PIENE, Ragni. Courbes sur un trait et morphismes de contraction. Mathematica Scandinavica, Berkeley, CA, United States, v. 35, p. 5–15, 1974. DOI: 10.7146/math.scand.a-11528. Disponível em: https://journals.msp.org/mscand/article/view/2118. Acesso em: 25 jul. 2026.