Katharina Hübner, Alexander Schmidt: The tame site of a scheme

       Authors: Katharina Hübner, Alexander Schmidt
       Title: The tame site of a scheme
       in: Invent. math., 223, 379-443 (2021)

	Abstract: Étale cohomology with non-invertible coefficients has some unpleasant properties, e.g., it is not A^1-homotopy invariant and for constructible coefficients the expected finiteness properties do not hold. In this paper we introduce the `tame site' which is slightly coarser than the étale site. Tame cohomology coincides with étale cohomology for invertible coefficients but is better behaved in the general case. The fundamental group of the tame site is the (curve-)tame fundamental group of Wiesend and Kerz/Schmidt. The higher tame homotopy groups hopefully have a better behaviour than the higher étale homotopy groups, which vanish for affine schemes in positive characteristic by a result of Achinger.

Abstract: Étale cohomology with non-invertible coefficients has some unpleasant properties, e.g., it is not A^1-homotopy invariant and for constructible coefficients the expected finiteness properties do not hold. In this paper we introduce the `tame site' which is slightly coarser than the étale site. Tame cohomology coincides with étale cohomology for invertible coefficients but is better behaved in the general case. The fundamental group of the tame site is the (curve-)tame fundamental group of Wiesend and Kerz/Schmidt. The higher tame homotopy groups hopefully have a better behaviour than the higher étale homotopy groups, which vanish for affine schemes in positive characteristic by a result of Achinger.

pdf-file: Tame site
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