1. I. Newton, Philosophiae Naturalis Principia Mathematica (1687).
2. I. Newton, The Principia: Mathematical Principles of Natural Philosophy, 3rd ed. (1726). The authoritative translation by I. B. Cohen and A. Whitman, preceeded by A guide to Newton’s Principia (UC Press, San Francisco, CA, 1999).
3. R. Borghi, “Corrigendum: On Newton’s shell theorem,” Eur. J. Phys. 35, 028003 (2014).
4. A. K. T. Assis and R. A. S. Karam, “The free fall of an apple: conceptual subtleties and implications for physics teaching,” Eur. J. Phys. 39, 035003 (2018).
5. A. Einstein, “Zür allgemeinen Relativitätstheorie” [On general theory of relativity], Sitzber. Preuss. Akad. Wiss. (part 2), 778 (1915). See also the addendum on p. 801.