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. 2020 Mar 25;20(7):1831. doi: 10.3390/s20071831

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© 2020 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Zone 1, zone 2, zone 3, $r_{1}$ , $r_{2}$ , and $r_{s}$ . We take $r_{2} = \sqrt{3} r_{1}$ .