Let A be a two-dimensional abelian variety defined over a number field K. Fix a prime number $\ell $ and suppose $\#A({\mathbf {F}_{\mathfrak {p}}}) \equiv 0 \pmod {\ell ^2}$ for a set of primes ${\mathfrak {p}} \subset {\mathcal {O}_{K}}$ of density 1. When $\ell =2$ Serre has shown that there does not necessarily exist a K-isogenous $A'$ such that $\#A'(K)_{{tor}} \equiv 0 \pmod {4}$ . We extend those results to all odd $\ell $ and classify the abelian varieties that fail this divisibility principle for torsion in terms of the image of the mod- $\ell ^2$ representation.

Let be a commutative algebraic group defined over a number field K. For a prime ℘ in K where has good reduction, let N℘,n be the number of n-torsion points of the reduction of modulo ℘ where n is a positive integer. When is of dimension one and n is relatively prime to a fixed finite set of primes depending on , we determine the average values of N℘,n as the prime ℘ varies. This average value as a function of n always agrees with a divisor function.

Soit A une variété abélienne définie sur un corps de nombres K, le nombre de points de torsion définis sur une extension finie L est borné polynomialement en terme du degré [L : K]. Lorsque A est isogène à un produit de variétés abéliennes simples de type GSp, c'est-à-dire dont le groupe de Mumford–Tate est « générique » (isomorphe au groupe des similitudes symplectiques) et vérifiant la conjecture de Mumford–Tate, nous calculons l'exposant optimal dans cette borne, en terme de la dimension des sous-variétés abéliennes de A. Le résultat est inconditionnel pour un produit de variétés abéliennes simples dont l'anneau d'endomorphismes est ℤ et la dimension n'appartient pas à un ensemble exceptionnel explicite S = {4, 10, 16, 32, …}. Par ailleurs nous prouvons, suivant une stratégie de Serre, que si la conjecture de Mumford–Tate est vraie pour des variétés abéliennes de type GSp, alors la conjecture de Mumford–Tate est vraie pour un produit de telles variétés abéliennes.

A theorem of Kuyk says that every Abelian extension of a Hilbertian field is Hilbertian. We conjecture that for an Abelian variety A defined over a Hilbertian field K every extension L of K in K(Ator) is Hilbertian. We prove our conjecture when K is a number field. The proof applies a result of Serre about l-torsion of Abelian varieties, information about l-adic analytic groups, and Haran's diamond theorem.

We construct families of hyperelliptic curves over ${\mathbb Q}$ of arbitrary genus g with (at least) g integral elements in K2. We also verify the Beilinson conjectures about K2 numerically for several curves with g = 2, 3, 4 and 5. The first few sections of the paper also provide an elementary introduction to the Beilinson conjectures for K2 of curves.

We present the details of a model-theoretic proof of an analogue of the Manin–Mumford conjecture for semiabelian varieties in positive characteristic. As a by-product of the proof we reduce the general positive-characteristic Mordell–Lang problem to a question about purely inseparable points on subvarieties of semiabelian varieties.