This is achieved by making two approximations, 1. The FGR correlation function C i→f (t, s) has a short lifetime (in terms of s) compared to the resulting dynamics, such that the integral limit can be extended to infinity. 2. The t-dependence in C i→f (t, s) is also eliminated. This is equivalent to setting ρb (0) = exp(-β Ĥi ) in Eq. 13, meaning the bath is at equilibrium in the initial system state i at t = 0, which is why the Markovian FGR is referred to equilibrium FGR (E-FGR) in this context.