ScalingStacks

4.2 [035K]

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4.2

We will define some local invariants in x∈Xanx\in{X^{\rm an}}. On an open affine neighbourhood U=Spec⁡(A)U={\rm Spec}(A), the point xx is given by a multiplicative seminorm pp on AA and we often write |f⁡(x)|:=p⁡(f)|f(x)|:=p(f) for f∈Af\in A. Dividing out the prime ideal I:={f∈A∣p⁡(f)=0}I:=\{f\in A\mid p(f)=0\}, we get a multiplicative norm on the integral domain B:=A/IB:=A/I which extends to an absolute value ||x|\phantom{a}|_{x} on the quotient field of BB. The completion of this field is denoted by ℋ⁡(x){\mathscr{H}}(x). It does not depend on the choice of UU and it may be also constructed analytically. The absolute value of ℋ⁡(x){\mathscr{H}}(x) is denoted by |⁣||\phantom{a}| as it extends the given absolute value on KK. Note that the completed residue field ℋ⁡(x){\mathscr{H}}(x) of xx remains the same if we replace the ambient variety XX by the Zariski closure of xx in XX.

Let s⁡(x)s(x) be the transcendence degree of the residue field of ℋ⁡(x){\mathscr{H}}(x) over K~{\tilde{K}}. The quotient of the value group of ℋ⁡(x){\mathscr{H}}(x) by Γ\Gamma is a finitely generated abelian group and we denote its ℚ{\mathbb{Q}}-rank by t⁡(x)t(x). Finally, we set d⁡(x):=s⁡(x)+t⁡(x)d(x):=s(x)+t(x). By [Be90], Proposition 9.1.3, we have dim(X)=dim(V)=supx∈Vd⁡(x)\dim(X)=\dim(V)=\sup_{x\in V}d(x) for every open subset VV of Xan{X^{\rm an}}.

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