# Asymptotic behavior of semilinear parabolic equations on the circle with time almost-periodic/recurrent dependence

## TL;DR

This paper investigates the long-term behavior of solutions to semilinear parabolic equations on a circle with almost-periodic time dependence, revealing differences from periodic cases and conditions for embedding limit sets into forced circle flows.

## Contribution

It establishes conditions under which omega-limit sets can be embedded into almost-periodically forced circle flows, highlighting differences from time-periodic scenarios and introducing new phenomena.

## Key findings

- Omega-limit sets are either spatially homogeneous or inhomogeneous when center dimension ≤ 1.
- Inhomogeneous omega-limit sets can be embedded into time-recurrent or almost-periodic forced circle flows.
- Embedding properties fail when the center dimension exceeds 1, especially for dimension 2 with odd unstable dimension.

## Abstract

We study topological structure of the $\omega$-limit sets of the skew-product semiflow generated by the following scalar reaction-diffusion equation \begin{equation*} u_{t}=u_{xx}+f(t,u,u_{x}),\,\,t>0,\,x\in S^{1}=\mathbb{R}/2\pi \mathbb{Z}, \end{equation*} where $f(t,u,u_x)$ is $C^2$-admissible with time-recurrent structure including almost-periodicity and almost-automorphy. Contrary to the time-periodic cases (for which any $\omega$-limit set can be imbedded into a periodically forced circle flow), it is shown that one cannot expect that any $\omega$-limit set can be imbedded into an almost-periodically forced circle flow even if $f$ is uniformly almost-periodic in $t$. More precisely, we prove that, for a given $\omega$-limit set $\Omega$, if ${\rm dim}V^c(\Omega)\leq 1$ ($V^c(\Omega)$ is the center space associated with $\Omega$), then $\Omega$ is either spatially-homogeneous or spatially-inhomogeneous; and moreover, any spatially-inhomogeneous $\Omega$ can be imbedded into a time-recurrently forced circle flow (resp. imbedded into an almost periodically-forced circle flow if $f$ is uniformly almost-periodic in $t$). On the other hand, when ${\rm dim}V^c(\Omega>1$, it is pointed out that the above embedding property cannot hold anymore. Furthermore, we also show the new phenomena of the residual imbedding into a time-recurrently forced circle flow (resp. into an almost automorphically-forced circle flow if $f$ is uniformly almost-periodic in $t$) provided that $\dim V^c(\Omega)=2$ and $\dim V^u(\Omega)$ is odd. All these results reveal that for such system there are essential differences between time-periodic cases and non-periodic cases.