Grothendieck-Serre in the quasi-split unramified case

Abstract

The Grothendieck--Serre conjecture predicts that every generically trivial torsor under a reductive group scheme $G$ over a regular local ring $R$ is trivial. We settle it in the case when $G$ is quasi-split and $R$ is unramified. Some of the techniques that allow us to overcome obstacles that have so far kept the mixed characteristic case out of reach include a version of Noether normalization over discrete valuation rings, as well as a suitable presentation lemma for smooth relative curves in mixed characteristic that facilitates passage to the relative affine line via excision and patching.

arXiv comment32 pages; appeared in Forum of Mathematics, Pi; a post-publication arXiv version: inserted a footnote on p. 20 to point out a minor oversight and to clarify the subsequent argument