really know what I am talking about here (a lower division ODE class, plus lots of scattered reading is my only background), so feel free to answer appropriately. Thanks!