holds, then the integrand becomes approximately zero before the interaction-picture density operator changes significantly. In this case, the so-called Markov approximation ρ ρ I ( t ′ ) ≈ ≈ ρ ρ I ( t ) holds. If we also move t 0 → → − − ∞ ∞ and change the integration variable t ′ → → τ τ = t − − t ′ , we end up with the Redfield master equation