Comparison of Kummer logarithmic topologies with classical topologies II

Abstract

We show that the higher direct images of smooth commutative group schemes from the Kummer log flat site to the classical flat site are torsion. For cases (1) smooth affine commutative schemes with geometrically connected fibers, (2) finite flat group schemes, and (3) extensions of abelian schemes by tori, we give explicit descriptions of the second higher direct images. If the rank of the log structure at any geometric point of the base is at most one, we show that the second higher direct image is zero for group schemes in case (1), case (3), and a certain subcase of case (2). If the underlying scheme of the base is of zero-characteristic or of positive characteristic, we can also give more explicit description of the second higher direct image of group schemes in case (1), case (3), and a certain subcase of case (2). Over standard Henselian log traits with finite residue field, we compute the first and the second Kummer log flat cohomology groups with coefficients in group schemes in case (1), case (3), and a certain subcase of case (2).

The authors' abstract, as published at the source. Journal of Algebra, 2026 · DOI ↗