KÖHLER, G. On Hsiao’s conjecture on Hecke groups. MATHEMATICA SCANDINAVICA, [S. l.], v. 48, p. 56–58, 1981. DOI: 10.7146/math.scand.a-11898. Disponível em: https://www.mscand.dk/article/view/11898. Acesso em: 14 apr. 2025.