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Write $\psi =\theta +\phi$, $U={\beta }_r+{\gamma }_r$ and $V={\beta }_i+{\gamma }_i$. The polar amplitude equations arewith their reflected counterparts. On the equal-amplitude standing wave branch $R=S>0$, equilibrium point requiresSquaring and adding eliminates the phase:For $U^2+V^2>0$ the candidate squared amplitudes areFor each positive root, set $\sin \psi =-Vz/\nu$ and $\cos \psi =(Uz-\mu )/\nu$. The quadratic identity ensures these define a phase. Conversely these phase equations imply the quadratic, so a nonzero phase-locked branch exists exactly when the discriminant is nonnegative and at least one root is positive.

Since $|\cos \chi |=|V|/\sqrt{U^2+V^2}$, the discriminant condition is the printed $|\mu \cos \chi /\nu |\le 1$ for $\nu \ne 0$. It is sufficient under the usual above-onset supercritical assumptions $\mu >0,U>0$, because then the plus root is positive. Without these assumptions the condition is not sufficient: $\mu =-1,U=1,V=0,\nu =1/2$ satisfies it but gives $z=-1\pm 1/2<0$. This is the positivity condition for a phase-locked standing wave, not a removable algebraic detail. The zero solution always exists and is distinct from the nonzero branch. For $\nu =0$, the undivided equations must be used; a nonzero equilibrium point in this fixed rotating frame requires $V=0$ and $z=\mu /U>0$. If $U=V=0$, nonzero equilibrium points require ${\mu }^2={\nu }^2$ and a compatible phase, with no cubic amplitude selection.

Strong enough forcing can overcome the nonlinear frequency shift, lock the sum phase to the external signal and produce a standing wave at half the forcing frequency. The difference phase remains free through spatial translation. The amplitude inequality is an existence condition, not a stability theorem: attraction requires the linearized forced amplitude equations, and nonlinear coefficients have not been restricted enough here to promise stability of every root.