1. C.J. Isham, Modern differential geometry for physicists (1999)
2. M. Nakahara, Geometry, Topology and Physics, ed. by D.F. Brewer, 2nd edn. (Taylor & Francis Ltd, 2003), p. 596. ISBN: 0750306068. http://www.ebook.de/de/product/3646467/mikio_nakahara_geometry_topology_and_physics.html
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5. R. Carballo-Rubio et al., Causal hierarchy in modified gravity. JHEP 12, 055 (2020). https://doi.org/10.1007/JHEP12(2020)055. (arXiv: 2005.08533 [gr-qc])