English

Separating measurable recurrence from strong recurrence via rigidity sequences

Dynamical Systems 2024-12-30 v4

Abstract

If GG is an abelian group, we say SGS\subset G is a set of recurrence if for every probability measure preserving GG-system (X,μ,T)(X,\mu,T) and every DXD\subset X having μ(D)>0\mu(D)>0, there is a gSg\in S such that μ(DTgD)>0\mu(D\cap T^{g}D)>0. We say SS is a set of strong recurrence if for every set DD having μ(D)>0\mu(D)>0 there is a c>0c>0 such that μ(DTgD)>c\mu(D\cap T^{g}D)>c for infinitely many gSg\in S. We call SS measure expanding if for all gGg\in G, the translate S+gS+g is a set of recurrence. A rigidity sequence for (X,μ,T)(X,\mu,T) is a sequence of elements snGs_n\in G satisfying limnμ(DTsnD)=0\lim_{n\to\infty} \mu(D\triangle T^{s_n}D)=0 for all measurable DXD\subset X. For all but countably many countable abelian groups GG, we prove that if SS is measure expanding, there is a sequence of elements snSs_n\in S such that {sn:nN}\{s_n:n\in \mathbb N\} is also measure expanding and every translate of (sn)(s_n) is a rigidity sequence for some free weak mixing measure preserving GG-system. The special case where S=GS=G proves a conjecture of Ackelsberg. As a consequence, we prove that for every countably infinite abelian group GG and every measure expanding set SGS\subset G there is a subset SSS'\subset S such that SS' is measure expanding and no translate of SS' is a set of strong recurrence.

Keywords

Cite

@article{arxiv.1808.05609,
  title  = {Separating measurable recurrence from strong recurrence via rigidity sequences},
  author = {John T. Griesmer},
  journal= {arXiv preprint arXiv:1808.05609},
  year   = {2024}
}

Comments

30 pages. v4 incorporates referee comments

R2 v1 2026-06-23T03:36:08.506Z