English

A strictly ergodic, positive entropy subshift uniformly uncorrelated to the Moebius function

Dynamical Systems 2019-02-13 v1

Abstract

A recent result of Downarowicz and Serafin (DS) shows that there exist positive entropy subshifts satisfying the assertion of Sarnak's conjecture. More precisely, it is proved that if y=(yn)n1y=(y_n)_{n\ge 1} is a bounded sequence with zero average along every infinite arithmetic progression (the M\"obius function is an example of such a \sq\ yy) then for every N2N\ge 2 there exists a subshift Σ\Sigma over NN symbols, with entropy arbitrarily close to logN\log N, uncorrelated to yy. In the present note, we improve the result of (DS). First of all, we observe that the uncorrelation obtained in (DS) is \emph{uniform}, i.e., for any continuous function f:ΣRf:\Sigma\to {\mathbb R} and every ϵ>0\epsilon>0 there exists n0n_0 such that for any nn0n\ge n_0 and any xΣx\in\Sigma we have 1ni=1nf(Tix)yi<ϵ. \left|\frac1n\sum_{i=1}^{n}f(T^ix)\,y_i\right|<\epsilon. More importantly, by a fine-tuned modification of the construction from (DS) we create a \emph{strictly ergodic} subshift, with all the desired properties of the example in (DS) (uniformly uncorrelated to yy and with entropy arbitrarily close to logN\log N). The question about these two additional properties (uniformity of uncorrelation and strict ergodicity) has been posed by Mariusz Lemanczyk in the context of the so-called strong MOMO (M\"obius Orthogonality on Moving Orbits) property. Our result shows, among other things, that strong MOMO is essentially stronger than uniform uncorrelation, even for strictly ergodic systems.

Keywords

Cite

@article{arxiv.1902.04162,
  title  = {A strictly ergodic, positive entropy subshift uniformly uncorrelated to the Moebius function},
  author = {Tomasz Downarowicz and Jacek Serafin},
  journal= {arXiv preprint arXiv:1902.04162},
  year   = {2019}
}
R2 v1 2026-06-23T07:38:12.652Z