Doubled patterns with reversal and square-free doubled patterns

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 $h:{\Delta }^{\ast }\to {\Sigma }^{\ast }$ 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.