A Classification of Genus 0 Modular Curves with Rational Points

Abstract

Let $E$ be a non-CM elliptic curve defined over $\mathbb {Q}$. Fix an algebraic closure $\overline{\mathbb {Q}}$ of $\mathbb {Q}$. We get a Galois representation \[\rho_E \colon Gal(\overline{\mathbb {Q}}/\mathbb {Q}) \to GL_2(\hat{\mathbb {Z}})\] associated to $E$ by choosing a compatible bases for the $N$-torsion subgroups of $E(\overline{\mathbb {Q}}).$ Associated to an open subgroup $G$ of $GL_2(\hat{\mathbb {Z}})$ satisfying $-I \in G$ and $det(G)=\hat{\mathbb {Z}}^{\times}$, we have the modular curve $(X_G,\pi_G)$ over $\mathbb {Q}$ which loosely parametrises elliptic curves $E$ such that the image of $\rho_E$ is conjugate to a subgroup of $G^t.$ In this article we give a complete classification of all such genus $0$ modular curves that have a rational point. This classification is given in finitely many families. Moreover, each such modular curve can be explicitly computed.