= Absolute hydrodynamic instability
An <absolute hydrodynamic instability> makes the response to a localized impulse grow at a fixed spatial point in a specified frame. This differs from <convective hydrodynamic instability>, which amplifies a moving <wave packet> while its response decays at fixed positions. For analytic <dispersion relations>, the relevant <absolute frequency> is associated with an accessible zero-<group velocity> saddle, with the spatial-branch selection checked rather than assumed for an arbitrary dispersion relation.
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