1. M. Sachs, ?On the meaning of E = mc2?,Int. J. Theoret. Phys. 8, 377 (1973).
2. A. Einstein, ?Does the inertia of a body depend on its energy content??, English translation in: A. Einsteinet al, The Principle of Relativity (Dover, New York, 1923), p. 69.
3. See, for example, M. Born,Einstein's Theory of Relativity (Dover, New York, 1965), p. 273.
4. See, for example, L. D. Landau and E. M. Lifshitz,The Classical Theory of Fields (Pergamon, Oxford, 1962), revised 2nd edition.
5. M. Sachs,Quantum Mechanics from General Relativity (Reidel, Dordrecht, 1986).