Frobenius projective and affine geometry of varieties in positive characteristic

Abstract: The goal of the present paper is to lay the foundations for a theory of projective and affine structures on higher-dimensional varieties in positive characteristic. This theory deals with Frobenius-projective and Frobenius-affine structures, which have been previously investigated only in the case where the underlying varieties are curves. As the first step in expanding the theory, we prove various basic properties on Frobenius-projective and Frobenius-affine structures and study (the positive characteristic version of) the classification problem, starting with S. Kobayashi and T. Ochiai, of varieties admitting projective or affine structures. In the first half of the present paper, we construct bijective correspondences with several types of generalized indigenous bundles; one of them is defined in terms of Berthelot's higher-level differential operators. We also prove the positive characteristic version of Gunning's formulas, which give necessary conditions on Chern classes for the existence of Frobenius-projective or Frobenius-affine structures respectively. The second half of the present paper is devoted to studying, from the viewpoint of Frobenius-projective and Frobenius-affine structures, some specific classes of varieties, i.e., projective and affine spaces, abelian varieties, curves, and surfaces. For example, it is shown that the existence of Frobenius-projective structures of infinite level gives a characterization of ordinariness for abelian varieties. Also, we prove that any smooth projective curve of genus $>1$ admits a Frobenius-projective structure of infinite level, which induces a representation of the stratified fundamental group.