Rational points on $X_0(N)^*$ when $N$ is non-squarefree

Sachi Hashimoto; Timo Keller; Samuel Le Fourn

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arXiv · 2505.00680

Rational points on $X_0(N)^*$ when $N$ is non-squarefree

Sachi Hashimoto·Timo Keller·Samuel Le Fourn

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Abstract

Let $N$ be a non-squarefree integer such that the quotient $X_0(N)^*$ of the modular curve $X_0(N)$ by the full group of Atkin-Lehner involutions has positive genus. Elkies conjectures that the rational points on $X_0(N)^*$ are only cusps or CM points when $N$ is large enough. We establish an integrality result for the $j$-invariants of non-cuspidal rational points on $X_0(N)^*$, representing a significant step toward resolving a key subcase of Elkies' conjecture. To this end, we prove the existence of rank-zero quotients of certain modular Jacobians $J_0(pq)$. Furthermore, we provide a complete classification of the rational points on $X_0(N)^*$ of genus $1 \leq g \leq 5$, when they are finite. In the process we identify exceptional rational points on $X_0(147)^*$ and $X_0(75)^*$ which were not known before.

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Sachi Hashimoto, Timo Keller, Samuel Le Fourn. 2025-05-01. Rational points on $X_0(N)^*$ when $N$ is non-squarefree. https://arxiv.org/abs/2505.00680

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