 Maxwell balance for a scalar reaction-diffusion front

A stationary front solution of $u_t=u_{xx}+f(u)$ connecting equilibria $u_{-}$ and $u_{+}$ requires ${\int }_{u_{-}}^{u_{+}}f(s)ds=0$. Multiply the stationary equation by $u_x$ and integrate: the derivative term has zero endpoint contribution. This equal-potential condition is the Maxwell balance. For $f(u)=\mu u+\alpha u^3-u^5$, a front from zero to a nonzero constant requires $u_{+}^2=3\alpha /4$ and $\mu =-3{\alpha }^2/16$.