ScalingStacks

1.1.5. [0252]

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1.1.5.

Given a continuous metric hh on LL, the metric induces for each integer n⩾1n\geqslant 1 a continuous metric on L⊗nL^{\otimes n} which we denote by hnh^{n}: for any point x∈Xanx\in X^{\mathrm{an}} and any local basis ω\omega of LL over a Zariski open neighborhood of xx one has

|ω⊗n|hn​(x)=|ω|h​(x)n.|\omega^{\otimes n}|_{h^{n}}(x)=|\omega|_{h}(x)^{n}.

Note that for any section s∈H0​(X,L)s\in H^{0}(X,L) one has ‖s⊗n‖hn=‖s‖hn\|s^{\otimes n}\|_{h^{n}}=\|s\|_{h}^{n}. By convention, h0h^{0} denotes the trivial metric on L⊗0=𝒪XL^{\otimes 0}=\mathscr{O}_{X}, namely |𝟏|h0​(x)=1|\mathbf{1}|_{h^{0}}(x)=1 for any x∈Xanx\in X^{\mathrm{an}}, where 𝟏\mathbf{1} denotes the section of unity of 𝒪X\mathscr{O}_{X}.

Conversely, given a continuous metric g={|.|g​(x)}x∈Xang=\{|\raisebox{1.72218pt}{.}|_{g}(x)\}_{x\in X^{\mathrm{an}}} on L⊗nL^{\otimes n}, there is a unique continuous metric hh on LL such that hn=gh^{n}=g. We denote by g1/ng^{1/n} this metric. This observation allows to define continuous metrics on an element in Pic⁡(X)⊗ℚ\mathrm{Pic}(X)\otimes\mathbb{Q} as follows. Given M∈Pic⁡(X)⊗ℚM\in\operatorname{Pic}(X)\otimes\mathbb{Q}, we denote by Γ⁡(M)\Gamma(M) the subsemigroup of ℕ≥1\mathbb{N}_{\geq 1} of all positive integers nn such that M⊗n∈Pic⁡(X)M^{\otimes n}\in\operatorname{Pic}(X). We call continuous metric on MM any family g=(gn)n∈Γ⁡(M)g=(g_{n})_{n\in\Gamma(M)} with gng_{n} being a continuous metric on M⊗nM^{\otimes n}, such that gnm=gm​ng_{n}^{m}=g_{mn} for any n∈Γ⁡(M)n\in\Gamma(M) and any m∈ℕ≥1m\in\mathbb{N}_{\geq 1}. Note that the family g=(gn)n∈Γ⁡(M)g=(g_{n})_{n\in\Gamma(M)} is uniquely determined by any of its elements. In fact, given an element n∈Γ⁡(M)n\in\Gamma(M), one has gm=gm​n1/n=(gnm)1/ng_{m}=g_{mn}^{1/n}=(g_{n}^{m})^{1/n} for any m∈Γ⁡(M)m\in\Gamma(M). In particular, for any positive rational number p/qp/q, the family gp/q=(gN​n​p1/N​q)n∈Γ⁡(M⊗(p/q))g^{p/q}=(g_{Nnp}^{1/Nq})_{n\in\Gamma(M^{\otimes(p/q)})} is a continuous metric on M⊗(p/q)M^{\otimes(p/q)}, where NN is a positive integer such that M⊗N∈Pic⁡(X)M^{\otimes N}\in\operatorname{Pic}(X), and the metric gp/qg^{p/q} does not depend on the choice of the positive integer NN.

Let MM be an element in Pic⁡(X)⊗ℚ\operatorname{Pic}(X)\otimes\mathbb{Q} equipped with a continuous metric g=(gn)n∈Γ⁡(M)g=(g_{n})_{n\in\Gamma(M)}. By abuse of notation, for n∈Γ⁡(M)n\in\Gamma(M) we also use the expression gng^{n} to denote the continuous metric gng_{n} on M⊗nM^{\otimes n}.

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