Bramson correction and convergence to the critical wave for Fisher-KPP equations on $\mathbb Z^d$

Abstract

We consider Fisher-KPP equations with nearest-neighbor diffusion on $\mathbb Z^d$, $d\geq2$, with nonzero finitely supported initial data. We prove a logarithmic delay of the front along each signed coordinate axis and show that the transition region has uniformly bounded width. On every fixed-width half-tube around an axis, the solution converges to translates of the minimal-speed lattice traveling wave, with a bounded phase. We also obtain an upper bound with a logarithmic correction in every direction. The proof uses weighted estimates, bounds for tilted random walks, and a product lower solution. Concavity of the reaction is not assumed.