Klepikov, P.N., and E.D. Rodionov. “Eigenvalues of Ricci Operator of Four-Dimensional Locally Homogeneous Riemannian Manifolds With Nontrivial Isotropy Subgroup”. Izvestiya of Altai State University0, no. 1(129) (March 28, 2023): 100-105. Accessed May 19, 2024. http://izvestiya.asu.ru/article/view/%282023%291-16.