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arXiv · 2607.16844

Duality in Biperiodic Fibonacci Words Substitution Frequencies and Combinatorial Invariants

Jasem Hamoud
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Abstract

In this paper, a natural duality on the family of biperiodic Fibonacci words $\mathfrak{F}^{(a,b)}$ generated by the directive sequence $(a,b,a,b,\dots)$. Both of $\mathfrak{F}^{(a,b)}$ and $\mathfrak{F}^{(b,a)}$ are related by the explicit morphism $\sigma_a:0\mapsto 0^a1$, $1\mapsto 0$, establishing a precise substitutional correspondence between them. We compute the exact letter frequencies, give a complete description of the return words for each letter, prove the existence of arbitrarily long palindromic prefixes, and determine the continued fraction expansion of the slope $\theta^{(a,b)}$. These findings reveal that the apparent asymmetry in several invariants arises uniformly from the length-redistribution mechanism induced by the morphism $\sigma_a$.

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Jasem Hamoud. 2026-07-18. Duality in Biperiodic Fibonacci Words Substitution Frequencies and Combinatorial Invariants. https://arxiv.org/abs/2607.16844

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