Quadratic Chabauty for Modular Curves

Abstract: Let $X/\mathbb{Q}$ be a curve of genus $g \ge 2$ with Jacobian $J$ and let $\ell$ be a prime of good reduction. Using Selmer varieties, Kim defines a decreasing sequence [ X(\mathbb{Q}\ell) \supseteq X(\mathbb{Q}\ell)1 \supseteq X(\mathbb{Q}\ell)2 \supseteq \cdots ] all containing the rational points of $X$. Thanks to the work of Coleman, the `Chabauty set' $X(\mathbb{Q}\ell)1$ is known to be finite provided the Mordell--Weil rank of $J$ is smaller than $g$. In this case one has a practical strategy that often succeeds in computing the set of rational points of $X$. Balakrishnan and Dogra have recently shown that the `quadratic Chabauty set' $X(\mathbb{Q}\ell)_2$ is finite provided the Mordell--Weil rank is less than $g + \rho-1$, where $\rho$ is the N\'eron-Severi rank of $J/\mathbb{Q}$. In view of this it is interesting to give families of curves where $\rho \ge 2$ and where therefore quadratic Chabauty is more likely to succeed than classical Chabauty. In this note we show that this is indeed the case for all modular curves of genus at least 3.