= Vertical slowness sextic of an anisotropic solid
{title2=$\det[Tq^2+(R+R^{\mathsf T})q+Q-\rho I]=0$}
Fix horizontal <slowness> $p_\alpha$ and define $T_{ip}=c_{i3p3}$, $R_{ip}=c_{i3p\alpha}p_\alpha$ and $Q_{ip}=c_{i\alpha p\beta}p_\alpha p_\beta$. Under <strong ellipticity in elasticity>, $T$ is positive definite, so the vertical-slowness determinant has degree six with real coefficients. Real roots are intersections of a vertical line with the <elastic slowness surface>; complex-conjugate pairs give <evanescent waves>.
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