Torsion bounds for a fixed abelian variety and varying number field

TL;DR

This paper investigates bounds on the size of torsion groups of abelian varieties over number fields, establishing a conjecturally optimal exponent under the Mumford--Tate conjecture and providing related bounds for torsion point orders.

Contribution

It determines the minimal exponent for torsion bounds of abelian varieties over number fields assuming the Mumford--Tate conjecture, aligning with prior conjectures.

Findings

The exponent $eta_A$ matches the conjectured value under the Mumford--Tate conjecture.

Provides bounds for the maximal order of torsion points in abelian varieties.

Establishes a uniform bound on torsion subgroup sizes depending on extension degree.

Abstract

Let $A$ be an abelian variety defined over a number field $K$. For a finite extension $L/K$, the cardinality of the group $A(L)_{\operatorname{tors}}$ of torsion points in $A(L)$ can be bounded in terms of the degree $[L:K]$. We study the smallest real number $\beta_A$ such that for any finite extension $L/K$ and $\varepsilon>0$, we have $|A(L)_{\operatorname{tors}}| \leq C \cdot [L:K]^{\beta_A+\varepsilon}$, where the constant $C$ depends only on $A$ and $\varepsilon$ (and not $L$). Assuming the Mumford--Tate conjecture for $A$, we will show that $\beta_A$ agrees with the conjectured value of Hindry and Ratazzi. We also give a similar bound for the maximal order of a torsion point in $A(L)$.