NAKAI, M.; SARIO, L. Classification and Deformation of Riemannian Spaces. MATHEMATICA SCANDINAVICA, [S. l.], v. 20, p. 193–208, 1967. DOI: 10.7146/math.scand.a-10830. Disponível em: https://www.mscand.dk/article/view/10830. Acesso em: 6 may. 2024.