Let T:M→M\documentclass[12pt]{minimal}
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\begin{document}$${T : M \to M}$$\end{document} be a nonuniformly expanding dynamical system, such as logistic or intermittent map. Let v:M→Rd\documentclass[12pt]{minimal}
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\begin{document}$${v : M \to \mathbb{R}^d}$$\end{document} be an observable and vn=∑k=0n-1v∘Tk\documentclass[12pt]{minimal}
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\begin{document}$${v_n = \sum_{k=0}^{n-1} v \circ T^k}$$\end{document} denote the Birkhoff sums. Given a probability measure μ\documentclass[12pt]{minimal}
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\begin{document}$${\mu}$$\end{document} on M, we consider vn as a discrete time random process on the probability space (M,μ)\documentclass[12pt]{minimal}
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\begin{document}$${(M, \mu)}$$\end{document}. In smooth ergodic theory there are various natural choices of μ\documentclass[12pt]{minimal}
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\begin{document}$${\mu}$$\end{document}, such as the Lebesgue measure, or the absolutely continuous T-invariant measure. They give rise to different random processes. We investigate relation between such processes. We show that in a large class of measures, it is possible to couple (redefine on a new probability space) every two processes so that they are almost surely close to each other, with explicit estimates of “closeness”. The purpose of this work is to close a gap in the proof of the almost sure invariance principle for nonuniformly hyperbolic transformations by Melbourne and Nicol.
