ScalingStacks

Example 2.25 . [02J7]

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Example 2.25.

Let X=β„™KnX=\mathbb{P}_{K}^{n} and L=π’ͺ⁑(1)L=\mathcal{O}(1), the universal line bundle of β„™Kn{\mathbb{P}_{K}^{n}}. As a model for (X,L)(X,L) we consider 𝒳=β„™K∘n\mathcal{X}=\mathbb{P}_{K^{\circ}}^{n}, the projective space over Spec⁑(K∘)\operatorname{Spec}(K^{\circ}), β„’=π’ͺβ„™K∘n​(1)\mathcal{L}=\mathcal{O}_{\mathbb{P}_{K^{\circ}}^{n}}(1), and e=1e=1. A rational section ss of LL can be identified with a homogeneous rational function ρs∈K⁑(x0,…,xn)\rho_{s}\in K(x_{0},\dots,x_{n}) of degree 1.

Let p=(p0:…:pn)∈(β„™Kn)anβˆ–div(s)p=(p_{0}:\dots:p_{n})\in(\mathbb{P}_{K}^{n})^{\text{\rm an}}\setminus\operatorname{div}(s) and set H=ℋ⁑(p)H=\mathscr{H}(p). Let i0i_{0} be such that |pi0|=maxi⁑{|pi|}|p_{i_{0}}|=\max_{i}\{|p_{i}|\}. Take U≃𝔸KnU\simeq\mathbb{A}_{K}^{n} (respectively 𝒰≃𝔸K∘n\mathcal{U}\simeq\mathbb{A}_{K^{\circ}}^{n}) as the affine set xi0β‰ 0x_{i_{0}}\not=0 over HH (respectively H∘H^{\circ}). The point pp corresponds to the algebraic morphism

pβˆ—:K⁑[X0,…,Xi0βˆ’1,Xi0+1,…,Xn]⟢Hp^{\ast}\colon K[X_{0},\dots,X_{i_{0}-1},X_{i_{0}+1},\dots,X_{n}]\longrightarrow H

that sends XiX_{i} to pi/pi0p_{i}/p_{i_{0}}. The extension p~{\widetilde{p}} factors through the algebraic morphism

p~βˆ—:Kβˆ˜β€‹[X1,…,Xi0βˆ’1,Xi0+1,…,Xn]⟢H∘,{\widetilde{p}}^{\ast}\colon K^{\circ}[X_{1},\dots,X_{i_{0}-1},X_{i_{0}+1},\dots,X_{n}]\longrightarrow H^{\circ},

with the same definition. Then

β€–s⁑(p)β€–\displaystyle||s(p)|| =inf{|z||z∈HΓ—,zβˆ’1p~βˆ—s∈p~βˆ—β„’}\displaystyle=\inf\big\{|z|\ \big|z\in H^{\times},z^{-1}{\widetilde{p}}^{\ast}s\in{\widetilde{p}}^{*}{\mathcal{L}}\big\}
=inf{|z||z∈HΓ—,zβˆ’1ρs(p0/pi0,…,1,…,pn/pi0)∈H∘}\displaystyle=\inf\big\{|z|\ \big|z\in H^{\times},z^{-1}\rho_{s}(p_{0}/p_{i_{0}},\dots,1,\dots,p_{n}/p_{i_{0}})\in H^{\circ}\big\}
=|ρr​(p0,…,pn)pi0|\displaystyle=\left|\frac{\rho_{r}(p_{0},\dots,p_{n})}{p_{i_{0}}}\right|
=|ρr​(p0,…,pn)|maxi⁑{|pi|}.\displaystyle=\frac{|\rho_{r}(p_{0},\dots,p_{n})|}{\max_{i}\{|p_{i}|\}}.

We call this the canonical metric of π’ͺ​(1)an{\mathcal{O}}(1)^{{\text{\rm an}}} and we denote it by βˆ₯β‹…βˆ₯can\|\cdot\|_{{\operatorname{can}}}.

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