Doubled patterns with reversal and square-free doubled patterns

TL;DR

This paper proves that doubled patterns with reversal are 3-avoidable and conjectures that square-free doubled patterns are 2-avoidable, with confirmation for patterns with up to four variables, advancing understanding of pattern avoidance in words.

Contribution

It establishes the 3-avoidability of doubled patterns with reversal and supports the conjecture that square-free doubled patterns are 2-avoidable, with partial proof for small variable counts.

Findings

Doubled patterns with reversal are 3-avoidable.

Square-free doubled patterns are 2-avoidable for patterns with up to 4 variables.

Ternary words avoiding doubled patterns grow at least as fast as square-free words.

Abstract

In combinatorics on words, a word $w$ over an alphabet $\Sigma$ is said to avoid a pattern $p$ over an alphabet $\Delta$ if there is no factor $f$ of $w$ such that $f=h(p)$ where $h:\Delta^*\to\Sigma^*$ is a non-erasing morphism. A pattern $p$ is said to be $k$-avoidable if there exists an infinite word over a $k$-letter alphabet that avoids $p$. A pattern is \emph{doubled} if every variable occurs at least twice. Doubled patterns are known to be $3$-avoidable. Currie, Mol, and Rampersad have considered a generalized notion which allows variable occurrences to be reversed. That is, $h(V^R)$ is the mirror image of $h(V)$ for every $V\in\Delta$. We show that doubled patterns with reversal are $3$-avoidable. We also conjecture that (classical) doubled patterns that do not contain a square are $2$-avoidable. We confirm this conjecture for patterns with at most 4 variables. This implies that for every doubled pattern $p$, the growth rate of ternary words avoiding $p$ is at least the growth rate of ternary square-free words. A previous version of this paper containing only the first result has been presented at WORDS 2021.