Three characterizations of a self-similar aperiodic 2-dimensional subshift

Sébastien Labbé

Published 2020-12-07Version 1

The goal of this chapter is to illustrate a generalization of the Fibonacci word to the case of 2-dimensional configurations on $\mathbb{Z}^2$. More precisely, we consider a particular subshift of $\mathcal{A}^{\mathbb{Z}^2}$ on the alphabet $\mathcal{A}=\{0,\dots,18\}$ for which we give three characterizations: as the subshift $\mathcal{X}_\phi$ generated by a 2-dimensional morphism $\phi$ defined on $\mathcal{A}$; as the Wang shift $\Omega_\mathcal{U}$ defined by a set $\mathcal{U}$ of 19 Wang tiles; as the symbolic dynamical system $\mathcal{X}_{\mathcal{P}_\mathcal{U},R_\mathcal{U}}$ representing the orbits under some $\mathbb{Z}^2$-action $R_\mathcal{U}$ defined by rotations on $\mathbb{T}^2$ and coded by some topological partition $\mathcal{P}_\mathcal{U}$ of $\mathbb{T}^2$ into 19 polygonal atoms. We prove their equality $\mathcal{X}_\phi=\Omega_\mathcal{U} =\mathcal{X}_{\mathcal{P}_\mathcal{U},R_\mathcal{U}}$ by showing they are self-similar with respect to the substitution $\phi$. This chapter provides a transversal reading of results divided into four different articles obtained through the study of the Jeandel-Rao Wang shift. It gathers in one place the methods introduced to desubstitute Wang shifts and to desubstitute codings of $\mathbb{Z}^2$-actions by focussing on a simple 2-dimensional self-similar subshift. Algorithms to find marker tiles and compute the Rauzy induction of $\mathbb{Z}^2$-rotations are provided as well as the SageMath code to reproduce the computations.