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arXiv · 1904.00248

On the longest common subsequence of Thue-Morse words

Joakim Blikstad
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Abstract

The length $a(n)$ of the longest common subsequence of the $n$'th Thue-Morse word and its bitwise complement is studied. An open problem suggested by Jean Berstel in 2006 is to find a formula for $a(n)$. In this paper we prove new lower bounds on $a(n)$ by explicitly constructing a common subsequence between the Thue-Morse words and their bitwise complement. We obtain the lower bound $a(n) = 2^{n}(1-o(1))$, saying that when $n$ grows large, the fraction of omitted symbols in the longest common subsequence of the $n$'th Thue-Morse word and its bitwise complement goes to $0$. We further generalize to any prefix of the Thue-Morse sequence, where we prove similar lower bounds.

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Joakim Blikstad. 2019-03-30. On the longest common subsequence of Thue-Morse words. https://arxiv.org/abs/1904.00248

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