Growth-rate solvability for conducting-square Darcy convection ($\lambda =(2q_1/q_c)\Vert \nabla {\theta }_0{\Vert }_2^2/\Vert {\theta }_0{\Vert }_2^2$)

For $q=q_c+ϵq_1$, ${\theta }_t=ϵ\lambda \theta$, the first-order equations are $\Delta {\psi }_1-q_c^2{\theta }_{1x}=2q_c{q}_1{\theta }_{0x}$ and ${\psi }_{1x}+\Delta {\theta }_1=\lambda {\theta }_0$. Multiply them by ${\psi }_0,q_c^2{\theta }_0$ and integrate. Integration by parts and the leading equations cancel the homogeneous correction terms, giving $2q_c{q}_1⟨{\psi }_0{\theta }_{0x}⟩+q_c^2\lambda ⟨{\theta }_0^2⟩=0$. Since $⟨{\psi }_0{\theta }_{0x}⟩=-⟨|\nabla {\theta }_0{|}^2⟩$, the displayed title formula follows. The reciprocal norm ratio is incompatible with this solvability condition. Reflection parity diagonalizes the first-order splitting of the two marginal modes.