Shift in the velocity of a front due to a cutoff

We consider the effect of a small cutoff $\varepsilon$ on the velocity of a traveling wave in one dimension. Simulations done over more than ten orders of magnitude as well as a simple theoretical argument indicate that the effect of the cutoff $\varepsilon$ is to select a single velocity that converges when $\varepsilon \to 0$ to the one predicted by the marginal stability argument. For small $\varepsilon$, the shift in velocity has the form $K(\ln \varepsilon {)}^{-2}$ and our prediction for the constant $K$ agrees very well with the results of our simulations. A very similar logarithmic shift appears in more complicated situations, in particular in finite-size effects of some microscopic stochastic systems. Our theoretical approach can also be extended to give a simple way of deriving the shift in position due to initial conditions in the Fisher-Kolmogorov or similar equations.