Validity of the linear marginal stability principle for monotonic fronts of the extended Fisher-Kolmogorov equation

The extended Fisher Kolmogorov equation $u_t = u_{xx} - \gamma u_{xxxx} + f(u)$ with arbitrary positive $f(u)$, satisfying $f(0) = f(1) =0$, has monotonic traveling fronts for $\gamma < 1/12$. We find a simple lower bound on the speed of the fronts which allows to assess the validity of linear marginal stability.