The geometric torsion conjecture asserts that the torsion part of the Mordell-Weil group of a family of abelian varieties over a complex quasiprojective curve is uniformly bounded in terms of the genus of the curve. We prove the conjecture for abelian varieties with real multiplication, uniformly in the field of multiplication. Fixing the field, we furthermore show that the torsion is bounded in terms of the gonality of the base curve, which is the closer analog of the arithmetic conjecture. The proof is a hybrid technique employing both the hyperbolic and algebraic geometry of the toroidal compactifications of the Hilbert modular varieties X(1) parametrizing such abelian varieties. We show that only finitely many torsion covers X1(n) contain d-gonal curves outside of the boundary for any fixed d. We further show the same is true for entire curves C → X1(n).
Statement of ResultsFor any elliptic curve E/Q, the group of rational points E(Q) is finitely generated by Mordell's theorem. The free part behaves wildly; it is expected that there are elliptic curves E/Q with arbitrarily large rank rk E(Q), and the record to date is an elliptic curve E with rk E(Q) ≥ 28 found by Elkies. On the other hand, by a celebrated theorem of Mazur [MG78] the torsion part E(Q) tor is uniformly bounded:Theorem (Mazur). For any elliptic curve E/Q, |E(Q) tor | ≤ 16.Mazur's theorem was subsequently generalized to arbitrary number fields K/Q by Merel [Mer96] (building on partial results of [Kam92]) who showed a stronger uniformity: there is an integer N = N (d) such that for any degree d number field K and any elliptic curve E/K, every K-rational torsion point has order dividing N , i.e. E(K) tor ⊂ E(K) [N ].Similarly, it is expected that the torsion part of the Mordell-Weil group of an abelian variety A/K is uniformly bounded, though there are few results in this direction. The same question can be asked for K = k(C) the function field of a curve C over any field k, and though k = F p is most closely analogous to the number field case, k = Q is also interesting:Conjecture (Geometric torsion conjecture). Let k be an algebraically closed field of characteristic 0. There is an integer N = N (g, n) such that for any Date: April 9, 2015.