1. G. C. Debney, R. P. Kerr, and A. Schild, J. Math. Phys 10 (1969), 1842.
2. J. F. Plebaáski, “Spinors, Tetrads and Forms”; a monograph of the Centro de Investigación, 297 pages, (1974), unpublished.
3. J. F. Plebatíski, J. Math. Phys. 16 (1975), 2395.
4. J. Goldberg and R. Sachs, Acta Phys. Polon. Suppl. 22, 13 (1962), see also I. Robinson and A. Schild, J. Math. Phys. 4, 484 (1963).
5. I. Robinson and A. Trautman, Phys. Rev. Lett. 431 (1960), and I. Robinson and A. Trautman in Conference Int. sur les Theories Relativistes, July 25–31, 1962, Gauthier-Villars, and PWN, Paris-Warszawa, 1964, p. 107–114.