ScalingStacks

Construction 2.6 . [059G]

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Construction 2.6.

Let 𝔇\mathfrak{D} be such a subdivision. We will construct a canonical formal scheme 𝔛′′\mathfrak{X}^{\prime\prime} over K∘K^{\circ} associated to 𝔇\mathfrak{D} together with a morphism ΞΉ:𝔛′′→𝔛\iota:\mathfrak{X}^{\prime\prime}\rightarrow\mathfrak{X} which induces the identity on the generic fibre such that there is a one to one correspondence between the open faces of 𝔇\mathfrak{D} and the strata of 𝔛~β€²β€²\tilde{\mathfrak{X}}^{\prime\prime}. First of all we choose a covering of 𝔛\mathfrak{X} as in Proposition 2.5. Let π”˜\mathfrak{U} be a member of this covering with an Γ©tale morphism ψ:π”˜β†’π”›β‘(𝒏,𝒂,m)\psi:\mathfrak{U}\rightarrow\mathfrak{X}(\boldsymbol{n},\boldsymbol{a},m) and let SS be the distinguished stratum of π”˜\mathfrak{U}. For Ξ”βˆˆπ”‡βˆ©Ξ”S\Delta\in\mathfrak{D}\cap\Delta_{S} we set

Aβ€²:={βˆ‘ΞΌaΞΌTμ∈K((T00,…,Tp,np))|βˆ€uβˆˆΞ”:limv(aΞΌ)+uβ‹…ΞΌ=∞}A^{\prime}:=\left\{\sum_{\mu}a_{\mu}T^{\mu}\in K((T_{00},...,T_{p,n_{p}}))\;\Big|\;\forall_{u\in\Delta}:\;\lim v(a_{\mu})+u\cdot\mu=\infty\right\}

and A:=Aβ€²/(T00​…​T0,n0βˆ’a0,…,Tp​0​…​Tp,npβˆ’ap)A:=A^{\prime}/(T_{00}...T_{0,n_{0}}-a_{0},...,T_{p0}...T_{p,n_{p}}-a_{p}) and define

AΞ”:={βˆ‘ΞΌaΞΌTμ∈A|βˆ€uβˆˆΞ”,ΞΌβˆˆβ„€π’+𝟏:v(aΞΌ)+ΞΌβ‹…uβ‰₯0}A^{\Delta}:=\left\{\sum_{\mu}a_{\mu}T^{\mu}\in A\;\Big|\;\forall_{u\in\Delta,\mu\in\mathbb{Z}^{\boldsymbol{n}+\boldsymbol{1}}}:\;v(a_{\mu})+\mu\cdot u\geq 0\right\}

and π”˜Ξ”:=Spf⁑AΞ”\mathfrak{U}_{\Delta}:=\Spf A^{\Delta}. If Ξ”1,Ξ”2βˆˆπ”‡βˆ©Ξ”S\Delta_{1},\Delta_{2}\in\mathfrak{D}\cap\Delta_{S} then Ξ”1βˆ©Ξ”2\Delta_{1}\cap\Delta_{2} is a face of both and by transferring the arguments in [Gub13, Proposition 6.12] to the analytic situation, we obtain that the canonical morphisms π”˜Ξ”1βˆ©Ξ”2β†’π”˜Ξ”i\mathfrak{U}_{\Delta_{1}\cap\Delta_{2}}\rightarrow\mathfrak{U}_{\Delta_{i}} are open immersions. Hence we can glue the π”˜Ξ”\mathfrak{U}_{\Delta} along this data to obtain a formal scheme which we denote by 𝔛​(𝒏,𝒂)β€²\mathfrak{X}(\boldsymbol{n},\boldsymbol{a})^{\prime} together with a morphism ΞΉβ€²:𝔛​(𝒏,𝒂)′→𝔛⁑(𝒏,𝒂)\iota^{\prime}:\mathfrak{X}(\boldsymbol{n},\boldsymbol{a})^{\prime}\rightarrow\mathfrak{X}(\boldsymbol{n},\boldsymbol{a}). Let Οˆβ€²:π”˜β€²β€²β†’π”›β€‹(𝒏,𝒂)′×𝔛⁑(m)\psi^{\prime}:\mathfrak{U}^{\prime\prime}\rightarrow\mathfrak{X}(\boldsymbol{n},\boldsymbol{a})^{\prime}\times\mathfrak{X}(m) be the base change of ψ\psi with respect to ΞΉβ€²Γ—Id\iota^{\prime}\times\Id. The construction of π”˜β€²β€²\mathfrak{U}^{\prime\prime} does not depend on the choice of ψ\psi up to isomorphism: Let ρ:π”˜β†’π”›β‘(𝒏,𝒂,m)\rho:\mathfrak{U}\rightarrow\mathfrak{X}(\boldsymbol{n},\boldsymbol{a},m) be another Γ©tale morphism. Then up to reordering the coordinates, Οβˆ—β€‹xi=uiβ€‹Οˆβˆ—β€‹xi\rho^{\ast}x_{i}=u_{i}\psi^{\ast}x_{i} for some ui∈π’ͺ​(π”˜)Γ—u_{i}\in\mathcal{O}(\mathfrak{U})^{\times}. Then we have canonical K∘K^{\circ}-algebra isomorphisms:

π’ͺ⁑(π”˜)β€‹βŠ—^Οˆβˆ—β€‹AΞ”\displaystyle\mathcal{O}(\mathfrak{U})\hat{\otimes}_{\psi^{\ast}}A^{\Delta} β†’π’ͺ⁑(π”˜)β€‹βŠ—^Οβˆ—β€‹AΞ”,\displaystyle\rightarrow\mathcal{O}(\mathfrak{U})\hat{\otimes}_{\rho^{\ast}}A^{\Delta},
aβŠ—xi\displaystyle a\otimes x_{i} ↦ui​aβŠ—xi,\displaystyle\mapsto u_{i}a\otimes x_{i},

which yield an isomorphism of the π”˜β€²β€²\mathfrak{U}^{\prime\prime} constructed with ψ\psi respectively ρ\rho.
We glue the π”˜β€²β€²\mathfrak{U}^{\prime\prime} to obtain our formal scheme 𝔛′′\mathfrak{X}^{\prime\prime}. Although 𝔛′′\mathfrak{X}^{\prime\prime} might not be admissible, we can define its generic fibre and reduction map in the usual way as the algebras AΞ”βŠ—K∘KA^{\Delta}\otimes_{K^{\circ}}K are strictly KK-affinoid (see [Gub13, Proposition 6.17]). Then ΞΉ\iota induces the identity on the generic fibres and we set p𝔛′′:=p𝔛p_{\mathfrak{X}^{\prime\prime}}:=p_{\mathfrak{X}}. Note that 𝔛′′\mathfrak{X}^{\prime\prime} is admissible if the vertices of the polytopes in 𝔇\mathfrak{D} are Ξ“\Gamma-rational, in particular the base change of 𝔛′′\mathfrak{X}^{\prime\prime} to the valuation ring of the completion of an algebraic closure of KK is admissible, see [Gub13, Proposition 6.7].

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