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A monodromy criterion for existence of Néron models and a result on semi-factoriality

Abstract : This thesis is subdivided in two parts. In the first part, we introduce a new condition, called toric-additivity, on a family of abelian varieties degenerating to a semi-abelian scheme over a normal crossing divisor. The condition depends only on the Tate module TlA(Ksep) of the generic fibre, for a prime l invertible on the base. We show that toric-additivity is a sufficient condition for the existence of a Néron model if the base is a Q-scheme. In the case of the jacobian of a smooth curve with semi-stable reduction, we obtain the same result without assumptions on the base characteristic; and we show that toric-additivity is also necessary for the existence of a Néron model, when the base is a Q-scheme. In the second part, we consider the case of a family of nodal curves over a discrete valuation ring, having split singularities. We say that such a family is semi factorial if every line bundle on the generic fibre extends to a line bundle on the total space. We give a necessary and sufficient condition for semi- factoriality, in terms of combinatorics of the dual graph of the special fibre. In particular, we show that performing one blow-up with center the non regular closed points yields a semi-factorial model of the generic fibre. As an application, we extend the result of Raynaud relating Néron models of smooth curves and Picard functors of their regular models to the case of nodal curves having a semi-factorial model.
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Giulio Orecchia. A monodromy criterion for existence of Néron models and a result on semi-factoriality. Algebraic Geometry [math.AG]. Université de Bordeaux; Universiteit Leiden (Leyde, Pays-Bas), 2018. English. ⟨NNT : 2018BORD0017⟩. ⟨tel-02355562⟩

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