1. S.W. Hawking and G.F.R. Ellis, The large scale structure of spacetime, Cambridge University Press, Cambridge, U.K. (1973) [ISBN-13:978-0521099066] [ISBN-10:0521099064].
2. R.K. Sachs and H.H. Wu, General relativity for mathematicians, Springer graduate texts in mathematics 48, Springer-Verlag, New York, NY, U.S.A. (1977) [ISBN-13:978-1461299059] [ISBN-10:1461299055] [e-ISBN:978-1-4612-9903-5].
3. R.M. Wald, General relativity, University of Chicago Press, Chicago, IL, U.S.A. (1984) [ISBN-10:0-226-87032-4] [ISBN-10:0-226-87033-2] [INSPIRE].
4. Causal structure Wikipedia page, https://en.wikipedia.org/wiki/Causal_structure.
5. Causality conditions Wikipedia page, https://en.wikipedia.org/wiki/Causality_conditions.