BECK, István; TROSBORG, Peter J. A note on free direct summands. Mathematica Scandinavica, Berkeley, CA, United States, v. 42, p. 34–38, 1978. DOI: 10.7146/math.scand.a-11733. Disponível em: https://journals.msp.org/mscand/article/view/1916. Acesso em: 25 jul. 2026.