1. Arnowitt, R., Deser, S., Misner, C.W.: The dynamics of general relativity, In: Witten, L. (ed.) Gravitation: An Introduction to Current Research, pp. 227–265. Wiley, New York (1962) (online version: gr-qc/0405109)
2. Bäckdahl, T., Valiente-Kroon, J.A.: Approximate twistors and positive mass. Class. Quantum Gravity 28, 075010 (2011) (online version: arXiv:1011.3712 [gr-qc])
3. Bondi, H., van der Burg, M.G.J., Metzner, A.W.K.: Gravitational waves in general relativity. VII. Waves from axi-symmetric isolated systems, Proc. R. Soc. Lond. Ser. A 269, 21–52 (1962)
4. Bramson, B.D.: The alignment of frames of reference at null infinity for asymptotically flat Einstein–Maxwell manifolds. Proc. R. Soc. Lond. Ser. A 341, 451–461 (1975)
5. Exton, A.R., Newman, E.T., Penrose, R.: Conserved quantities in the Einstein–Maxwell theory. J. Math. Phys. 10, 1566–1570 (1969)