Stanilov, G. and Petrova, I. (1993) “A generalized Jacobi operator in the 4-dimensional Riemannian geometry”, Ann. Sofia Univ. Fac. Math. and Inf, 85, pp. 55–64. Available at: https://ftl5.uni-sofia.bg/index.php/fmi/article/view/454 (Accessed: 5 December 2024).