1. Senovilla, J.M.M.: Class. Quantum Gravity 17, 2799 (2000) (and references therein)
2. Szabados, L.B.: Living Rev. Relativ. 7, 4 (2004) (and references therein)
3. Einstein, A.: Sitz.ber. Preuss. Akad. Wiss. Berl. Philos.-Hist. Kl. 47, 778 (1915)
4. Einstein, A.: Sitz.ber. Preuss. Akad. Wiss. Berl. Philos.-Hist. Kl. 47, 799 (1915)
5. Trautman, A.: In: Witten, L. (ed.) Gravitation: an Introduction to Current Research, p. 169. Wiley, New York (1962)