Trigonal modular curves \(X_0^{+d}(N)\)
The curve \(X_0^{+d}(N)\) is the quotient of the modular curve \(X_0(N)\) by the (not necessarily full) Atkin-Lehner involution \(W_d\) for \(d \parallel N\). The authors determine which of these curves is trigonal, i.e., admits a morphism of degree 3 to the projective line. The arguments are similar to those applied for \(d=1\) by the authors in [Acta Arith. 88, No. 2, 129-140 (1999; Zbl 0947.11018)].
scientific article; zbMATH DE number 1443393
scientific article