Doubled patterns are $3$-avoidable

TL;DR

This paper proves that doubled patterns with 4 and 5 variables are avoidable over a 3-letter alphabet, extending known results for patterns with fewer or more variables, thus advancing understanding in combinatorics on words.

Contribution

It establishes that all doubled patterns with 4 and 5 variables are 3-avoidable, filling a gap in the classification of pattern avoidability.

Findings

Doubled patterns with 4 variables are 3-avoidable.

Doubled patterns with 5 variables are 3-avoidable.

Extends previous results to include patterns with 4 and 5 variables.

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 said to be doubled if no variable occurs only once. Doubled patterns with at most 3 variables and patterns with at least 6 variables are $3$-avoidable. We show that doubled patterns with 4 and 5 variables are also $3$-avoidable.