On the purity conjecture of Nisnevich for torsors under reductive group schemes

Abstract

Let $R$ be a regular semilocal integral domain containing an infinite field $k$. Let $f\in R$ be an element such that for all maximal ideals $\mathfrak{m}$ of $R$ we have $f\notin {\mathfrak{m}}^2$. Let $\mathbf{G}$ be a reductive group scheme over $R$. Under an isotropy assumption on $\mathbf{G}$ we show that a $\mathbf{G}$-torsor over the localization $R_f$ is trivial, provided it is rationally trivial. We show that it is not true without the isotropy assumption. Finally, if $\mathbf{G}$ is a commutative group scheme of multiplicative type and the regular semilocal ring contains a field of characteristic zero, we prove an analogue of Nisnevich purity conjecture for higher étale cohomology groups. The first statement is derived from its abstract version concerning presheaves of pointed sets satisfying some properties. The counterexample is constructed by providing a torsor over a local family of affine lines that cannot be extended to the family of projective lines. The latter is accomplished using the technique of affine Grassmannians.

Final version accepted for publication in Ann. Sci. Ec. Norm. Super

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