= Briggs-Bers criterion
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{wiki=Briggs–Bers_criterion}
= Briggs-Bers technique
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{synonym}
A criterion for <absolute wave-packet instability> based on analytic continuation of the spatial roots of a <dispersion relation>. Start the temporal inversion contour above the spectrum, then lower it while deforming the spatial contour. A <spatial pinch point> occurs when branches originating in opposite spatial half-planes obstruct that deformation. A double root $D=D_k=0$ is only an algebraic candidate. The opposite-half-plane condition is stated in the primary study https://doi.org/10.1017/jfm.2016.195, section 4.2.
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