BECK, I.; TROSBORG, P. J. A note on free direct summands. MATHEMATICA SCANDINAVICA, [S. l.], v. 42, p. 34–38, 1978. DOI: 10.7146/math.scand.a-11733. Disponível em: https://www.mscand.dk/article/view/11733. Acesso em: 5 may. 2024.