ERIKSSON, F. Planes of Curvature in Riemannian Spaces. MATHEMATICA SCANDINAVICA, [S. l.], v. 11, p. 5–21, 1962. DOI: 10.7146/math.scand.a-10645. Disponível em: https://www.mscand.dk/article/view/10645. Acesso em: 2 may. 2024.