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. 2019 Jul 9;113(4):357–372. doi: 10.1007/s00422-019-00803-y

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© The Author(s) 2019

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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Fig. 2 — Explicitly writing $X_{i} (t)$ as a function of time $t$ is used to indicate that $X_{i}$ is nonstationary and changes slowly