The open-access journal Living Reviews in Relativity has published an updated review article on 15 December 2020:
Massimo Tinto and Sanjeev V. Dhurandhar,
Living Rev Relativ 24, 1 (2021)
This article is a revised version of https://doi.org/10.12942/lrr-2014-6
Major revision, updated and expanded (for details see Appendix B).
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