= Roll instability from broken up-down symmetry
{title2=$\lambda_\perp=\pm|\gamma|K|A|$}
In the <hexagonal convection amplitude equations>, a pure <convection roll> has $|A|^2=\mu_2/(3K)$ and $B=C=0$. The transverse diagonal linear growth cancels because self and cross cubic coefficients coincide. Broken up-down symmetry leaves the off-diagonal coupling $\gamma K A$, giving eigenvalues $\pm|\gamma|K|A|$ and an unstable orientation perturbation whenever $\gamma\ne0$. With zero quadratic coupling these directions are neutral at the retained order, so that degenerate case cannot be declared strictly unstable by this calculation.
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