On a generalization of the Jacobi operator in the Riemannian geometry. Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics, [S. l.], v. 86, n. 1, p. 27–34, 1994. Disponível em: https://annual.uni-sofia.bg/index.php/fmi/article/view/432. Acesso em: 12 jan. 2026.