An upper bound of the number of distinct powers in binary words

Abstract

A power is a word of the form $\underset{k\text{times}}{\underbrace{uu...u}}$, where $u$ is a word and $k$ is a positive integer and a square is a word of the form $uu$. Fraenkel and Simpson conjectured in 1998 that the number of distinct squares in a word is bounded by the length of the word. This conjecture was proven recently by Brlek and Li. Besides, there exists a stronger upper bound for binary words conjectured by Jonoska, Manea and Seki stating that for a word of length $n$ over the alphabet $\left\{a,b\right\}$, if we let $k$ be the least of the number of a's and the number of b's and $k\ge 2$, then the number of distinct squares is upper bounded by $\frac{2k-1}{2k+2}n$. In this article, we prove this conjecture by giving a stronger statement on the number of distinct powers in a binary word.