A generalized Jacobi operator in the 4-dimensional Riemannian geometry. Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics, [S. l.], v. 85, p. 55–64, 1993. Disponível em: https://annual.uni-sofia.bg/index.php/fmi/article/view/454. Acesso em: 12 jan. 2026.