PETROV, P. I. Characterization of types of conformally plane four-dimensional Riemannian Spaces . Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 18, n. 2, p. 42–49, 1966. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/8715. Acesso em: 14 apr. 2025.