ScalingStacks

Subsection [04X5]

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(2.4) Let xx be a point in XanX^{\mathrm{an}} and let red𝒳​(x)\mathrm{red}_{\mathscr{X}}(x) be its reduction on 𝒳k\mathscr{X}_{k} (see [MN15, 2.2.2]). Let ZZ be the unique minimal stratum of 𝒳k\mathscr{X}_{k} that contains red𝒳​(x)\mathrm{red}_{\mathscr{X}}(x). By our assumption (2), ZZ is a log canonical center of (𝒳,𝒳k,red)(\mathscr{X},\mathscr{X}_{k,\mathrm{red}}). Then Zβˆ©π’³sncZ\cap\mathscr{X}^{\mathrm{snc}} is a non-empty stratum of 𝒳ksnc\mathscr{X}^{\mathrm{snc}}_{k} by the definition of a dlt pair. Thus, it determines a unique face Ο„\tau of the dual intersection complex Δ⁑(𝒳snc)\Delta(\mathscr{X}^{\mathrm{snc}}). Let E1,…,ErE_{1},\ldots,E_{r} be the prime components of 𝒳k\mathscr{X}_{k} that contain ZZ, and let N1,…,NrN_{1},\ldots,N_{r} be their multiplicities in 𝒳k\mathscr{X}_{k}. Then E1,…,ErE_{1},\ldots,E_{r} correspond precisely to the vertices v1,…,vrv_{1},\ldots,v_{r} of Ο„\tau. We choose a positive integer mm such that m​EimE_{i} is Cartier at the point red𝒳​(x)\mathrm{red}_{\mathscr{X}}(x) for every ii, and we choose a local equation fi=0f_{i}=0 for m​EimE_{i} at red𝒳​(x)\mathrm{red}_{\mathscr{X}}(x). Then ρ𝒳​(x)\rho_{\mathscr{X}}(x) is the point of the simplex Ο„\tau with barycentric coordinates

Ξ±=1m​(βˆ’N1​ln⁑|f1​(x)|,…,βˆ’Nr​ln⁑|fr​(x)|)\alpha=\frac{1}{m}(-N_{1}\ln|f_{1}(x)|,\ldots,-N_{r}\ln|f_{r}(x)|)

with respect to the vertices (v1,…,vr)(v_{1},\ldots,v_{r}). Under the embedding of Δ⁑(𝒳snc)\Delta(\mathscr{X}^{\mathrm{snc}}) into XanX^{\mathrm{an}}, the point ρ𝒳​(x)\rho_{\mathscr{X}}(x) corresponds to the monomial point represented by (𝒳,(E1,…,Er),ΞΎ)(\mathscr{X},(E_{1},\ldots,E_{r}),\xi) and the tuple

1m​(βˆ’ln⁑|f1​(x)|,…,βˆ’ln⁑|fr​(x)|),\frac{1}{m}(-\ln|f_{1}(x)|,\ldots,-\ln|f_{r}(x)|),

in the terminology of [MN15, 2.4.5]. It is obvious that this definition does not depend on the choices of mm and the local equations fif_{i}. It is also straightforward to check that ρ𝒳\rho_{\mathscr{X}} is continuous, and that it is a retraction onto Δ⁑(𝒳snc)=Sk⁑(X)\Delta(\mathscr{X}^{\mathrm{snc}})=\mathrm{Sk}(X).

Definition 2.5.

Let XX be a Calabi-Yau variety over KK and let 𝒳\mathscr{X} be a good minimal dlt-model of XX that satisfies assumption (2). Then we call the map ρ𝒳:Xanβ†’Sk⁑(X)\rho_{\mathscr{X}}\colon X^{\mathrm{an}}\to\mathrm{Sk}(X) constructed in (2) the non-archimedean SYZ fibration associated with 𝒳\mathscr{X}.

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