Intermediate modular curves with infinitely many quartic points

Abstract

For every group $\{\pm1\}\subseteq \Delta\subseteq (\mathbb Z/N\mathbb Z)^\times$, there exists an intermediate modular curve $X_\Delta(N)$. In this paper we determine all curves $X_\Delta(N)$ with infinitely many points of degree $4$ over $\mathbb Q$. To do that, we developed a method to compute possible degrees of rational morphisms from $X_\Delta(N)$ to an elliptic curve.