KHADSHIIVANOV, N. On a theorem of Hopf. Ann. Sofia Univ. Fac. Math. and Inf., [S. l.], v. 85, p. 65–69, 1993. Disponível em: https://admin.uni-sofia.bg/index.php/fmi/article/view/455. Acesso em: 16 jan. 2025.