Pink’s theory of Hodge structures and the Hodge conjecture over function fields

  • Urs Hartl

    Westfälische Wilhelms-Universität Münster, Germany
  • Ann-Kristin Juschka

    Universität Heidelberg, Germany
Pink’s theory of Hodge structures and the Hodge conjecture over function fields cover
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In 1997 Richard Pink has clarified the concept of Hodge structures over function fields in positive characteristic, which today are called Hodge-Pink structures. They form a neutral Tannakian category over the underlying function field. He has defined Hodge realization functors from the uniformizable abelian tt-modules and tt-motives of Greg Anderson to Hodge-Pink structures. This allows one to associate with each uniformizable tt-motive a Hodge-Pink group, analogous to the Mumford-Tate group of a smooth projective variety over the complex numbers. It further enabled Pink to prove the analog of the Mumford-Tate Conjecture for Drinfeld modules. Moreover, based on unpublished work of Pink and the first author, the second author proved in her Diploma thesis that the Hodge-Pink group equals the motivic Galois group of the tt-motive as defined by Papanikolas and Taelman. This yields a precise analog of the famous Hodge Conjecture, which is an outstanding open problem for varieties over the complex numbers.

In this report we explain Pink's results on Hodge structures and the proof of the function field analog of the Hodge conjecture. The theory of tt-motives has a variant in the theory of dual tt-motives. We clarify the relation between tt-motives, dual tt-motives and tt-modules. We also construct cohomology realizations of abelian tt-modules and (dual) tt-motives and comparison isomorphisms between them generalizing Gekeler's de Rham isomorphism for Drinfeld modules.