No extremal square-free words over large alphabets

L Hong, S Zhang - arXiv preprint arXiv:2107.13123, 2021 - arxiv.org
arXiv preprint arXiv:2107.13123, 2021arxiv.org
A word is square-free if it does not contain any square (a word of the form $ XX $), and is
extremal square-free if it cannot be extended to a new square-free word by inserting a single
letter at any position. Grytczuk, Kordulewski, and Niewiadomski proved that there exist
infinitely many ternary extremal square-free words. We establish that there are no extremal
square-free words over any alphabet of size at least 17.
A word is square-free if it does not contain any square (a word of the form ), and is extremal square-free if it cannot be extended to a new square-free word by inserting a single letter at any position. Grytczuk, Kordulewski, and Niewiadomski proved that there exist infinitely many ternary extremal square-free words. We establish that there are no extremal square-free words over any alphabet of size at least 17.
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