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arXiv · 1810.04228

On motivic obstructions to Witt cancellation for quadratic forms over schemes

Matthias Wendt
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Abstract

The paper provides computations of the first non-vanishing $\mathbb{A}^1$-homotopy sheaves of the orthogonal Stiefel varieties which are relevant for the unstable isometry classification of quadratic forms over smooth affine schemes over perfect fields of characteristic $\neq 2$. Together with the $\mathbb{A}^1$-representability for quadratic forms, this provides the first obstructions for rationally trivial quadratic forms to split off a hyperbolic plane. For even-rank quadratic forms, this first obstruction is a refinement of the Euler class of Edidin and Graham. A couple of consequences are discussed, such as improved splitting results over algebraically closed base fields as well as examples where the obstructions are nontrivial.

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Matthias Wendt. 2018-10-09. On motivic obstructions to Witt cancellation for quadratic forms over schemes. https://arxiv.org/abs/1810.04228

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