Absolute wave-packet instability
= Absolute wave-packet instability
A localized perturbation has positive asymptotic exponential growth at a fixed observation point in the chosen reference frame. The relevant <spatial pinch point> of the <Briggs-Bers criterion> has positive imaginary <frequency>. Merely finding a growing temporal mode or a formal complex saddle is not sufficient; the saddle must contribute to the localized impulse response.