LAFFEY, Thomas 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://tidsskrift.dk/math/article/view/11653. Acesso em: 14 aug. 2026.