# Identifying Invariant Ergodic Subsets and Barriers to Mixing by Cutting and Shuffling: Study in a Bi-rotated Hemisphere

## TL;DR

This paper investigates the structure of invariant sets and barriers to mixing in cutting-and-shuffling dynamics on a hemispherical shell, revealing how ergodic properties influence mixing potential through computational and analytical methods.

## Contribution

It introduces new methods to analyze the structure of exceptional sets in PWIs, linking their ergodic properties to mixing behavior in a hemispherical domain.

## Key findings

- Invariant ergodic subsets can either prevent or enable mixing.
- Some PWIs have non-ergodic exceptional sets, blocking mixing.
- Connectivity of orbits is essential for mixing in ergodic cases.

## Abstract

Mixing by cutting-and-shuffling can be mathematically described by the dynamics of piecewise isometries (PWIs), higher dimensional analogs of one-dimensional interval exchange transformations. In a two-dimensional domain under a PWI, the exceptional set, $\bar{E}$, which is created by the accumulation of cutting lines (the union of all iterates of cutting lines and all points that pass arbitrarily close to a cutting line), defines where mixing is possible but not guaranteed. There is structure within $\bar{E}$ that directly influences the mixing potential of the PWI. Here we provide new computational and analytical formalisms for examining this structure by way of measuring the density and connectivity of $\varepsilon$-fattened cutting lines that form an approximation of $\bar{E}$. For the example of a PWI on a hemispherical shell studied here, this approach reveals the subtle mixing behaviors and barriers to mixing formed by invariant ergodic subsets (confined orbits) within the fractal structure of the exceptional set. Some PWIs on the shell have provably non-ergodic exceptional sets, which prevent mixing, while others have potentially ergodic exceptional sets where mixing is possible since ergodic exceptional sets have uniform cutting line density. For these latter exceptional sets, we show the connectivity of orbits in the PWI map through direct examination of orbit position and shape and through a two-dimensional return plot to explain the necessity of orbit connectivity for mixing.