Sharp Propagation and the Local-to-Nonlocal Transition for Fractional Fisher-KPP Equations on Integer Lattices

Abstract: Let $P$ denote normalized nearest-neighbor averaging on $\mathbb{Z}<sup>d$ and set $A=I-P$. We consider $\partial_tu+A^{su=f(u),t&gt;0,} x\in\mathbb{Z}^d, 0&lt;s\&lt;1.$ For a Fisher-KPP reaction with $a=f&#39;(0)\&gt;0$, compactly supported data have logarithmic propagation rate $a/(d+2s)$. In the logistic case, each fixed level set lies between two constant multiples of $\exp(at/(d+2s))$; this localization remains valid for data with the critical algebraic tail. Nondecreasing front-like data in one dimension instead propagate at rate $a/(2s)$, and the resulting acceleration rules out planar traveling fronts of finite speed in every dimension. We also analyze the singular limit $s\uparrow1$. If $τ_s=-\log(1-s)$, then throughout $at&lt;τ_s$ compactly supported solutions follow the Wulff shape of the local nearest-neighbor equation. After a constant multiple of $τ_s$, their propagation is exponential. Half-space data exhibit the corresponding transition from the directional local speed to the exponent $a/(2s)$. The analysis rests on sharp bounds for the subordinated kernel, algebraic barriers, a compactness--Liouville argument, and a decomposition of the fractional walk into its nearest-neighbor and rare long-jump parts.