LAFFEY, T. J. Commutative subrings of periodic rings. MATHEMATICA SCANDINAVICA, [S. l.], v. 39, p. 161–166, 1976. DOI: 10.7146/math.scand.a-11653. Disponível em: https://www.mscand.dk/article/view/11653. Acesso em: 2 may. 2024.