ScalingStacks

Example 3.5 . [04XH]

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Example 3.5.

We use the tropicalization map to identify the canonical skeleton Δ⁡(T)\Delta(T) with NℝN_{\mathbb{R}}. We denote by CC the open cone (Nℝ×ℝ>0)∪{0}(N_{\mathbb{R}}\times\mathbb{R}_{>0})\cup\{0\} in Nℝ⊕ℝN_{\mathbb{R}}\oplus\mathbb{R}. Let Σ\Sigma be a locally finite fan of strongly convex rational polyhedral cones in CC. We denote by Σ1\Sigma_{1} the rational polyhedral complex in NℝN_{\mathbb{R}} obtained by intersecting the cones in Σ\Sigma with Nℝ×{1}N_{\mathbb{R}}\times\{1\}. Consider the torus embedding T→𝒳T\to\mathscr{X} over RR associated with Σ\Sigma as in [Kü98, 1.13]. The RR-scheme 𝒳\mathscr{X} is separated and locally of finite type, and it is quasi-compact if and only if Σ\Sigma is finite. Since Σ\Sigma is supported in CC, the generic fiber of 𝒳\mathscr{X} is canonically isomorphic to the split KK-torus TT. Assume that 𝒳\mathscr{X} is regular; this is equivalent to the property that the fan Σ\Sigma is simple, and it implies that the special fiber 𝒳k\mathscr{X}_{k} is a strict normal crossings divisor. Denote by 𝔛\mathfrak{X} the formal tt-adic completion of 𝒳\mathscr{X}. The generic fiber 𝔛η\mathfrak{X}_{\eta} is a KK-analytic space endowed with a natural injective morphism of KK-analytic spaces i:𝔛η→Tani:\mathfrak{X}_{\eta}\to T^{\mathrm{an}}. The morphism ii embeds 𝔛η\mathfrak{X}_{\eta} as an analytic domain in TanT^{\mathrm{an}}.

The construction of the Berkovich skeleton Sk⁡(𝒳)\mathrm{Sk}(\mathscr{X}) and the retraction map ρ𝒳\rho_{\mathscr{X}} in [MN15, §3] are local on 𝒳\mathscr{X}, so that they extend immediately to schemes that are locally of finite type. This yields a canonical embedding of the dual intersection complex Δ⁡(𝒳)\Delta(\mathscr{X}) of 𝒳k\mathscr{X}_{k} into 𝔛η\mathfrak{X}_{\eta}. The image of this embedding is called the Berkovich skeleton of 𝒳\mathscr{X}. The embedding has a canonical retraction ρ𝒳:𝔛η→Δ⁡(𝒳)\rho_{\mathscr{X}}:\mathfrak{X}_{\eta}\to\Delta(\mathscr{X}). It follows directly from the definitions that Δ⁡(𝒳)\Delta(\mathscr{X}) is contained in Δ⁡(T)=Nℝ\Delta(T)=N_{\mathbb{R}} and coincides with the support of Σ1\Sigma_{1}. In particular, if Σ\Sigma is a subdivision of CC, then Δ⁡(𝒳)=Δ⁡(T)\Delta(\mathscr{X})=\Delta(T). Moreover, the Δ\Delta-structure on Δ⁡(𝒳)\Delta(\mathscr{X}) is precisely the polyhedral decomposition Σ1\Sigma_{1}. We have 𝔛η=ρT−1​(|Σ1|)\mathfrak{X}_{\eta}=\rho_{T}^{-1}(|\Sigma_{1}|), and the retraction map ρ𝒳\rho_{\mathscr{X}} is the restriction of ρT\rho_{T} to 𝔛η\mathfrak{X}_{\eta}.

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