ScalingStacks

The case of good reduction [01JW]

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The case of good reduction

When the variety X\mathrm{X} has good reduction, the canonical metrics and the associated measures are fairly easy to describe. Indeed, let 𝔛\mathfrak{X} be the Néron model of X\mathrm{X} over K∘K^{\circ}, an Abelian scheme. For any line bundle LL on X\mathrm{X} there is a unique line bundle 𝔏\mathfrak{L} on 𝔛\mathfrak{X} which extends LL and which admits a trivialization at the 00 section extending the given one over KK. By the theorem of the cube for the Abelian scheme 𝔛\mathfrak{X}, the isomorphism [m]∗​L≃L⊗ma[m]^{*}L\simeq L^{\otimes m^{a}} (with a=1a=1 or 22, according to whether LL is odd or even) extends uniquely to an isomorphism [m]∗​𝔏≃𝔏⊗ma[m]^{*}\mathfrak{L}\simeq\mathfrak{L}^{\otimes m^{a}}. This implies that the canonical metrics are algebraic, induced by these models.

The description of the canonical measures on 𝔛\mathfrak{X} follows at once. Let ξ\xi be the point of X\mathrm{X} whose reduction is the generic point of the special fiber of 𝔛\mathfrak{X}. Then, for any family (L1,…,Ln)(L_{1},\dots,L_{n}) of line bundles on X\mathrm{X}, one has

c1​(L¯1)​…​c1​(L¯n)=deg⁡(c1​(L1)​…​c1​(Ln))​δξ.c_{1}(\overline{L}_{1})\dots c_{1}(\overline{L}_{n})=\deg(c_{1}(L_{1})\dots c_{1}(L_{n}))\delta_{\xi}.

We see in particular that they only depend on the classes of the line bundles LjL_{j} modulo numerical equivalence.

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