Transition semi-wave solutions of reaction diffusion equations with free boundaries

Abstract

In this paper, we define the transition semi-wave solution of the following reaction diffusion equation with free boundaries \begin{equation}\label{0.1} \left\{ \begin{aligned} u_{t}=u_{xx}+f(t,x,u),\ \ &t\in\Real, x<h(t), u(t,h(t))=0,\ \ &t\in\Real, h^{\prime}(t)=-μu_{x}(t,h(t)),\ \ &t\in\Real, \end{aligned} \right. \end{equation} In the homogeneous case, i.e., $f(t,x,u)=f(u)$, under the hypothesis $$f(u)\in C^1([0,1]),f(0)=f(1)=0,f^{\prime }(1)<0,f(u)<0\text{for}u>1,$$ we prove that the semi-wave connecting $1$ and $0$ is unique provided it exists. Furthermore, we prove that any bounded transition semi-wave connecting $1$ and 0 is exactly the semi-wave. In the cases where $f$ is KPP-Fisher type and almost periodic in time (space), i.e., $f(t,x,u)=u(c(t)-u)$ (resp. $u(a(x)-u)$) with $c(t)$ (resp. $a(x)$) being almost periodic, applying totally different method, we also prove any bounded transition semi-wave connecting the unique almost periodic positive solution of $u_t=u(c(t)-u)$ (resp. $u_{xx}+u(a(x)-u)=0$) and $0$ is exactly the unique almost periodic semi-wave. Finally, we provide an example of the heterogeneous equation to show the existence of the transition semi-wave without any global mean speeds.