Petrov, P. I. “Characterization of Types of Conformally Plane Four-Dimensional Riemannian Spaces ”.
Ukrains’kyi Matematychnyi Zhurnal
, vol. 18, no. 2, Mar. 1966, pp. 42-49,
https://umj.imath.kiev.ua/index.php/umj/article/view/8715
.