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More on Square-free Words Obtained from Prefixes by Permutations

Ochem P.

Fundamenta Informaticae

2014

Vol. 132, nr 1

109--112

An infinite square-free word w over the alphabet Σ3 = {0, 1, 2} is said to have a k-stem σ if |σ| = k and w = σw1w2· · · where for each i, there exists a permutation πi of Σ3 which extended to a morphism gives wi= πi (σ). Harju proved that there exists an infinite k-stem word for k = 1, 2, 3, 9 and 13 ≤ k ≤ 19, but not for 4 ≤ k ≤ 8 and 10 ≤ k ≤ 12. He asked whether k-stem words exist for each k ≥ 20. We give a positive answer to this question. Currie has found another construction that answers Harju’s question.