# On Anti-Powers in Aperiodic Recurrent Words

## TL;DR

This paper determines that the maximum anti-power length guaranteed in aperiodic recurrent words is five, and explores bounds on anti-power blocks in morphic words, extending previous results on specific words like Thue-Morse.

## Contribution

It proves the maximum anti-power length in aperiodic recurrent words is five and characterizes bounds for binary morphic words, generalizing prior work on Thue-Morse.

## Key findings

- Maximum anti-power length in aperiodic recurrent words is 5.
- Binary morphic words have linear bounds on anti-power blocks.
- Identifies exceptional cases where bounds do not hold.

## Abstract

Fici, Restivo, Silva, and Zamboni define a $\textit{$k$-anti-power}$ to be a concatenation of $k$ consecutive words that are pairwise distinct and have the same length. They ask for the maximum $k$ such that every aperiodic recurrent word must contain a $k$-anti-power, and they prove that this maximum must be 3, 4, or 5. We resolve this question by demonstrating that the maximum is 5. We also conjecture that if $W$ is a reasonably nice aperiodic morphic word, then there is some constant $C = C(W)$ such that for all $i,k\geq 1$, $W$ contains a $k$-anti-power with blocks of length at most $Ck$ beginning at its $i^\text{th}$ position. We settle this conjecture for binary words that are generated by a uniform morphism, characterizing the small exceptional set of words for which such a constant cannot be found. This generalizes recent results of the second author, Gaetz, and Narayanan that have been proven for the Thue-Morse word, which also show that such a linear bound is the best one can hope for in general.