Strong Borel--Cantelli Lemmas for Recurrence

Tomas Persson, Alejandro Rodriguez Sponheimer

Published 2025-02-06Version 1

Let $(X,T,\mu,d)$ be a metric measure-preserving system for which $3$-fold correlations decay exponentially for Lipschitz continuous observables. Suppose that $(M_k)$ is a sequence satisfying some weak decay conditions and suppose there exist open balls $B_k(x)$ around $x$ such that $\mu(B_k(x)) = M_k$. Under a short return time assumption, we prove a strong Borel--Cantelli lemma, including an error term, for recurrence, i.e., for $\mu$-a.e. $x \in X$, \[ \sum_{k=1}^{n} \mathbf{1}_{B_k(x)} (T^k x) = \Phi(n) + O \bigl( \Phi(n)^{1/2} (\log \Phi(n))^{3/2 + \varepsilon} \bigr), \] where $\Phi(n) = \sum_{k=1}^{n} \mu(B_k(x))$. Applications to systems include some non-linear piecewise expanding interval maps and hyperbolic automorphisms of $\mathbf{T}^2$.