On the absolute value of the autocorrelations of the Thue-Morse sequence

Abstract

Recently, Baake and Coons proved several results on the average size of the autocorrelations of the Thue--Morse sequence. They also considered the absolute value of the autocorrelations, and showed that the average value of the autocorrelations is zero. In particular, they showed that $\sum_{n\leqslant x}|\eta(n)|=o(x^\alpha)$ for any $\alpha>\log(3)/\log(4)$. In this paper, we sharpen this result, providing upper and lower bounds for $\alpha$. On the way to our lower bounds, we obtain the structure of the linear representation of the point-wise product of two $k$-regular sequences, which may be of independent interest.