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Chow groups, pull back and class groups

  • Kalyan Banerjee,
  • Azizul Hoque

摘要

Let S be a certain affine algebraic surface over \({\mathbb Q}\) Q such that it admits a regular map to \({\mathbb A}^2/{\mathbb Q}\) A 2 / Q . We show that any non-trivial torsion element in the Chow group \({ {CH}}^1(S)\) CH 1 ( S ) can be pulled back to ideal classes of quadratic fields whose order can be made as large as possible. This gives an affirmative answer to a question analogous to one raised by Agboola and Pappas, in the case of certain affine algebraic surfaces. Spreading out S over \({\mathbb Z}\) Z and for a closed point \(P\in {\mathbb A}^2/{\mathbb Z}\) P A 2 / Z , we show that the cardinality of a subgroup of the Picard group of the fiber \(S_P\) S P remains unchanged when P varies over a Zariski open subset in \({\mathbb A}^2/{\mathbb Z}\) A 2 / Z . We also show by constructing an element of odd order \(n\ge 3\) n 3 in the class group of certain imaginary quadratic fields that the Picard group of \(S_P\) S P has a subgroup isomorphic to \({\mathbb Z}/n{\mathbb Z}\) Z / n Z .