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Torsors under tori and Néron models. (English. French summary) Zbl 1251.14013

The main object of the paper under review is an \(X\)-torsor \(Y\) under \(G\), where \(X\) is a smooth variety over a local field \(K\) (the field of fractions of a Henselian discrete valuation ring \(R\)) and \(G\) is an algebraic \(K\)-torus. The residue field \(k\) of \(K\) is assumed perfect. The author’s goal is to show that, under certain conditions, the evaluation map \(X(K)\to H^1(K,G)\), which associates to each point \(P\) the isomorphism class of the fibre \(Y_P\), factors through reduction to the special fibre, i.e., for a suitable \(R\)-model \(\mathcal X\) of \(X\), the evaluation map comes from \(\mathcal X_s(k)\to H^1(k,\mathcal G_s)\), where \(\mathcal G\) is the Néron–Raynaud model of \(G\) and the subscript \(s\) refers to the special fibres of \(\mathcal X\) and \(\mathcal G\). This is proved when the torus \(G\) is split by a tamely ramified extension of \(K\). The author presents an example showing that one should not expect such a result in wildly ramified cases. Some intermediate results on extending torsors are formulated for more general algebraic groups \(G\) and are interesting by their own right. Finally, the author discusses some applications to the case where \(X\) is a rational surface.

MSC:

14G20 Local ground fields in algebraic geometry
14G05 Rational points
14F20 Étale and other Grothendieck topologies and (co)homologies
11G25 Varieties over finite and local fields

Keywords:

torsor; Néron model

References:

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