Eckhaus boundary for a subcritical quintic amplitude equation ($Q^2\le R^4-\alpha R^2/2$)

For the upper plane-wave branch of the quintic real Ginzburg-Landau equation, put $\Lambda =2R^2(2R^2-\alpha )>0$. A modulation of wavenumber $p$ has growth rates $-p^2-\Lambda /2\pm \sqrt{{\Lambda }^2/4+4Q^2{p}^2}$. Long-wave phase diffusion is positive when $4Q^2<\Lambda$, giving the displayed stability boundary. At equality the leading phase growth is $-p^4/\Lambda$, so the boundary is marginal in the diffusion coefficient but damped at the next spatial order. In the $(\mu ,Q)$ plane the boundary is $\mu =2Q^2-\alpha [\alpha +\sqrt{{\alpha }^2+16Q^2}]/8$.