Primitive algebraic points on curves

Abstract

A number field $K$ is primitive if $K$ and $\mathbb{Q}$ are the only subextensions of $K$. Let $C$ be a curve defined over $\mathbb{Q}$. We call an algebraic point $P\in C(\overline{\mathbb{Q}})$ primitive if the number field $\mathbb{Q}(P)$ is primitive. We present several sets of sufficient conditions for a curve $C$ to have finitely many primitive points of a given degree $d$. For example, let $C/\mathbb{Q}$ be a hyperelliptic curve of genus $g$, and let $3\le d\le g-1$. Suppose that the Jacobian $J$ of $C$ is simple. We show that $C$ has only finitely many primitive degree $d$ points, and in particular it has only finitely many degree $d$ points with Galois group $S_d$ or $A_d$. However, for any even $d\ge 4$, a hyperelliptic curve $C/\mathbb{Q}$ has infinitely many imprimitive degree $d$ points whose Galois group is a subgroup of $S_2\wr S_{d/2}$.

Theorem 2 and Corollary 5 are strengthened and are now more easily applicable to modular curves