number-theory / Arithmetic geometry: Hodge classes, reduction and rational Tate classes

Milne's rationality conjecture for abelian varieties

Claims Theorem 1.1: for every abelian variety over Q‾\overline{\mathbb Q} with good reduction at a pp-adic place, every residue characteristic pp including 22, every rational Hodge class γ\gamma of codimension rr and every divisors D1,…,Dd−rD_1,\dots,D_{d-r} on the reduction, the ℓ\ell-adic traces (ℓ≠p\ell\ne p) and the crystalline trace of γ0⋅D1⋯Dd−r\gamma_0\cdot D_1\cdots D_{d-r} equal one rational number. Also claims each specialized Hodge class lies in one fixed good theory of rational Tate classes, and (using the CM Hodge theorem) is represented by a rational algebraic cycle in all realizations. It does not prove the Hodge or Tate conjecture for general abelian varieties.

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number-theorySep 23, 2026Significance 28/100Registry: unreviewed

Milne's rationality conjecture for abelian varieties

Prior state unknown→proved

Claims Theorem 1.1: for every abelian variety over Q‾\overline{\mathbb Q} with good reduction at a pp-adic place, every residue characteristic pp including 22, every rational Hodge class γ\gamma of codimension rr and every divisors D1,…,Dd−rD_1,\dots,D_{d-r} on the reduction, the ℓ\ell-adic traces (ℓ≠p\ell\ne p) and the crystalline trace of γ0⋅D1⋯Dd−r\gamma_0\cdot D_1\cdots D_{d-r} equal one rational number. Also claims each…

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Claims Theorem 1.1: for every abelian variety over Q‾\overline{\mathbb Q} with good reduction at a pp-adic place, every residue characteristic pp including 22, every rational Hodge class γ\gamma of codimension rr and every divisors D1,…,Dd−rD_1,\dots,D_{d-r} on the reduction, the ℓ\ell-adic traces (ℓ≠p\ell\ne p) and the crystalline trace of γ0⋅D1⋯Dd−r\gamma_0\cdot D_1\cdots D_{d-r} equal one rational number. Also claims each specialized Hodge class lies in one fixed good theory of rational Tate classes, and (using the CM Hodge theorem) is represented by a rational algebraic cycle in all realizations. It does not prove the Hodge or Tate conjecture for general abelian varieties.

Claims Theorem 1.1: for every abelian variety over Q‾\overline{\mathbb Q} with good reduction at a pp-adic place, every residue characteristic pp including 22, every rational Hodge class γ\gamma of codimension rr and every divisors D1,…,Dd−rD_1,\dots,D_{d-r} on the reduction, the ℓ\ell-adic traces (ℓ≠p\ell\ne p) and the crystalline trace of γ0⋅D1⋯Dd−r\gamma_0\cdot D_1\cdots D_{d-r} equal one rational number. Also claims each specialized Hodge class lies in one fixed good theory of rational Tate classes, and (using the CM Hodge theorem) is represented by a rational algebraic cycle in all realizations. It does not prove the Hodge or Tate conjecture for general abelian varieties.

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Milne's rationality conjecture for abelian varieties — Mathematical Frontier Network