IVANOVA-KARATOPRAKLIEVA, I. Infinitesimal bendings of rotational surfaces with changing signs curvature. Ann. Sofia Univ. Fac. Math. and Inf., [S. l.], v. 87, p. 179–188, 1995. Disponível em: https://admin.uni-sofia.bg/index.php/fmi/article/view/412. Acesso em: 24 jan. 2025.