Semistable abelian varieties with small division fields, by Armand Brumer and Kenneth Kramer

Let $A$ be a semistable abelian variety defined over ${\bf Q}$ with bad reduction only at one prime $p$. Let $L= {\bf Q}(A[\ell])$ be the $\ell$-division field of $A$ for a prime $\ell$ not equal to $p$ and let $F={\bf Q}(\mu_\ell)$ be the cyclotomic field generated by the group of $\ell^{th}$-roots of unity. We study the varieties $A$ for which $H={\rm Gal(L/F)}$ is ``small" in the sense that $H$ is an $\ell$-group or, more generally, that $H$ is nilpotent. We show that if $\ell=2$ or 3 and $H$ is nilpotent then the reduction of $A$ at $p$ is totally toroidal, so its conductor is $p^{\dim A}$. The Jacobian of the modular curve $X_0(41)$ is a simple semistable abelian variety of dimension 3, with bad reduction only at $p=41$ and the Galois group of its 2-division field is a 2-group. For $\ell=2$, 3 or 5, there exist elliptic curves $E$ of prime conductor such that ${\bf Q}(E[\ell]) = {\bf Q}(\mu_{2 \ell})$. We characterize the abelian varieties that are isogenous to products $E^d$.

Armand Brumer and Kenneth Kramer <kramer@forbin.qc.edu>