Spreading and vanishing in nonlinear diffusion problems with free boundaries

We study nonlinear diffusion problems of the form $u_t=u_{xx}+f(u)$ with free boundaries. Such problems may be used to describe the spreading of a biological or chemical species, with the free boundary representing the expanding front. For special $f(u)$ of the Fisher-KPP type, the problem was investigated by Du and Lin [8]. Here we consider much more general nonlinear terms. For any $f(u)$ which is $C^1$ and satisfies $f(0)=0$, we show that the omega limit set $ω(u)$ of every bounded positive solution is determined by a stationary solution. For monostable, bistable and combustion types of nonlinearities, we obtain a rather complete description of the long-time dynamical behavior of the problem; moreover, by introducing a parameter $σ$ in the initial data, we reveal a threshold value $σ^*$ such that spreading ($\lim_{t\to\infty}u= 1$) happens when $σ>σ^*$, vanishing ($\lim_{t\to\infty}u=0$) happens when $σ<σ^*$, and at the threshold value $σ^*$, $ω(u)$ is different for the three different types of nonlinearities. When spreading happens, we make use of "semi-waves" to determine the asymptotic spreading speed of the front.

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