Boundary orders and geometry of the signed Thom-Smale complex for Sturm global attractors

Abstract

We embark on a detailed analysis of the close relations between combinatorial and geometric aspects of the scalar parabolic PDE \begin{equation}\label{eq:*} u_t = u_{xx} + f(x,u,u_x) \tag{$*$} \end{equation} on the unit interval $0 < x<1$ with Neumann boundary conditions. We assume $f$ to be dissipative with $N$ hyperbolic equilibria $v\in\mathcal{E}$. The global attractor $\mathcal{A}$ of \eqref{eq:*}, also called \emph{Sturm global attractor}, consists of the unstable manifolds of all equilibria $v$. As cells, these form the \emph{Thom-Smale complex} $\mathcal{C}$. Based on the fast unstable manifolds of $v$, we introduce a refinement $\mathcal{C}^s$ of the regular cell complex $\mathcal{C}$, which we call the \emph{signed Thom-Smale complex}. Given the signed cell complex $\mathcal{C}^s$ and its underlying partial order, only, we derive the two total boundary orders $h_\iota:\{1,\ldots , N\}\rightarrow\mathcal{E}$ of the equilibrium values $v(x)$ at the two Neumann boundaries $\iota=x=0,1$. In previous work we have already established how the resulting Sturm permutation \[\sigma:=h_{0}^{-1} \circ h_1,\] conversely, determines the global attractor $\mathcal{A}$ uniquely, up to topological conjugacy.