Petrov, P.I. (1966) “Characterization of types of conformally plane four-dimensional Riemannian Spaces ”, Ukrains’kyi Matematychnyi Zhurnal, 18(2), pp. 42–49. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/8715 (Accessed: 14 April 2025).