# Sturm 3-ball global attractors 2: Design of Thom-Smale complexes

## TL;DR

This paper characterizes the geometric and combinatorial structure of 3-ball Sturm attractors, establishing a cycle of implications between meander properties, Thom-Smale complexes, and Sturm attractors, and enabling their construction from prescribed complexes.

## Contribution

It proves the equivalence between meander properties and Sturm attractors for 3-balls, enabling the construction of attractors from Thom-Smale complexes with specific geometric features.

## Key findings

- Thom-Smale complex is a regular cell complex with bipolar orientation.
- Meander properties characterize Sturm attractors in 3-balls.
- Construction of Sturm 3-ball attractors from prescribed Thom-Smale complexes.

## Abstract

This is the second of three papers on the geometric and combinatorial characterization of global Sturm attractors which consist of a single closed 3-ball. The underlying scalar PDE is parabolic, $$ u_t = u_{xx} + f(x,u,u_x)\,, $$ on the unit interval $0 < x<1$ with Neumann boundary conditions. Equilibria are assumed to be hyperbolic.\\ \newline Geometrically, we study the resulting Thom-Smale dynamic complex with cells defined by the fast unstable manifolds of the equilibria. The Thom-Smale complex turns out to be a regular cell complex. Our geometric description involves a bipolar orientation of the 1-skeleton, a hemisphere decomposition of the boundary 2-sphere by two polar meridians, and a meridian overlap of certain 2-cell faces in opposite hemispheres.\\ \newline The combinatorial description is in terms of the Sturm permutation, alias the meander properties of the shooting curve for the equilibrium ODE boundary value problem.\\ \newline In the first paper we showed the implications $$ \text{Sturm attractor}\quad \Longrightarrow \quad \text{Thom-Smale complex} \quad \Longrightarrow \quad \text{meander}\,.$$ The present part 2, closes the cycle of equivalences by the implication $$ \text{meander} \quad \Longrightarrow \quad \text{Sturm attractor}\,.$$ In particular this cycle allows us to construct a unique Sturm 3-ball attractor for any prescribed Thom-Smale complex which satisfies the geometric properties of the bipolar orientation and the hemisphere decomposition. Many explicit examples and illustrations will be discussed in part 3. The present 3-ball trilogy, however, is just another step towards the still elusive geometric and combinational characterization of all Sturm global attractors in arbitrary dimensions.