The rationality conjecture for specializations of Hodge classes

Let AA be an abelian variety over an algebraic closure of Q\mathbb{Q} with good reduction to an abelian variety A0A_{0} over an algebraic closure of a finite field. A Hodge class on AA specializes to a cohomology class on A0A_{0}, and a Lefschetz class is a class generated by divisor classes and the Lefschetz operations. Rationality conjecture. The cup product of the specialization to A0A_{0} of any Hodge class on AA with any Lefschetz class of complementary dimension lies in Q\mathbb{Q}. Equivalently, its ll-adic component is a rational number independent of ll. The conjecture follows for a given Hodge class when its specialization is algebraic, and is therefore implied by the Hodge conjecture for abelian varieties; it is known for CM abelian varieties with simple ordinary reduction and their powers.

Sources & referencesView supporting material

Primary source

J. S. Milne, “Rational Tate classes”, arXiv:0707.3167 (2008).

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