Alternately, the condition for oscillation can be expressed using the reflection coefficient. The voltage waveform at the reference plane can be divided into a component V 1 travelling toward the negative resistance device and a component V 2 travelling in the opposite direction, toward the resonator part. The reflection coefficient of the active device Γ Γ N = V 2 / V 1 is greater than one, while that of the resonator part Γ Γ L = V 1 / V 2 is less than one. During operation the waves are reflected back and forth in a round trip so the circuit will oscillate only if | Γ Γ N Γ Γ L | ≥ ≥ 1 As above, the equality gives the condition for steady oscillation, while the inequality is required during startup to provide excess negative resistance. The above conditions are analogous to the Barkhausen criterion for feedback oscillators; they are necessary but not sufficient, so there are some circuits that satisfy the equations but do not oscillate. Kurokawa also derived more complicated sufficient conditions, which are often used instead.