Infinite square-free self-shuffling words

Abstract

In this paper we answer two recent questions from Charlier et al. and Harju about self-shuffling words. An infinite word $w$ is called self-shuffling, if $w={\prod }_{i=0}^{\infty }{U}_i{V}_i={\prod }_{i=0}^{\infty }{U}_i={\prod }_{i=0}^{\infty }{V}_i$ for some finite words $U_i$, $V_i$. Harju recently asked whether square-free self-shuffling words exist. We answer this question affirmatively. Besides that, we build an infinite word such that no word in its shift orbit closure is self-shuffling, answering positively a question from Charlier et al.