IVANOV, V. ON LOWER BOUNDS OF THE SECOND-ORDER DINI DIRECTIONAL DERIVATIVES. Ann. Sofia Univ. Fac. Math. and Inf., [S. l.], v. 96, p. 163–171, 2004. Disponível em: https://admin.uni-sofia.bg/index.php/fmi/article/view/170. Acesso em: 15 jan. 2025.