On parameterizations of cyclic $N$-isogenies and strict $K$-curves lying above rational points of $Y_0^+(N)$

Abstract

Elliptic $K$-curves are elliptic curves defined over some field extension $L/K$ that are isogenous to all of their Galois conjugates. We present a new result on $K$-curves $E$ that are given by a $K$-rational orbit $\{\tau, -1/N\tau\}$ of the Fricke involution on $Y_0(N)$, giving a simple Diophantine condition on the extension $L/K$ that determines which twists of $E$ allow the isogeny between Galois conjugates to be defined over $L$. To support and illustrate this result, we also discuss parameterizations of cyclic $N$-isogenies corresponding to points on modular curves $X_0(N)$ of genus $0$. These modular curves admit parameterizations in terms of a distinguished Hauptmodul. We provide an exposition on the derivation of these Hauptmoduln as products of the Dedekind eta function based on the approach of Ligozat. As an application, we provide a complete tabulation of explicit formulas for the coefficients of cyclic $N$-isogenous curves in terms of the Hauptmodul for all $N$ such that $X_0(N)$ has genus $0$. We also include an abbreviated table of rational functions for the $j$-invariant in terms of Hauptmoduln, and we discuss a classical application of these expressions to finding special values of the $j$-invariant at CM points.