POLETAEVA, E. Penrose’s tensors. II. Mathematica Scandinavica, Berkeley, CA, United States, v. 72, p. 191–211, 1993. DOI: 10.7146/math.scand.a-12444. Disponível em: https://journals.msp.org/mscand/article/view/1179. Acesso em: 24 jul. 2026.