A Unique Extension of Rich Words

Abstract

A word $w$ is called rich if it contains $| w|+1$ palindromic factors, including the empty word. We say that a rich word $w$ can be extended in at least two ways if there are two distinct letters $x,y$ such that $wx,wy$ are rich. Let $R$ denote the set of all rich words. Given $w\in R$, let $K(w)$ denote the set of all words such that if $u\in K(w)$ then $wu\in R$ and $wu$ can be extended in at least two ways. Let $\omega(w)=\min\{| u| \mid u\in K(w)\}$ and let $\phi(n)=\max\{\omega(w)\mid w\in R\mbox{ and }| w|=n\}$, where $n>0$. Vesti (2014) showed that $\phi(n)\leq 2n$. In other words, it says that for each $w\in R$ there is a word $u$ with $| u|\leq 2| w|$ such that $wu\in R$ and $wu$ can be extended in at least two ways. We prove that $\phi(n)\leq n$. In addition we prove that for each real constant $c>0$ and each integer $m>0$ there is $n>m$ such that $\phi(n)\geq (\frac{2}{9}-c)n$. The results hold for each finite alphabet having at least two letters.