STANILOV, G.; PETROVA, I. A generalized Jacobi operator in the 4-dimensional Riemannian geometry. Ann. Sofia Univ. Fac. Math. and Inf., [S. l.], v. 85, p. 55–64, 1993. Disponível em: https://stipendii.uni-sofia.bg/index.php/fmi/article/view/454. Acesso em: 5 dec. 2024.