The automorphism group of the modular curve $X_0^{\ast }(N)$ with square-free level

Francesc Bars, Josep González

We determine the automorphism group of the modular curve $X_0^{\ast }(N)$, obtained as the quotient of the modular curve $X_0(N)$ by the group of its Atkin-Lehner involutions, for all sq…