= Saddle growth rate along a ray
{title2=$\sigma(V)=\operatorname{Im}(\omega_\star-Vk_\star)$}
On a ray $x/t=V$, a localized <wave packet> is governed by a physically selected <saddle point> satisfying $d\omega/dk=V$ in its analytic <dispersion relation>. The exponent's real growth is $\operatorname{Im}(\omega_\star-Vk_\star)$. The fixed-frame case $V=0$ gives the <absolute growth rate>. This ray formulation makes the frame dependence of <convective hydrodynamic instability> explicit.
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