Tero Harju and Dirk Nowotka
Periodicity and Unbordered Words: A Proof of the Extended Duval Conjecture

Journal of the ACM, 54(4):20, 2007.


The relationship between the length of a word and the maximum length of its unbordered factors is investigated in this paper.

Consider a finite word w of length n. Let μ(w) denote the maximum length of its unbordered factors, and let (w) denote the period of w. Clearly, μ(w) ≤ (w).

We establish that μ(w) = (w), if w has an unbordered prefix of length μ(w) and n ≥ 2μ(w)-1. This bound is tight and solves a 21 year old conjecture by Duval. It follows from this result that, in general, n ≥ 3μ(w) implies μ(w) = (w) which gives an improved bound for the question asked by Ehrenfeucht and Silberger in 1979.

Keywords: combinatorics on words, Duval's conjecture, periodicity, unbordered words

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