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6. J. Tafel, Lett. Math. Phys. 10, 33 (1985)
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8. A.Z. Petrov, Sci. Not. 114, 55 (1954)
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16. E.T. Newman, E.Couch, K. Chinnapered, A. Exton, A. Prakas, R. Torrence, J. Math. Phys. 6,
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49
6. J. Tafel, Lett. Math. Phys. 10, 33 (1985)
7. E.T. Newman, R. Penrose, J. Math. Phys. 3, 566 (1962)
8. A.Z. Petrov, Sci. Not. 114, 55 (1954)
9. F.A.E. Pirani, Phys. Rev. 105, 1089 (1957)
10. R. Debever, C. R. Acad. Sci. 249, 1324 (1959)
11. R. Penrose, Ann. Phys. 10, 171 (1960)
12. J.N. Goldberg, R.K. Sachs, Acta Phys. Polon. 22, 13 (1962)
13. I. Robinson, A. Schild, J. Math. Phys. 4, 484 (1963)
14. R.P. Kerr, J. Math. Phys. 11, 237 (1963)
15. G.C. Debney, R.P. Kerr, A. Schild, J. Math. Phys. 10, 1842 (1969)
16. E.T. Newman, E.Couch, K. Chinnapered, A. Exton, A. Prakas, R. Torrence, J. Math. Phys. 6,
918 (1965)
