Isotropic Torsors on Smooth Algebras over Prüfer Rings

The Grothendieck–Serre conjecture predicts that every generically trivial torsor under a reductive group over a regular semilocal ring is itself trivial. Extending the work of Česnavičius and Fedorov, we prove a non-noetherian analogue of this conjecture for rings $A$ that are semilocalisations of smooth schemes over valuation rings of rank one, and for reductive $A$-group schemes $G$ that are totally isotropic. Roughly speaking, such group schemes are characterised by the existence of a parabolic subgroup of their adjoint quotients. Since quasi-split groups are totally isotropic, our result, in particular, generalises the Grothendieck–Serre result of Guo–Liu and the author’s thesis. Our proof relies on a new instance of Gabber’s presentation lemma, obtained by extending techniques developed in the author’s thesis.