1. R. Penrose, Phys. Rev. Lett. 14, 57 (1965). https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.14.57
2. R. Penrose, Rivista del Nuovo Cimento, Numero Speziale I 252, (1969). https://ui.adsabs.harvard.edu/abs/1969NCimR...1..252P/abstract
3. F.J. Tipler, C.J.S. Clarke, G.F.R. Ellis, A. Held (Plenu, New York, 1980)
4. R. Geroch, G. Horowitz (Cambridge University Press, Cambridge, United Kingdom, 1979)
5. S.W. Hawking, W. Israel (eds.) (Cambridge University Press, Cambridge, United Kingdom, 1979). https://www.cambridge.org/us/universitypress/subjects/physics/cosmology-relativity-and-gravitation/general-relativity-einstein-centenary-survey?format=PB&isbn=9780521299282