Dynamics of time-periodic reaction-diffusion equations with front-like initial data on $\mathbb{R}$

This paper is concerned with the Cauchy problem $$u_t=u_{xx} +f(t,u), \,\,\, x\in\mathbb{R},\,t>0,$$ $$u(0,x)= u_0(x), \,\,\, x\in\mathbb{R},$$ where $f$ is a rather general nonlinearity that is periodic in $t$, and satisfies $f(\cdot,0)\equiv 0$ and that the corresponding ODE has a positive periodic solution $p(t)$. Assuming that $u_0$ is front-like, that is, $u_0(x)$ is close to $p(0)$ for $x\approx -\infty$ and close to $0$ for $x\approx \infty$, we aim to determine the long-time dynamical behavior of the solution $u(t,x)$ by using the notion of propagation terrace introduced by Ducrot, Giletti and Matano (2014). We establish the existence and uniqueness of propagating terrace for a very large class of nonlinearities, and show the convergence of the solution $u(t,x)$ to the terrace as $t\to\infty$ under various conditions on $f$ or $u_0$. We first consider the special case where $u_0$ is a Heaviside type function, and prove the converge result without requiring any non-degeneracy on $f$. Furthermore, if $u_0$ is more general such that it can be trapped between two Heaviside type functions, but not necessarily monotone, we show that the convergence result remains valid under a rather mild non-degeneracy assumption on $f$. Lastly, in the case where $f$ is a non-degenerate multistable nonlinearity, we show the global and exponential convergence for a much larger class of front-like initial data.