Characterization of iterated palindromes in the intersection of two clustering classes

Let A={a,b,c}A=\{a,b,c\} be the ternary alphabet, let P1P_1 and P2P_2 denote the two classes of perfectly clustering words, and let Pal(u)\operatorname{Pal}(u) be the iterated palindrome directed by a word uAu\in A^*. A word ww belongs to P1P2P_1\cap P_2 exactly when there is a word u{a,c}bu\in\{a,c\}^*\cdot b^* such that

Pal(u)=w.\operatorname{Pal}(u)=w.

Characterization conjecture. A word wP1P2w\in P_1\cap P_2 if and only if Pal(u)=w\operatorname{Pal}(u)=w for some u{a,c}bu\in\{a,c\}^*\cdot b^*. The preceding proposition establishes this characterization for iterated palindromes, and the conjecture is presented as supported by computer exploration; no resolution is supplied here.

Sources & referencesView supporting material

Primary source

Mélodie Lapointe and Nathan Plourde-Hébert, “Perfectly Clustering Words and Iterated Palindromes over a Ternary Alphabet”, arXiv:2406.16410 (2024).

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