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