On anti-powers in aperiodic recurrent words

Abstract

Fici, Restivo, Silva, and Zamboni define a 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≥1, W contains a k-anti-power with blocks of length at most Ck beginning at its ith 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.