On Circular Threshold Words and Other Stronger Versions of Dejean's conjecture

TL;DR

This paper explores stronger versions of Dejean's conjecture through the concept of threshold words, proposing new proof methods, constructions, and computational verification techniques for certain cases.

Contribution

It introduces novel methods for proving Dejean's conjecture for some cases, constructs cyclic threshold words, and proposes a new approach to generate stronger threshold words.

Findings

Proposed computer verification methods for odd n≥5 cases.

Constructed cyclic threshold words (TWs) for certain n.

Introduced a TW tree with exponential growth for stronger TWs.

Abstract

Let the root of the word $w$ be the smallest prefix $v$ of $w$ such that $w$ is a prefix of $vvv...$. $per(w)$ is the length of the root of $w$. For any $n\ge5$, an $n$-ary threshold word is a word $w$ such that for any factor (subword) $v$ of $w$ the condition $\frac{|v|}{per(v)}\le\frac{n}{n-1}$ holds. Dejean conjecture (completely proven in 2009) states for $n\ge5$ that exists infinitely many of $n$-ary TWs. This manuscript is based on the author's student works (diplomas of 2011 (bachelor's thesis) and 2013 (master's thesis) years) and presents an edited version (in Russian) of these works with some improvements. In a 2011 work proposed new methods of proving of the Dejean conjecture for some odd cases $n\ge5$, using computer verification in polynomial time (depending on $n$). Moreover, the constructed threshold words (TWs) are ciclic/ring TWs (any cyclic shift is a TW). In the 2013 work, the proof method (of 2011) was improved by reducing the verification conditions. A solution for some even cases $n\ge6$ is also proposed. A 2013 work also proposed a method to construct stronger TWs, using a TW tree with regular exponential growth. Namely, the TWs, where all long factors have an exponent close to 1.