Stanilov, Grozio, and Irina Petrova. “A Generalized Jacobi Operator in the 4-Dimensional Riemannian Geometry”. Ann. Sofia Univ. Fac. Math. and Inf. 85 (December 12, 1993): 55–64. Accessed December 5, 2024. https://ftl5.uni-sofia.bg/index.php/fmi/article/view/454.