STANILOV, G.; VIDEV, V. On a generalization of the Jacobi operator in the Riemannian geometry. Ann. Sofia Univ. Fac. Math. and Inf., [S. l.], v. 86, n. 1, p. 27–34, 1994. Disponível em: https://admin.uni-sofia.bg/index.php/fmi/article/view/432. Acesso em: 16 jan. 2025.