This equation is non-local in time: To get the derivative of the reduced density operator at time t, we need its values at all past times. As such, it cannot be easily solved. To construct an approximate solution, note that there are two time scales: a typical relaxation time τ τ r that gives the time scale on which the environment affects the system time evolution, and the coherence time of the environment, τ τ c that gives the typical time scale on which the correlation functions decay. If the relation