I've got a function

PV(t) = PV_{0}* (1 + i_{e})^{(D - t}) * (1 - i_{i})^{(D - t)}

which feeds into a step function, and which I have to integrate

F(t) =PV_{0}I^{D}_{0}* (1 + i_{e})^{(D - [t]}) * (1 - i_{i})^{(D - [t])}dt

(...Integral; [t]...ceiling t)_{0}I^{D}