GARCIA, Juan J.; DEL RIO, Angel. On Flatness and Projectivity of a Ring as a Module over a Fixed Subring. Mathematica Scandinavica, Berkeley, CA, United States, v. 76, p. 179–193, 1995. DOI: 10.7146/math.scand.a-12533. Disponível em: https://journals.msp.org/mscand/article/view/1091. Acesso em: 25 jul. 2026.