ScalingStacks

Proof. [027W]

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Proof.

Clearly we may assume that l≠0l\not=0. Let us begin with the following claim:

Claim 4.2.1.

For any ϵ′>0{\epsilon^{\prime}}>0, there are a positive integer NN and sN∈H0​(X,L⊗N)s_{N}\in H^{0}(X,L^{\otimes N}) such that

sN|Y=l⊗Nand‖sN‖hN≤eN​ϵ′​(‖l‖Y,h)N.\left.{s_{N}}\right|_{{Y}}=l^{\otimes N}\quad\text{and}\quad\|s_{N}\|_{h^{N}}\leq e^{N{\epsilon^{\prime}}}(\|l\|_{Y,h})^{N}.
Proof.

By using Proposition 3.10, we can find a positive integer aa such that L⊗aL^{\otimes a} is very ample and

|.|ha​(x)≤|.|haquot​(x)≤ea​ϵ′/2​|.|ha​(x)|\raisebox{1.72218pt}{.}|_{h^{a}}(x)\leq|\raisebox{1.72218pt}{.}|^{\mathrm{quot}}_{h^{a}}(x)\leq e^{a{\epsilon^{\prime}/2}}|\raisebox{1.72218pt}{.}|_{h^{a}}(x)

for all x∈Xanx\in X^{\mathrm{an}}. We set h′={|.|haquot​(x)}h^{\prime}=\{|\raisebox{1.72218pt}{.}|^{\mathrm{quot}}_{h^{a}}(x)\}. Then, the above inequalities means that

(9) |.|ha​(x)≤|.|h′​(x)≤ea​ϵ′/2​|.|ha​(x)|\raisebox{1.72218pt}{.}|_{h^{a}}(x)\leq|\raisebox{1.72218pt}{.}|_{h^{\prime}}(x)\leq e^{a{\epsilon^{\prime}/2}}|\raisebox{1.72218pt}{.}|_{h^{a}}(x)

for all x∈Xanx\in X^{\mathrm{an}}. Further, by Theorem 4.1, there are a positive integer bb and sa​b∈H0​(X,L⊗a​b)s_{ab}\in H^{0}(X,L^{\otimes ab}) such that sa​b|Y=l⊗a​b\left.{s_{ab}}\right|_{{Y}}=l^{\otimes ab} and

‖sa​b‖h′b≤ea​b​ϵ′/2​(‖l⊗a‖Y,h′)b.\|s_{ab}\|_{{h^{\prime}}^{b}}\leq e^{ab{\epsilon^{\prime}/2}}(\|l^{\otimes a}\|_{Y,h^{\prime}})^{b}.

By (9),

‖l⊗a‖Y,h′≤ea​ϵ′/2​‖l⊗a‖Y,ha=ea​ϵ′/2​(‖l‖Y,h)a.\|l^{\otimes a}\|_{Y,h^{\prime}}\leq e^{a{\epsilon^{\prime}/2}}\|l^{\otimes a}\|_{Y,h^{a}}=e^{a{\epsilon^{\prime}/2}}(\|l\|_{Y,h})^{a}.

Moreover, as |.|ha​b​(x)≤|.|h′b​(x)|\raisebox{1.72218pt}{.}|_{h^{ab}}(x)\leq|\raisebox{1.72218pt}{.}|_{{h^{\prime}}^{b}}(x) by (9), we have ‖sa​b‖ha​b≤‖sa​b‖h′b\|s_{ab}\|_{h^{ab}}\leq\|s_{ab}\|_{{h^{\prime}}^{b}}, so that

‖sa​b‖ha​b\displaystyle\|s_{ab}\|_{h^{ab}} ≤‖sa​b‖h′b≤ea​b​ϵ′/2​(‖l⊗a‖Y,h′)b\displaystyle\leq\|s_{ab}\|_{{h^{\prime}}^{b}}\leq e^{ab{\epsilon^{\prime}/2}}(\|l^{\otimes a}\|_{Y,h^{\prime}})^{b}
≤ea​b​ϵ′/2​(ea​ϵ′/2​(‖l‖Y,h)a)b≤ea​b​ϵ′​(‖l‖Y,h)a​b.\displaystyle\leq e^{ab{\epsilon^{\prime}/2}}(e^{a{\epsilon^{\prime}/2}}(\|l\|_{Y,h})^{a})^{b}\leq e^{ab{\epsilon^{\prime}}}(\|l\|_{Y,h})^{ab}.

Therefore, if we set N=a​bN=ab, then we have the assertion of the claim. ∎

Since LL is ample, by Corollary 1.2, the above claim is actually equivalent to the assertion of the theorem. Thus the theorem is proved. ∎

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