OLSZAK, Z. On nondegenerate umbilical affine hypersurfaces in recurrent affine manifolds. Annales Universitatis Paedagogicae Cracoviensis Studia Mathematica, [S. l.], v. 4, p. 171–179, 2004. Disponível em: https://studmath.uken.krakow.pl/article/view/13316. Acesso em: 31 oct. 2026.