The complexity of finite smooth words over binary alphabets

Fuente: arXiv
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Main Authors: Cassaigne, Julien, Henry, Raphaël
Format: Preprint
Published: 2026
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author Cassaigne, Julien
Henry, Raphaël
author_facet Cassaigne, Julien
Henry, Raphaël
contents Smooth words over an alphabet of non-negative integers $\{a,b\}$ are infinite words that are infinitely derivable, the most famous example being the Oldenburger-Kolakoski word over $\{1,2\}$. The main way to study their language is to consider a finite version of smooth words that we call f-smooth words. In this paper we prove that the f-smooth words are exactly the factors of smooth words, and we make progress towards the conjecture of Sing that the complexity of f-smooth words over $\{a,b\}$ grows like $Θ\left(n^{\log(a+b)/\log((a+b)/2)}\right)$: we prove it over even alphabets, we prove the lower bound over any binary alphabet and we improve the known upper bound over odd alphabets.
format Preprint
id arxiv_https___arxiv_org_abs_2603_10733
institution arXiv
publishDate 2026
record_format arxiv
spellingShingle The complexity of finite smooth words over binary alphabets
Cassaigne, Julien
Henry, Raphaël
Formal Languages and Automata Theory
Combinatorics
Dynamical Systems
Smooth words over an alphabet of non-negative integers $\{a,b\}$ are infinite words that are infinitely derivable, the most famous example being the Oldenburger-Kolakoski word over $\{1,2\}$. The main way to study their language is to consider a finite version of smooth words that we call f-smooth words. In this paper we prove that the f-smooth words are exactly the factors of smooth words, and we make progress towards the conjecture of Sing that the complexity of f-smooth words over $\{a,b\}$ grows like $Θ\left(n^{\log(a+b)/\log((a+b)/2)}\right)$: we prove it over even alphabets, we prove the lower bound over any binary alphabet and we improve the known upper bound over odd alphabets.
title The complexity of finite smooth words over binary alphabets
topic Formal Languages and Automata Theory
Combinatorics
Dynamical Systems
url https://arxiv.org/abs/2603.10733