An effective open image theorem for abelian varieties (1910.14171v1)

Abstract: Fix an abelian variety $A$ of dimension $g\geq 1$ defined over a number field $K$. For each prime $\ell$, the Galois action on the $\ell$-power torsion points of $A$ induces a representation $\rho_{A,\ell}\colon Gal_K \to GL_{2g}(\mathbb{Z}\ell)$. The $\ell$-adic monodromy group of $A$ is the Zariski closure $G{A,\ell}$ of the image of $\rho_{A,\ell}$ in $GL_{2g,\mathbb{Q}\ell}$. The image of $\rho{A,\ell}$ is open in $G_{A,\ell}(\mathbb{Q}\ell)$ with respect to the $\ell$-adic topology and hence the index $[G{A,\ell}(\mathbb{Q}\ell)\cap GL{2g}(\mathbb{Z}\ell): \rho{A,\ell}(Gal_K)]$ is finite. We prove that this index can be bounded in terms of $g$ for all $\ell$ larger then some constant depending on certain invariants of $A$.