An explicit condition for boundedly supermultiplicative subshifts

TL;DR

This paper establishes a sufficient condition for the growth rate of certain forbidden-factor word languages to be boundedly supermultiplicative, enabling computation of growth constants and advancing understanding of power-free and square-free circular words.

Contribution

It introduces a new condition ensuring boundedly supermultiplicative growth in forbidden-factor languages, with applications to power-free and circular words, and progress on a conjecture about square-free circular words.

Findings

Provided a computable bound for growth rate constants.

Applied the condition to power-free words for improved bounds.

Made progress on Shur's conjecture regarding square-free circular words.

Abstract

We study some properties of the growth rate of $\mathcal{L}(\mathcal{A},\mathcal{F})$, that is, the language of words over the alphabet $\mathcal{A}$ avoiding the set of forbidden factors $\mathcal{F}$. We first provide a sufficient condition on $\mathcal{F}$ and $\mathcal{A}$ for the growth of $\mathcal{L}(\mathcal{A},\mathcal{F})$ to be boundedly supermultiplicative. That is, there exist constants $C>0$ and $\alpha\ge0$, such that for all $n$, the number of words of length $n$ in $\mathcal{L}(\mathcal{A},\mathcal{F})$ is between $\alpha^n$ and $C\alpha^n$. In some settings, our condition provides a way to compute $C$, which implies that $\alpha$, the growth rate of the language, is also computable whenever our condition holds. We also apply our technique to the specific setting of power-free words where the argument can be slightly refined to provide better bounds. Finally, we apply a similar idea to $\mathcal{F}$-free circular words and in particular we make progress toward a conjecture of Shur about the number of square-free circular words.