Maximal and minimal spreading speeds for reaction diffusion equations in nonperiodic slowly varying media

This paper investigates the asymptotic behavior of the solutions of the Fisher-KPP equation in a heterogeneous medium, $\partial_t u = \partial_{xx} u + f(x,u),$ associated with a compactly supported initial datum. A typical nonlinearity we consider is $f(x,u) = μ_0 (ϕ(x)) u(1-u)$, where $μ_0$ is a 1-periodic function and $ϕ$ is a $\mathcal{C}^1$ increasing function that satisfies $\lim_{x\to +\infty} ϕ(x) = +\infty$ and $\lim_{x\to +\infty} ϕ' (x) = 0$. Although quite specific, the choice of such a reaction term is motivated by its highly heterogeneous nature. We exhibit two different behaviors for $u$ for large times, depending on the speed of the convergence of $ϕ$ at infinity. If $ϕ$ grows sufficiently slowly, then we prove that the spreading speed of $u$ oscillates between two distinct values. If $ϕ$ grows rapidly, then we compute explicitly a unique and well determined speed of propagation $w_\infty$, arising from the limiting problem of an infinite period. We give a heuristic interpretation for these two behaviors.

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