FORELLI, Frank. Two-sheeted coverings of the disc. MATHEMATICA SCANDINAVICA, [S. l.], v. 61, p. 17–38, 1987. DOI: 10.7146/math.scand.a-12189. Disponível em: https://tidsskrift.dk/math/article/view/12189. Acesso em: 14 aug. 2026.