Constrained order-parameter boundary stability
= Constrained order-parameter boundary stability
A <stationary point> on the domain $\phi\geq0$ must be tested using allowed one-sided variations. For $F=A\phi^2/2-B\phi^3+\lambda\phi^4/4$ with $B>0$, the boundary is a <local minimum> when $A>0$ and a <local maximum> when $A<0$. At $A=0$, the leading negative cubic term makes it a one-sided <local maximum> even though $F''(0)=0$; on an unrestricted real line it is instead a stationary inflection point.