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4. Measures [05AE]

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4. Measures

We recall the real Monge-Ampère operator which associates to a convex function a positive Borel measure. Then we introduce the Chambert-Loir measure on the generic fibres of admissible formal schemes and on paracompact strictly KK-analytic spaces. Chambert-Loir introduced these measures in [Cha06] on the analytification XanX^{\textup{an}} of a proper variety XX over KK under the assumption that KK has a countable dense subfield and associates to a family of semipositive metrized line bundles a positive Radon measure. This was later extended by Gubler to the case of an algebraically closed base field in [Gub07]. Using the local approach to metrics from section 3, it is now possible to define Monge-Ampère measures locally. Note that there is also a local approach by Chambert-Loir and Ducros in [CD] which associates a measure to a metric which is locally psh-approximable. However it is not known whether a semipositive metric is locally psh-approximable. In this section we assume that the non-archimedean complete base field KK is algebraically closed which is no restriction as one can always reduce to this case by base change (see Remark 4.16).

Definition 4.1.

Let Ω⊆ℝn\Omega\subseteq\mathbb{R}^{n} be bounded, open and convex and denote by λ\lambda the standard Lebesgue measure on ℝn\mathbb{R}^{n} and by ⟨⋅,⋅⟩\langle\cdot,\cdot\rangle the standard scalar product on ℝn\mathbb{R}^{n}. Let hh be a convex function on Ω\Omega and x0∈Ωx_{0}\in\Omega. We define the gradient image of x0x_{0} under hh to be

∇h(x0):={p∈ℝn|∀x∈Ω:h(x0)+⟨x−x0,p⟩≤h(x)}\nabla h(x_{0}):=\left\{p\in\mathbb{R}^{n}\;\Big|\;\forall x\in\Omega\;:\;h(x_{0})+\langle x-x_{0},p\rangle\leq h(x)\right\}

and for E⊆ΩE\subseteq\Omega

∇h​(E):=⋃x0∈E∇h​(x0).\nabla h(E):=\bigcup_{x_{0}\in E}\nabla h(x_{0}).

Note that if EE is a Borel set, the same is true for ∇h​(E)\nabla h(E). Finally we define the Monge-Ampère measure associated to hh by

MA⁡(h)​(E):=λ⁡(∇h​(E))\MA(h)(E):=\lambda(\nabla h(E))

for all Borel sets E⊆ΩE\subseteq\Omega. It is indeed a measure on the Borel σ\sigma-algebra, for details see [RT77, Section 2]. The real Monge-Ampère operator is continuous in the sense that if (un)n∈ℕ(u_{n})_{n\in\mathbb{N}} is a sequence of convex functions on Ω\Omega converging pointwise to a convex function uu then (MA⁡(un))n∈ℕ(\MA(u_{n}))_{n\in\mathbb{N}} converges weakly to MA⁡(u)\MA(u). If hh is two times continuously differentiable then MA⁡(h)=detD2​h⋅λ\MA(h)=\det D^{2}h\cdot\lambda.

Definition 4.2.

In [Con99, Definition 2.2.2] Conrad defined the notion of irreducibility for analytic spaces which we recall here. Let XX be a paracompact strictly KK-analytic space and p:X~→Xp:\tilde{X}\rightarrow X the normalization of XX ([Con99, 2.1]). Then the irreducible components of XX are defined to be the sets Xi:=p⁡(X~i)X_{i}:=p(\tilde{X}_{i}) where X~i\tilde{X}_{i} are the connected components of X~\tilde{X}. The space XX is said to be irreducible if it has a unique irreducible component. By [Con99, Lemma 2.2.3] XX is irreducible if and only if it can not non trivially be written as a union of two closed strictly KK-analytic subsets.
Let YY be an irreducible component of XX and V=ℳ⁡(𝒜)V=\mathscr{M}(\mathscr{A}) an affinoid domain with Y∩V≠∅Y\cap V\neq\emptyset. Then by [Con99, Corollary 2.2.9] there is an irreducible component Y′Y^{\prime} of VV which is contained in V∩YV\cap Y. Then Y′Y^{\prime} corresponds to a minimal prime ideal 𝔭\mathfrak{p} of 𝒜\mathscr{A} and hence to an irreducible component of Spec⁡(𝒜)\Spec(\mathscr{A}). We define the multiplicity of YY to be the multiplicity of this component. Note that this does not depend on the choice of VV and Y′Y^{\prime}: If V′=ℳ⁡(ℬ)⊆VV^{\prime}=\mathscr{M}(\mathscr{B})\subseteq V and 𝔭′\mathfrak{p}^{\prime} is a minimal prime ideal of ℬ\mathscr{B} lying over 𝔭\mathfrak{p} then ℬ/𝔭​ℬ\mathscr{B}/\mathfrak{p}\mathscr{B} is reduced by [BGR84, Corollary 7.3.2/10] as it induces an affinoid domain in ℳ⁡(𝒜/𝔭)\mathscr{M}(\mathscr{A}/\mathfrak{p}) which is reduced. Hence also ℬ𝔭′/𝔭​ℬ𝔭′\mathscr{B}_{\mathfrak{p}^{\prime}}/\mathfrak{p}\mathscr{B}_{\mathfrak{p}^{\prime}} is reduced and since ℬ𝔭′\mathscr{B}_{\mathfrak{p}^{\prime}} is a local ring of dimension 0, this implies 𝔭′​ℬ𝔭′=𝔭​ℬ𝔭′\mathfrak{p}^{\prime}\mathscr{B}_{\mathfrak{p}^{\prime}}=\mathfrak{p}\mathscr{B}_{\mathfrak{p}^{\prime}}. Hence by [Ful98, Lemma A.4.1] the multiplicity of the irreducible component corresponding to 𝔭\mathfrak{p} is equal to that of the irreducible component corresponding to 𝔭′\mathfrak{p}^{\prime}.
Let φ:X→Y\varphi:X\rightarrow Y be a proper surjective morphism of irreducible and reduced strictly KK-analytic spaces. If dim(Y)<dim(X)\dim(Y)<\dim(X) we set deg⁡(φ)=0\deg(\varphi)=0. Otherwise φ\varphi is a finite morphism outside a lower dimensional analytic subset WW of YY. Let ℳ⁡(𝒜′)\mathscr{M}(\mathscr{A}^{\prime}) be an affinoid domain in Y∖WY\setminus W, VV an irreducible component of Spec⁡(𝒜′)\Spec(\mathscr{A}^{\prime}) and ℳ⁡(𝒜):=φ−1​(ℳ⁡(𝒜′))\mathscr{M}(\mathscr{A}):=\varphi^{-1}(\mathscr{M}(\mathscr{A}^{\prime})) then Spec⁡(𝒜)→Spec⁡(𝒜′)\Spec(\mathscr{A})\rightarrow\Spec(\mathscr{A}^{\prime}) is finite and we define deg⁡(φ)\deg(\varphi) to be the sum of the degrees of the irreducible components of Spec⁡(𝒜)\Spec(\mathscr{A}) over VV. As explained in [Gub98, 2.6] this again does not depend on the choices.

4.3 Monge-Ampère measure for line bundles on admissible formal schemes
Let 𝔛\mathfrak{X} be an admissible formal scheme over K∘K^{\circ} of dimension n+1n+1 with generic fibre XX. Our goal is to introduce a Monge-Ampère measure on XX for formal line bundles 𝔏1,…,𝔏n\mathfrak{L}_{1},...,\mathfrak{L}_{n} on 𝔛\mathfrak{X}. We assume first that XX is irreducible and reduced and that the special fibre of 𝔛\mathfrak{X} is reduced. Then the non-archimedean Monge-Ampère measure on XX with respect to these metrized line bundles is defined as

c1​(𝔏1)∧…∧c1​(𝔏n):=∑Y∈irr⁡(𝔛~)Y​ properdeg𝔏1,…,𝔏n⁡(Y)⋅δζY,c_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n}):=\sum_{\begin{subarray}{c}Y\in\irr(\tilde{\mathfrak{X}})\\ Y\text{ proper}\end{subarray}}\Deg_{\mathfrak{L}_{1},...,\mathfrak{L}_{n}}(Y)\cdot\delta_{\zeta_{Y}},

where δζY\delta_{\zeta_{Y}} denotes the Dirac-measure at the unique point ζY\zeta_{Y} which is mapped to the generic point of the proper irreducible component YY under the reduction map (cf. [Ber90, Proposition 2.4.4]).

If 𝔛\mathfrak{X} has irreducible and reduced generic fibre but no longer reduced special fibre, there is a canonical admissible formal model 𝔛′\mathfrak{X}^{\prime} of XX with reduced special fibre together with a finite morphism ι:𝔛′→𝔛\iota:\mathfrak{X}^{\prime}\rightarrow\mathfrak{X} which restricts to the identity on XX which can be constructed as follows (cf. [Gub98, Definition 3.10]). Choose a cover (𝔘i=Spf⁡(Ai))i∈I(\mathfrak{U}_{i}=\Spf(A_{i}))_{i\in I} of 𝔛\mathfrak{X} by affine formal subschemes. Define 𝒜i:=A⊗K∘K\mathscr{A}_{i}:=A\otimes_{K^{\circ}}K. If Spf⁡(B)⊆Spf⁡(Ai)\Spf(B)\subseteq\Spf(A_{i}) is a formal open subscheme for some i∈Ii\in I then Ai→BA_{i}\rightarrow B induces a morphism 𝒜i∘→ℬ∘\mathscr{A}_{i}^{\circ}\rightarrow\mathscr{B}^{\circ} for ℬ:=B⊗K∘K\mathscr{B}:=B\otimes_{K^{\circ}}K. Hence by standard arguments we can glue the Spf⁡(𝒜i∘)\Spf(\mathscr{A}_{i}^{\circ}) to obtain 𝔛′\mathfrak{X}^{\prime} and the canonical morphisms Ai→𝒜i∘A_{i}\rightarrow\mathscr{A}_{i}^{\circ} induce the morphism 𝔛′→𝔛\mathfrak{X}^{\prime}\rightarrow\mathfrak{X}. We then define

c1​(𝔏1)∧…∧c1​(𝔏n):=(ιan)∗​(c1​(ι∗​𝔏1)∧…∧c1​(ι∗​𝔏n)).c_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n}):=(\iota^{\textup{an}})_{\ast}(c_{1}(\iota^{\ast}\mathfrak{L}_{1})\wedge...\wedge c_{1}(\iota^{\ast}\mathfrak{L}_{n})).

In the general case, let X=∑jmj​XjX=\sum_{j}m_{j}X_{j} be the decomposition of the generic fibre into prime cycles. By [Gub98, Proposition 3.3] the closure X¯j\overline{X}_{j} of XjX_{j} in 𝔛\mathfrak{X} is an admissible formal scheme with irreducible and reduced generic fibre XjX_{j}. We define

c1​(𝔏1)∧…∧c1​(𝔏n):=∑jmj⋅c1​(𝔏1|X¯j)∧…∧c1​(𝔏n|X¯j)c_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n}):=\sum_{j}m_{j}\cdot c_{1}\left(\mathfrak{L}_{1}\Big|_{\overline{X}_{j}}\right)\wedge...\wedge c_{1}\left(\mathfrak{L}_{n}\Big|_{\overline{X}_{j}}\right)

as a measure on XX.

Remark 4.4.

There is a close connection of the Monge-Ampère measure with the intersection product on formal schemes as defined in [Gub98]: Assume that 𝔛\mathfrak{X} has irreducible, reduced and boundaryless generic fibre and reduced special fibre. In addition to 𝔏1,…,𝔏n\mathfrak{L}_{1},...,\mathfrak{L}_{n} let 𝔏0\mathfrak{L}_{0} be a formal line bundle on 𝔛\mathfrak{X} which is trivial on the generic fibre and set f:=−log⁡‖1‖f:=-\log\|1\| where ∥⋅∥\|\cdot\| is the formal metric induced by 𝔏0\mathfrak{L}_{0}. Suppose that ff has compact support and let D:=div⁡(1)D:=\Div(1) be the Cartier divisor on 𝔛\mathfrak{X} induced by 11 as in [Gub98, Remark 3.1]. We examine the Weil divisor cyc⁡(D)\cyc(D) associated to DD as defined in [Gub98, §3]. Since 𝔏0\mathfrak{L}_{0} is trivial on the generic fibre, the horizontal part of cyc⁡(D)\cyc(D) is zero while the vertical part is by definition ([Gub98, 3.8]) given by ∑Y∈irr⁡(𝔛~)f⁡(ζY)⋅Y\sum_{Y\in\irr(\tilde{\mathfrak{X}})}f(\zeta_{Y})\cdot Y. Now since 𝔛an\mathfrak{X}^{\textup{an}} has no boundary, every irreducible component of 𝔛~\tilde{\mathfrak{X}} is proper by Corollary A.4 and together with the definition of the intersection product ([Gub98, §4]) we obtain

∫𝔛anf​c1​(𝔏1)∧…∧c1​(𝔏n)=∑Y∈irr⁡(𝔛~)f⁡(ζY)⋅deg𝔏1,…,𝔏n⁡(Y)=deg𝔏1,…,𝔏n⁡(cyc⁡(D)).\int_{\mathfrak{X}^{\textup{an}}}fc_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n})=\sum_{Y\in\irr(\tilde{\mathfrak{X}})}f(\zeta_{Y})\cdot\Deg_{\mathfrak{L}_{1},...,\mathfrak{L}_{n}}(Y)=\Deg_{\mathfrak{L}_{1},...,\mathfrak{L}_{n}}(\cyc(D)).
Proposition 4.5.

The measure defined above has the following properties:

  1. i)

    c1​(𝔏1)∧…∧c1​(𝔏n)c_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n}) is a discrete measure (i.e. of the form ∑x∈Sλx​δx\sum_{x\in S}\lambda_{x}\delta_{x} with S⊆XS\subseteq X a closed discrete subset, λx∈ℝ\lambda_{x}\in\mathbb{R} and δx\delta_{x} the Dirac-measure at xx) whose support is contained in the relative interior of XX over KK (in the sense of [Ber93, 1.5]).

  2. ii)

    c1​(𝔏1)∧…∧c1​(𝔏n)c_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n}) is multilinear and symmetric in 𝔏1,…,𝔏n\mathfrak{L}_{1},...,\mathfrak{L}_{n}.

  3. iii)

    Let φ:𝔛′→𝔛\varphi:\mathfrak{X}^{\prime}\rightarrow\mathfrak{X} be a proper morphism of admissible formal schemes over K∘K^{\circ} with irreducible and reduced generic fibres of dimension nn such that the induced morphism on the generic fibres is surjective. Then for formal line bundles 𝔏1,…,𝔏n\mathfrak{L}_{1},...,\mathfrak{L}_{n} on 𝔛\mathfrak{X} we have

    (φan)∗​(c1​(φ∗​𝔏1)∧…∧c1​(φ∗​𝔏n))=deg⁡(φan)​c1​(𝔏1)∧…∧c1​(𝔏n).(\varphi^{\textup{an}})_{\ast}\left(c_{1}(\varphi^{\ast}\mathfrak{L}_{1})\wedge...\wedge c_{1}(\varphi^{\ast}\mathfrak{L}_{n})\right)=\Deg(\varphi^{\textup{an}})c_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n}).
Proof.

ii) follows from symmetry and multilinearity of the intersection product ([Ful98, Proposition 2.5]). For iii) we reduce first to the case where 𝔛′\mathfrak{X}^{\prime} and 𝔛\mathfrak{X} have reduced special fibre. Let 𝔜′\mathfrak{Y}^{\prime} respectively 𝔜\mathfrak{Y} be the canonical formal models with reduced special fibre as in 4. This construction is functorial and we obtain a commutative diagram

𝔜′\textstyle{\mathfrak{Y}^{\prime}\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}ι′\scriptstyle{\iota^{\prime}}φ′\scriptstyle{\varphi^{\prime}}𝔜\textstyle{\mathfrak{Y}\ignorespaces\ignorespaces\ignorespaces\ignorespaces}ι\scriptstyle{\iota}𝔛′\textstyle{\mathfrak{X}^{\prime}\ignorespaces\ignorespaces\ignorespaces\ignorespaces}φ\scriptstyle{\varphi}𝔛\textstyle{\mathfrak{X}}

Assuming that we know the claim for reduced special fibres we obtain

deg⁡(φan)​c1​(𝔏1)∧…∧c1​(𝔏n)\displaystyle\Deg(\varphi^{\textup{an}})c_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n}) =deg⁡(φ′a​n)​(ιan)∗​(c1​(ι∗​𝔏1)∧…∧c1​(ι∗​𝔏n))\displaystyle=\Deg(\varphi^{\prime an})(\iota^{\textup{an}})_{\ast}(c_{1}(\iota^{\ast}\mathfrak{L}_{1})\wedge...\wedge c_{1}(\iota^{\ast}\mathfrak{L}_{n}))
=(ιan)∗​(φ′a​n)∗​(c1​(φ′⁣∗​ι∗​𝔏1)∧…∧c1​(φ′⁣∗​ι∗​𝔏n))\displaystyle=(\iota^{\textup{an}})_{\ast}(\varphi^{\prime an})_{\ast}(c_{1}(\varphi^{\prime\ast}\iota^{\ast}\mathfrak{L}_{1})\wedge...\wedge c_{1}(\varphi^{\prime\ast}\iota^{\ast}\mathfrak{L}_{n}))
=(φan)∗​(ι′a​n)∗​(c1​(ι′⁣∗​φ∗​𝔏1)∧…∧c1​(ι′⁣∗​φ∗​𝔏n))\displaystyle=(\varphi^{\textup{an}})_{\ast}(\iota^{\prime an})_{\ast}(c_{1}(\iota^{\prime\ast}\varphi^{\ast}\mathfrak{L}_{1})\wedge...\wedge c_{1}(\iota^{\prime\ast}\varphi^{\ast}\mathfrak{L}_{n}))
=(φan)∗​(c1​(φ∗​𝔏1)∧…∧c1​(φ∗​𝔏n)).\displaystyle=(\varphi^{\textup{an}})_{\ast}(c_{1}(\varphi^{\ast}\mathfrak{L}_{1})\wedge...\wedge c_{1}(\varphi^{\ast}\mathfrak{L}_{n})).

So from now on assume that 𝔛′\mathfrak{X}^{\prime} and 𝔛\mathfrak{X} have reduced special fibre. Let YY be an irreducible component of 𝔛~\tilde{\mathfrak{X}} with corresponding Shilov point ζY\zeta_{Y}. Let ζ1,…,ζr\zeta_{1},...,\zeta_{r} be the preimages of ζY\zeta_{Y} under φan\varphi^{\textup{an}} with corresponding irreducible components Y1,…,YrY_{1},...,Y_{r} of 𝔛′~\tilde{\mathfrak{X}^{\prime}}. If YY is proper then clearly all the YiY_{i} are proper. If on the other hand one of the YiY_{i} is proper then YY is proper by [GW10, Proposition 12.59]. In this case we can use the projection formula to calculate:

(φan)∗​(c1​(φ∗​𝔏1)∧…∧c1​(φ∗​𝔏n))​(ζY)\displaystyle(\varphi^{\textup{an}})_{\ast}\left(c_{1}(\varphi^{\ast}\mathfrak{L}_{1})\wedge...\wedge c_{1}(\varphi^{\ast}\mathfrak{L}_{n})\right)(\zeta_{Y}) =∑i=1rc1​(φ∗​𝔏1)∧…∧c1​(φ∗​𝔏n)​(ζi)\displaystyle=\sum_{i=1}^{r}c_{1}(\varphi^{\ast}\mathfrak{L}_{1})\wedge...\wedge c_{1}(\varphi^{\ast}\mathfrak{L}_{n})(\zeta_{i})
=∑i=1rdeg𝔏1,…,𝔏n⁡(φ~∗​Yi)\displaystyle=\sum_{i=1}^{r}\Deg_{\mathfrak{L}_{1},...,\mathfrak{L}_{n}}(\tilde{\varphi}_{\ast}Y_{i})
=∑i=1rdeg𝔏1,…,𝔏n(Y)⋅[K~(Yi):K~(Y)]\displaystyle=\sum_{i=1}^{r}\Deg_{\mathfrak{L}_{1},...,\mathfrak{L}_{n}}(Y)\cdot[\tilde{K}(Y_{i}):\tilde{K}(Y)]

As already mentioned in Definition 4.2, φan\varphi^{\textup{an}} is finite outside a lower dimensional analytic subset. Hence we may apply equation (3) in the proof of [Gub98, Proposition 4.5] to see that the last term in the display equals deg⁡(φan)⋅c1​(𝔏1)∧…∧c1​(𝔏n)​(ζY)\Deg(\varphi^{\textup{an}})\cdot c_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n})(\zeta_{Y}).
On the other hand, if YY is an irreducible component of 𝔛~′\tilde{\mathfrak{X}}^{\prime} whose image is not an irreducible component of 𝔛~\tilde{\mathfrak{X}} then its degree with respect to the line bundles φ∗​𝔏1,…,φ∗​𝔏n\varphi^{\ast}\mathfrak{L}_{1},...,\varphi^{\ast}\mathfrak{L}_{n} is 00 by the projection formula, as the image is of lower dimension. This proves iii).
For i) let 𝔛an=∑jmj​Xj\mathfrak{X}^{\textup{an}}=\sum_{j}m_{j}X_{j} be the decomposition into prime cycles. It is then enough to prove the claim for each XjX_{j} and by definition of the measure we may hence assume that 𝔛\mathfrak{X} has irreducible and reduced generic fibre and reduced special fibre. Let SS be the set of all ζY\zeta_{Y} where YY is a proper irreducible component of 𝔛~\tilde{\mathfrak{X}} with deg𝔏1,…,𝔏n⁡(Y)≠0\Deg_{\mathfrak{L}_{1},...,\mathfrak{L}_{n}}(Y)\neq 0. Then SS is discrete as red−1⁡(Y)\red^{-1}(Y) is an open neighbourhood of ζY\zeta_{Y} which does not contain any other points of SS. Furthermore XX is the union of all red−1⁡(Y)\red^{-1}(Y) where YY runs over all irreducible components of 𝔛~\tilde{\mathfrak{X}} and as all of these sets contain at most one point of SS and by paracompactness of XX, every x∉Sx\notin S has an open neighbourhood which does not intersect SS and hence SS is closed. By definition c1​(𝔏1)∧…∧c1​(𝔏n)c_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n}) is of the desired form and its support is contained in the relative interior of XX over KK by Corollary A.4. ∎

Lemma 4.6.

Let 𝔛\mathfrak{X} be an admissible formal scheme over K∘K^{\circ} of dimension n+1n+1 with boundaryless generic fibre 𝔛an\mathfrak{X}^{\textup{an}} and L0,…,LnL_{0},...,L_{n} line bundles on 𝔛an\mathfrak{X}^{\textup{an}} endowed with formal metrics corresponding to the models 𝔏0,…,𝔏n\mathfrak{L}_{0},...,\mathfrak{L}_{n} on 𝔛\mathfrak{X}. Suppose that L0=L1=𝒪𝔛anL_{0}=L_{1}=\mathcal{O}_{\mathfrak{X}^{\textup{an}}}, denote by ∥⋅∥0\|\cdot\|_{0} and ∥⋅∥1\|\cdot\|_{1} the metrics on L0L_{0} respectively L1L_{1} and set f0:=−log⁡‖1‖0f_{0}:=-\log\|1\|_{0}, f1:=−log⁡‖1‖1f_{1}:=-\log\|1\|_{1}. Suppose that f0f_{0} and f1f_{1} have compact support. Then

∫𝔛anf0​c1​(𝔏1)∧…∧c1​(𝔏n)=∫𝔛anf1​c1​(𝔏0)∧c1​(𝔏2)∧…∧c1​(𝔏n).\int_{\mathfrak{X}^{\textup{an}}}f_{0}\;c_{1}(\mathfrak{L_{1}})\wedge...\wedge c_{1}(\mathfrak{L}_{n})=\int_{\mathfrak{X}^{\textup{an}}}f_{1}\;c_{1}(\mathfrak{L}_{0})\wedge c_{1}(\mathfrak{L}_{2})\wedge...\wedge c_{1}(\mathfrak{L}_{n}).
Proof.

Let 𝔛an=∑jmj​Xj\mathfrak{X}^{\textup{an}}=\sum_{j}m_{j}X_{j} be the decomposition into prime cycles. It is enough to prove the claim for the closures X¯j\overline{X}_{j} of XjX_{j} in 𝔛\mathfrak{X}. We may hence assume that 𝔛an\mathfrak{X}^{\textup{an}} is irreducible and reduced. Furthermore by passing to a dominating model as in 4, we may assume that the special fibre 𝔛~\tilde{\mathfrak{X}} of 𝔛\mathfrak{X} is reduced. As 𝔛an\mathfrak{X}^{\textup{an}} has no boundary, every irreducible component of 𝔛~\tilde{\mathfrak{X}} is proper by Corollary A.4 and hence using commutativity of the intersection product ([Gub98, Theorem 5.9]) we obtain

∫𝔛anf0​c1​(𝔏1)∧…∧c1​(𝔏n)\displaystyle\int_{\mathfrak{X}^{\textup{an}}}f_{0}\;c_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n}) =∑Y∈irr⁡(𝔛~)f0​(ζY)⋅deg𝔏1,…,𝔏n⁡(Y)\displaystyle=\sum_{\begin{subarray}{c}Y\in\irr(\tilde{\mathfrak{X}})\end{subarray}}f_{0}(\zeta_{Y})\cdot\Deg_{\mathfrak{L}_{1},...,\mathfrak{L}_{n}}(Y)
=deg𝔏1,…,𝔏n⁡(cyc⁡(div𝔏0⁡(1)))\displaystyle=\Deg_{\mathfrak{L}_{1},...,\mathfrak{L}_{n}}\left(\cyc(\Div_{\mathfrak{L}_{0}}(1))\right)
=deg𝔏0,𝔏2,…,𝔏n⁡(cyc⁡(div𝔏1⁡(1)))\displaystyle=\Deg_{\mathfrak{L}_{0},\mathfrak{L}_{2},...,\mathfrak{L}_{n}}\left(\cyc(\Div_{\mathfrak{L}_{1}}(1))\right)
=∑Y∈irr⁡(𝔛~)f1​(ζY)⋅deg𝔏0,𝔏2,…,𝔏n⁡(Y)\displaystyle=\sum_{\begin{subarray}{c}Y\in\irr(\tilde{\mathfrak{X}})\end{subarray}}f_{1}(\zeta_{Y})\cdot\Deg_{\mathfrak{L}_{0},\mathfrak{L}_{2},...,\mathfrak{L}_{n}}(Y)
=∫𝔛anf1​c1​(𝔏0)∧c1​(𝔏2)∧…∧c1​(𝔏n).\displaystyle=\int_{\mathfrak{X}^{\textup{an}}}f_{1}\;c_{1}(\mathfrak{L}_{0})\wedge c_{1}(\mathfrak{L}_{2})\wedge...\wedge c_{1}(\mathfrak{L}_{n}).

∎

Definition 4.7.

Let XX be an nn-dimensional paracompact strictly KK-analytic space and L¯1,…,L¯n\overline{L}_{1},...,\overline{L}_{n} formally metrized line bundles on XX. Let 𝔛\mathfrak{X} be a formal model of XX on which there exist formal models 𝔏1,…,𝔏n\mathfrak{L}_{1},...,\mathfrak{L}_{n} of L¯1,…,L¯n\overline{L}_{1},...,\overline{L}_{n}. The existence of such a formal model follows from Remark 3.2. We then define

c1​(L¯1)∧…∧c1​(L¯n):=c1​(𝔏1)∧…∧c1​(𝔏n).c_{1}(\overline{L}_{1})\wedge...\wedge c_{1}(\overline{L}_{n}):=c_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n}).

Note that this definition is independent of the choice of 𝔛\mathfrak{X} and 𝔏1,…,𝔏n\mathfrak{L}_{1},...,\mathfrak{L}_{n} by the projection formula. If the metrics on L¯1,…,L¯n\overline{L}_{1},...,\overline{L}_{n} are semipositive then c1​(L¯1)∧…∧c1​(L¯n)c_{1}(\overline{L}_{1})\wedge...\wedge c_{1}(\overline{L}_{n}) is a positive measure.

Lemma 4.8.

Let W2W_{2} be a paracompact strictly KK-analytic space of dimension nn and W1⊆W2W_{1}\subseteq W_{2} a paracompact strictly KK-analytic subdomain of W2W_{2}. Then for formally metrized line bundles L¯1,…,L¯n\overline{L}_{1},...,\overline{L}_{n} on W2W_{2}, we have c1​(L¯1)∧…∧c1​(L¯n)=c1​(L¯1|W1)∧…∧c1​(L¯n|W1)c_{1}(\overline{L}_{1})\wedge...\wedge c_{1}(\overline{L}_{n})=c_{1}\left(\overline{L}_{1}\Big|_{W_{1}}\right)\wedge...\wedge c_{1}\left(\overline{L}_{n}\Big|_{W_{1}}\right) in the topological interior W1∘\overset{\circ}{W_{1}} of W1W_{1} in W2W_{2}.

Proof.

Let W2=∑jmj​XjW_{2}=\sum_{j}m_{j}X_{j} be the decomposition of W2W_{2} into prime cycles and for each jj let (Xi​j)i∈Ij(X_{ij})_{i\in I_{j}} be the irreducible components of W1W_{1} with Xi​j⊆Xj∩W1X_{ij}\subseteq X_{j}\cap W_{1}. Then W1=∑j,imj​Xi​jW_{1}=\sum_{j,i}m_{j}X_{ij} is the decomposition of W1W_{1} into prime cycles. Furthermore, the intersection of any two irreducible components of W1W_{1} does not contain a Shilov point as it is of lower dimension and hence does not meet the support of the measures of interest. By linearity in the irreducible components we may therefore assume that W1W_{1} and W2W_{2} are irreducible and reduced. Let 𝔛2\mathfrak{X}_{2} be a formal model of W2W_{2} with reduced special fibre on which there exist formal models of L¯1,…,L¯n\overline{L}_{1},...,\overline{L}_{n}. Let 𝔛1\mathfrak{X}_{1} be a formal model of W1W_{1} which exists by paracompactness of W1W_{1}, see Remark 3.2. After possibly blowing up, the inclusion W1↪W2W_{1}\hookrightarrow W_{2} induces a morphism ι:𝔛1→𝔛2\iota:\mathfrak{X}_{1}\rightarrow\mathfrak{X}_{2} ([Bos14, Theorem 8.4.3]). Let x∈W1∘x\in\overset{\circ}{W_{1}}. As both measures are discrete it is enough to show that they have the same mass at xx. Let Int⁡(Wi)\Int(W_{i}) denote the relative interior of WiW_{i} over KK in the sense of [Ber93, 1.5]. If x∈Int⁡(W2)x\in\Int(W_{2}) then x∈Int⁡(W1)x\in\Int(W_{1}) by [Ber93, Proposition 1.5.5 (ii)]. Conversely if x∈Int⁡(W1)x\in\Int(W_{1}) then there exists an affinoid neighbourhood VV of xx in W1W_{1} such that xx is in the relative interior of VV over KK. But VV is also a neighbourhood of xx in W2W_{2} as x∈W1∘x\in\overset{\circ}{W_{1}} and therefore x∈Int⁡(W2)x\in\Int(W_{2}). Hence x∈Int⁡(W1)x\in\Int(W_{1}) if and only if x∈Int⁡(W2)x\in\Int(W_{2}). If this is not the case then by definition of the measures and Corollary A.4, both of them are zero at xx. So assume that x∈Int⁡(W1)x\in\Int(W_{1}). Choose a locally finite cover (𝔘i)i∈I(\mathfrak{U}_{i})_{i\in I} of 𝔛1\mathfrak{X}_{1} by open affine formal subschemes and let 𝔘\mathfrak{U} be the union of all 𝔘i\mathfrak{U}_{i} which contain red⁡(x)\red(x). Then 𝔘\mathfrak{U} is an open and quasi-compact formal subscheme of 𝔛1\mathfrak{X}_{1}. Analogously choose a cover (𝔙i)i∈J(\mathfrak{V}_{i})_{i\in J} of 𝔛2\mathfrak{X}_{2} by open affine formal subschemes. As ι⁡(𝔘)\iota(\mathfrak{U}) is quasi-compact, there is a finite subcover of it. Let 𝔙\mathfrak{V} be the union of the sets in this subcover and add all 𝔙i\mathfrak{V}_{i} with red⁡(x)∈𝔙i\red(x)\in\mathfrak{V}_{i}. Then also 𝔙\mathfrak{V} is an open and quasi-compact formal subscheme of 𝔛2\mathfrak{X}_{2} and ι\iota induces a morphism 𝔘→𝔙\mathfrak{U}\rightarrow\mathfrak{V}. By [BL93, Corollary 5.4] there is an admissible formal blowing up 𝔙′→𝔙\mathfrak{V}^{\prime}\rightarrow\mathfrak{V} such that the induced morphism 𝔘′→𝔙′\mathfrak{U}^{\prime}\rightarrow\mathfrak{V}^{\prime} is an open immersion.
Let YY be an irreducible component of 𝔛~2\tilde{\mathfrak{X}}_{2} with corresponding divisorial point ζY=x\zeta_{Y}=x. Then Y⊆𝔙~Y\subseteq\tilde{\mathfrak{V}} by definition and hence we may calculate the mass of c1​(L¯1)∧…∧c1​(L¯n)c_{1}(\overline{L}_{1})\wedge...\wedge c_{1}(\overline{L}_{n}) at xx using 𝔙\mathfrak{V}. By Proposition 4.5 iii) we may also use 𝔙′\mathfrak{V}^{\prime}. So let Y′Y^{\prime} be the irreducible component of 𝔙~′\tilde{\mathfrak{V}}^{\prime} corresponding to xx. Since red⁡(x)∈𝔘~′\red(x)\in\tilde{\mathfrak{U}}^{\prime} we see that Y∩𝔘~′Y\cap\tilde{\mathfrak{U}}^{\prime} is an irreducible component of 𝔘~′\tilde{\mathfrak{U}}^{\prime}. Additionally, by Corollary A.4, Y′Y^{\prime} and Y′∩𝔘~′Y^{\prime}\cap\tilde{\mathfrak{U}}^{\prime} are proper and hence Y′=Y′∩𝔘~′Y^{\prime}=Y^{\prime}\cap\tilde{\mathfrak{U}}^{\prime} and it is an irreducible component of 𝔘~′\tilde{\mathfrak{U}}^{\prime}. It’s image in 𝔘~\tilde{\mathfrak{U}} is a proper irreducible component of 𝔘~\tilde{\mathfrak{U}} and hence also an irreducible component of 𝔛~1\tilde{\mathfrak{X}}_{1}. By the same argumentation as above we may use 𝔘′\mathfrak{U}^{\prime} instead of 𝔛1\mathfrak{X}_{1} to calculate the mass of c1​(L¯1|W1)∧…∧c1​(L¯n|W1)c_{1}\left(\overline{L}_{1}\Big|_{W_{1}}\right)\wedge...\wedge c_{1}\left(\overline{L}_{n}\Big|_{W_{1}}\right) at xx. This shows that the mass of the two measures is equal at xx in this case.
Conversely, if YY is an irreducible component of 𝔛~1\tilde{\mathfrak{X}}_{1} with corresponding divisorial point ζY=x\zeta_{Y}=x then Y⊆𝔘~Y\subseteq\tilde{\mathfrak{U}} by definition. Again we may use 𝔘′\mathfrak{U}^{\prime} to calculate the mass at xx and we denote the corresponding irreducible component by Y′Y^{\prime}. Then the closure Y¯′\overline{Y}^{\prime} of Y′Y^{\prime} in 𝔙~′\tilde{\mathfrak{V}}^{\prime} is an irreducible component of 𝔙~′\tilde{\mathfrak{V}}^{\prime} with corresponding divisorial point ζY¯′=x\zeta_{\overline{Y}^{\prime}}=x and hence by the above Y¯′=Y′\overline{Y}^{\prime}=Y^{\prime}. Therefore c1​(L¯1)∧…∧c1​(L¯n)c_{1}(\overline{L}_{1})\wedge...\wedge c_{1}(\overline{L}_{n}) and c1​(L¯1|W1)∧…∧c1​(L¯n|W1)c_{1}\left(\overline{L}_{1}\Big|_{W_{1}}\right)\wedge...\wedge c_{1}\left(\overline{L}_{n}\Big|_{W_{1}}\right) coincide at xx. ∎

Definition 4.9.

Let XX be a Hausdorff topological space. A measure μ\mu on the σ\sigma-algebra of Borel sets of XX is called a Radon measure if

  1. i)

    for every x∈Xx\in X there exists an open neighbourhood UU of XX with μ⁡(U)<∞\mu(U)<\infty,

  2. ii)

    for every open set U⊆XU\subseteq X we have μ(U)=sup{μ(K)|K⊆U,K compact}\mu(U)=\sup\left\{\mu(K)\;\Big|\;K\subseteq U,\;K\text{ compact}\right\},

  3. iii)

    for every Borel set BB of XX we have μ(B)=inf{μ(U)|B⊆U,U open}\mu(B)=\inf\left\{\mu(U)\;\Big|\;B\subseteq U,\;U\text{ open}\right\}.

Remark 4.10.

It follows from Proposition 4.5 i) that the measure defined in 4 is a Radon measure.

Definition 4.11.

Let VV be a strictly KK-analytic Hausdorff space of dimension nn and L¯1,…,L¯n\overline{L}_{1},...,\overline{L}_{n} semipositive piecewise ℚ\mathbb{Q}-linear metrized line bundles on VV. The assignment

Cc​(V)\displaystyle C_{c}(V) →ℝ≥0,\displaystyle\rightarrow\mathbb{R}_{\geq 0},
f\displaystyle f ↦1e1⋅…⋅en​∫Wf​c1​(L¯1e1|W)∧…∧c1​(L¯nen|W)\displaystyle\mapsto\frac{1}{e_{1}\cdot...\cdot e_{n}}\int_{W}f\;c_{1}\left(\overline{L}_{1}^{e_{1}}\Big|_{W}\right)\wedge...\wedge c_{1}\left(\overline{L}_{n}^{e_{n}}\Big|_{W}\right)

where WW is a compact strictly KK-analytic domain with supp⁡(f)⊆W∘\supp(f)\subseteq\overset{\circ}{W} and e1,…,en∈ℕe_{1},...,e_{n}\in\mathbb{N} are non-zero integers such that L¯iei|W\overline{L}_{i}^{e_{i}}\Big|_{W} is a formally metrized line bundle, yields a positive linear functional on the space Cc​(V)C_{c}(V) of continuous functions with compact support in VV and hence by the Riesz Representation Theorem (see [Rud87, Theorem 2.14]) a positive Radon measure on VV which we again denote by c1​(L¯1)∧…∧c1​(L¯n)c_{1}(\overline{L}_{1})\wedge...\wedge c_{1}(\overline{L}_{n}). Note that the integral does neither depend on the choice of WW by Lemma 4.8 nor on the choice of the eie_{i} by Proposition 4.5 and that we can always find such a WW together with the eie_{i} by choosing for every point in supp⁡(f)\supp(f) a compact strictly KK-analytic neighbourhood where some powers of the L¯i\overline{L}_{i} are formally metrized and using compactness of supp⁡(f)\supp(f).

Remark 4.12.

It is easy to see that Proposition 4.5, Lemma 4.6 and Lemma 4.8 remain true if we replace formal metrics by piecewise ℚ\mathbb{Q}-linear metrics.

Proposition 4.13.

Let XX be a separated scheme of finite type over KK of dimension nn with line bundles L1,…,LnL_{1},...,L_{n} on XX. Let VV be an open subset of XanX^{\textup{an}} and ∥⋅∥i\|\cdot\|_{i} a continuous metric on Lian|VL_{i}^{\textup{an}}\Big|_{V} for each ii. Denote by L¯1,…,L¯n\overline{L}_{1},...,\overline{L}_{n} the line bundles L1an|V,…,Lnan|VL_{1}^{\textup{an}}\Big|_{V},...,L_{n}^{\textup{an}}\Big|_{V}, endowed with these metrics. For i∈{1,…,n}i\in\{1,...,n\} let (∥⋅∥i,k)k∈ℕ(\|\cdot\|_{i,k})_{k\in\mathbb{N}} be piecewise ℚ\mathbb{Q}-linear metrics on Li|VL_{i}\Big|_{V} converging uniformly to the continuous metric ∥⋅∥i\|\cdot\|_{i} on Li|VL_{i}\Big|_{V}. Suppose that all ∥⋅∥i,k\|\cdot\|_{i,k} are semipositive in VV. Denote by L¯i,k\overline{L}_{i,k} the line bundle Lian|VL_{i}^{\textup{an}}\Big|_{V} endowed with the metric ∥⋅∥i,k\|\cdot\|_{i,k}. Then the measures c1​(L¯1,k)∧…∧c1​(L¯n,k)c_{1}\left(\overline{L}_{1,k}\right)\wedge...\wedge c_{1}\left(\overline{L}_{n,k}\right) converge weakly to a positive Radon measure on VV.

Proof.

By Vojta’s version of Nagata’s compactification theorem ([Voj, Theorem 5.7]) we may assume that XX is proper. We show by reverse induction over m∈{0,…,n}m\in\{0,...,n\} that the claim holds when for some choice of pairwise different i1,…,in∈{1,…,n}i_{1},...,i_{n}\in\{1,...,n\} the sequences (∥⋅∥i1,k)k∈ℕ,…,(∥⋅∥im,k)k∈ℕ\Big(\|\cdot\|_{i_{1},k}\Big)_{k\in\mathbb{N}},...,\Big(\|\cdot\|_{i_{m},k}\Big)_{k\in\mathbb{N}} are constant with respect to kk. The case m=nm=n is clear. So let 0≤m<n0\leq m<n and assume that the claim holds for m+1m+1. For j∈{m+1,…,n}j\in\{m+1,...,n\} we can write ∥⋅∥ij,k=∥⋅∥ij,1⊗∥⋅∥′j,k\|\cdot\|_{i_{j},k}=\|\cdot\|_{i_{j},1}\otimes\|\cdot\|^{\prime}_{j,k} for a sequence of piecewise ℚ\mathbb{Q}-linear metrics (∥⋅∥j,k′)k∈ℕ\Big(\|\cdot\|^{\prime}_{j,k}\Big)_{k\in\mathbb{N}} on 𝒪Xan|V\mathcal{O}_{X^{\textup{an}}}\Big|_{V} converging uniformly to a continuous metric ∥⋅∥′j\|\cdot\|^{\prime}_{j} on 𝒪Xan|V\mathcal{O}_{X^{\textup{an}}}\Big|_{V}. Denote by 𝒪¯j,k\overline{\mathcal{O}}_{j,k} the line bundle 𝒪Xan|V\mathcal{O}_{X^{\textup{an}}}\Big|_{V} endowed with the metric ∥⋅∥′j,k\|\cdot\|^{\prime}_{j,k}. We show that

(μm,k:=c1​(L¯i1,1)∧…∧c1​(L¯im,1)∧c1​(L¯im+1,k)∧…∧c1​(L¯in,k))k∈ℕ\left(\mu_{m,k}:=c_{1}(\overline{L}_{i_{1},1})\wedge...\wedge c_{1}(\overline{L}_{i_{m},1})\wedge c_{1}(\overline{L}_{i_{m+1},k})\wedge...\wedge c_{1}(\overline{L}_{i_{n},k})\right)_{k\in\mathbb{N}}

is a Cauchy sequence with respect to the weak topology on the space of Borel-measures on VV. Thus we have to show that for all continuous functions ff on XX with compact support in VV:

|∫Vf​μm,k−∫Vf​μm,k′|​⟶k,k′→∞​0.\left|\int_{V}f\;\mu_{m,k}-\int_{V}f\;\mu_{m,k^{\prime}}\right|\underset{k,k^{\prime}\rightarrow\infty}{\longrightarrow}0.

Let WW be a compact strictly KK-analytic domain with supp⁡(f)⊆W∘\supp(f)\subseteq\overset{\circ}{W} and W⊆VW\subseteq V. By [GM19, Proposition 2.7] we may extend the metrics from WW to XanX^{\textup{an}} and hence assume that they are defined on the whole space. Hence by Chow’s lemma and the projection formula we may assume that XX is projective. Then by [Gub03, Proposition 10.5] any formal model of XX is dominated by a projective model. Any formal line bundle on this model becomes semipositive after tensoring with 𝒪⁡(n)\mathcal{O}(n) for nn big enough by using Serre’s theorem ([Har77, Theorem II.5.17]) on the special fibre. As a consequence one can write any formal metric on any line bundle on XX as a quotient of two semipositive formal metrics (on possibly different line bundles). We will see below, that μm,k​(Z)\mu_{m,k}(Z) is bounded with respect to kk for every compact subset Z⊆VZ\subseteq V. Hence, as the set of piecewise ℚ\mathbb{Q}-linear metrics is dense in the space of continuous metrics on 𝒪Xan\mathcal{O}_{X^{\textup{an}}} with respect to uniform convergence (Proposition 3.13), we may assume that f=−log⁡‖1‖f=-\log\|1\| for a formal metric ∥⋅∥\|\cdot\| on 𝒪Xan\mathcal{O}_{X^{\textup{an}}}. Then we can write ∥⋅∥=∥⋅∥+/∥⋅∥−\|\cdot\|=\|\cdot\|_{+}/\|\cdot\|_{-} for two semipositive formal metrics ∥⋅∥+,∥⋅∥−\|\cdot\|_{+},\|\cdot\|_{-} on some line bundles L+L_{+} respectively L−L_{-} on XanX^{\textup{an}}. In fact L+=L−L_{+}=L_{-} but we will use the notation L¯+\overline{L}_{+} and L¯−\overline{L}_{-} to distinguish between the two metrics. Write 𝒪¯Xf\overline{\mathcal{O}}_{X}^{f} for the line bundle 𝒪Xan|V\mathcal{O}_{X^{\textup{an}}}\Big|_{V} endowed with the metric ‖1‖=e−f\|1\|=e^{-f} and to shorten notation μm:=c1​(L¯i1,1)∧…∧c1​(L¯im,1)\mu_{m}:=c_{1}(\overline{L}_{i_{1},1})\wedge...\wedge c_{1}(\overline{L}_{i_{m},1}) which is a purely formal notation. Furthermore without loss of generality assume i1=1,…,im=mi_{1}=1,...,i_{m}=m. We have

|\displaystyle\Big| ∫Vfμm,k−∫Vfμm,k′|\displaystyle\int_{V}f\;\mu_{m,k}-\int_{V}f\;\mu_{m,k^{\prime}}\Big|
=|∑i=1n−m∫Vf​μm∧c1​(L¯m+1,k)∧…∧c1​(L¯m+i,k)∧c1​(L¯m+i+1,k′)∧…∧c1​(L¯n,k′)\displaystyle=\Big|\sum_{i=1}^{n-m}\int_{V}f\;\mu_{m}\wedge c_{1}\left(\overline{L}_{m+1,k}\right)\wedge...\wedge c_{1}\left(\overline{L}_{m+i,k}\right)\wedge c_{1}\left(\overline{L}_{m+i+1,k^{\prime}}\right)\wedge...\wedge c_{1}\left(\overline{L}_{n,k^{\prime}}\right)
−∫Vfμm∧c1(L¯m+1,k)∧…∧c1(L¯m+i−1,k)∧c1(L¯m+i,k′)∧…∧c1(L¯n,k′)|\displaystyle\phantom{\Big|\sum_{i=1}^{n}}-\int_{V}f\;\mu_{m}\wedge c_{1}\left(\overline{L}_{m+1,k}\right)\wedge...\wedge c_{1}\left(\overline{L}_{m+i-1,k}\right)\wedge c_{1}\left(\overline{L}_{m+i,k^{\prime}}\right)\wedge...\wedge c_{1}\left(\overline{L}_{n,k^{\prime}}\right)\Big|
=|∑i=1n−m∫Vf​μm∧…∧c1​(L¯m+i−1,k)∧c1​(L¯m+i,1⊗𝒪¯m+i,k)∧c1​(L¯m+i+1,k′)∧…\displaystyle=\Big|\sum_{i=1}^{n-m}\int_{V}f\;\mu_{m}\wedge...\wedge c_{1}\left(\overline{L}_{m+i-1,k}\right)\wedge c_{1}\left(\overline{L}_{m+i,1}\otimes\overline{\mathcal{O}}_{m+i,k}\right)\wedge c_{1}\left(\overline{L}_{m+i+1,k^{\prime}}\right)\wedge...
−∫Vfμm∧…∧c1(L¯m+i−1,k)∧c1(L¯m+i,1⊗𝒪¯m+i,k′)∧c1(L¯m+i+1,k′)∧…|\displaystyle\phantom{\Big|\sum_{i=1}^{n}}-\int_{V}f\;\mu_{m}\wedge...\wedge c_{1}\left(\overline{L}_{m+i-1,k}\right)\wedge c_{1}\left(\overline{L}_{m+i,1}\otimes\overline{\mathcal{O}}_{m+i,k^{\prime}}\right)\wedge c_{1}\left(\overline{L}_{m+i+1,k^{\prime}}\right)\wedge...\Big|
=|∑i=1n−m∫Vf​μm∧…∧c1​(L¯m+i−1,k)∧c1​(𝒪¯m+i,k)∧c1​(L¯m+i+1,k′)∧…\displaystyle=\Big|\sum_{i=1}^{n-m}\int_{V}f\;\mu_{m}\wedge...\wedge c_{1}\left(\overline{L}_{m+i-1,k}\right)\wedge c_{1}\left(\overline{\mathcal{O}}_{m+i,k}\right)\wedge c_{1}\left(\overline{L}_{m+i+1,k^{\prime}}\right)\wedge...
−∫Vfμm∧…∧c1(L¯m+i−1,k)∧c1(𝒪¯m+i,k′)∧c1(L¯m+i+1,k′)∧…|\displaystyle\phantom{\Big|\sum_{i=1}^{n}}-\int_{V}f\;\mu_{m}\wedge...\wedge c_{1}\left(\overline{L}_{m+i-1,k}\right)\wedge c_{1}\left(\overline{\mathcal{O}}_{m+i,k^{\prime}}\right)\wedge c_{1}\left(\overline{L}_{m+i+1,k^{\prime}}\right)\wedge...\Big|

Since the support of ff is contained in VV and by Lemma 4.8 these last integrals depend only on the restrictions of the metrics to VV. Hence we may instead consider them as integrals over XanX^{\textup{an}} which allows us to use Lemma 4.6 as XanX^{\textup{an}} has no boundary ([Ber90, Theorem 3.4.1]). In combination with an index shift, the last term amounts to

|\displaystyle\Big| ∑i=m+1n∫Xan−log∥1∥i,k′μm∧…∧c1(L¯i−1,k)∧c1(𝒪¯Xf)∧c1(L¯i+1,k′)∧…\displaystyle\sum_{i=m+1}^{n}\int_{X^{\textup{an}}}-\log\|1\|^{\prime}_{i,k}\;\mu_{m}\wedge...\wedge c_{1}\left(\overline{L}_{i-1,k}\right)\wedge c_{1}\left(\overline{\mathcal{O}}_{X}^{f}\right)\wedge c_{1}\left(\overline{L}_{i+1,k^{\prime}}\right)\wedge...
−∫Xan−log∥1∥i,k′′μm∧…∧c1(L¯i−1,k)∧c1(𝒪¯Xf)∧c1(L¯i+1,k′)∧…|\displaystyle-\int_{X^{\textup{an}}}-\log\|1\|^{\prime}_{i,k^{\prime}}\;\mu_{m}\wedge...\wedge c_{1}\left(\overline{L}_{i-1,k}\right)\wedge c_{1}\left(\overline{\mathcal{O}}_{X}^{f}\right)\wedge c_{1}\left(\overline{L}_{i+1,k^{\prime}}\right)\wedge...\Big|

As any point in Xan∖supp⁡(f)X^{\textup{an}}\setminus\supp(f) has a strictly KK-analytic neighbourhood on which ff vanishes, the support of μm∧…∧c1​(L¯i−1,k)∧c1​(𝒪¯Xf)∧c1​(L¯i+1,k′)∧…\mu_{m}\wedge...\wedge c_{1}\left(\overline{L}_{i-1,k}\right)\wedge c_{1}\left(\overline{\mathcal{O}}_{X}^{f}\right)\wedge c_{1}\left(\overline{L}_{i+1,k^{\prime}}\right)\wedge... is contained in supp⁡(f)\supp(f) by Lemma 4.8. So the last display equals

|\displaystyle\Big| ∑i=m+1n∫supp⁡(f)−log∥1∥i,k′μm∧…∧c1(L¯i−1,k)∧c1(𝒪¯Xf)∧c1(L¯i+1,k′)∧…\displaystyle\sum_{i=m+1}^{n}\int_{\supp(f)}-\log\|1\|^{\prime}_{i,k}\;\mu_{m}\wedge...\wedge c_{1}\left(\overline{L}_{i-1,k}\right)\wedge c_{1}\left(\overline{\mathcal{O}}_{X}^{f}\right)\wedge c_{1}\left(\overline{L}_{i+1,k^{\prime}}\right)\wedge...
−∫supp⁡(f)−log∥1∥i,k′′μm∧…∧c1(L¯i−1,k)∧c1(𝒪¯Xf)∧c1(L¯i+1,k′)∧…|\displaystyle-\int_{\supp(f)}-\log\|1\|^{\prime}_{i,k^{\prime}}\;\mu_{m}\wedge...\wedge c_{1}\left(\overline{L}_{i-1,k}\right)\wedge c_{1}\left(\overline{\mathcal{O}}_{X}^{f}\right)\wedge c_{1}\left(\overline{L}_{i+1,k^{\prime}}\right)\wedge...\Big|
=|∑i=m+1n∫supp⁡(f)log⁡(‖1‖i,k′′/‖1‖i,k′)​…∧c1​(L¯i−1,k)∧c1​(𝒪¯Xf)∧c1​(L¯i+1,k′)∧…|\displaystyle=\Big|\sum_{i=m+1}^{n}\int_{\supp(f)}\log\Big(\|1\|^{\prime}_{i,k^{\prime}}/\|1\|^{\prime}_{i,k}\Big)\;...\wedge c_{1}\left(\overline{L}_{i-1,k}\right)\wedge c_{1}\left(\overline{\mathcal{O}}_{X}^{f}\right)\wedge c_{1}\left(\overline{L}_{i+1,k^{\prime}}\right)\wedge...\Big|
≤2⋅∑i=m+1nsupx∈supp⁡(f)|log⁡(‖1‖i,k′​(x)/‖1‖i,k′′​(x))|\displaystyle\leq 2\cdot\sum_{i=m+1}^{n}\sup_{x\in\supp(f)}\left|\log\Big(\|1\|^{\prime}_{i,k}(x)/\|1\|^{\prime}_{i,k^{\prime}}(x)\Big)\right|
⋅maxs∈{+,−}⁡μm∧…∧c1​(L¯i−1,k)∧c1​(L¯s)∧c1​(L¯i+1,k′)∧…​(supp⁡(f))​⟶k,k′→∞​0.\displaystyle\phantom{\leq\sum_{i=m+}^{n}}\cdot\max_{s\in\{+,-\}}\mu_{m}\wedge...\wedge c_{1}\left(\overline{L}_{i-1,k}\right)\wedge c_{1}\left(\overline{L}_{s}\right)\wedge c_{1}\left(\overline{L}_{i+1,k^{\prime}}\right)\wedge...(\supp(f))\underset{k,k^{\prime}\rightarrow\infty}{\longrightarrow}0.

Here the last term converges to zero as supx∈supp⁡(f)|log⁡(‖1‖i,k′​(x)/‖1‖i,k′′​(x))|\sup_{x\in\supp(f)}\left|\log\Big(\|1\|^{\prime}_{i,k}(x)/\|1\|^{\prime}_{i,k^{\prime}}(x)\Big)\right| tends to zero by uniform convergence of ∥⋅∥′i,k\|\cdot\|^{\prime}_{i,k} and compactness of supp⁡(f)\supp(f) and μm∧c1​(L¯m+1,k)∧…∧c1​(L¯i−1,k)∧c1​(L¯s)∧c1​(L¯i+1,k′)∧…∧c1​(L¯n,k′)\mu_{m}\wedge c_{1}\left(\overline{L}_{m+1,k}\right)\wedge...\wedge c_{1}\left(\overline{L}_{i-1,k}\right)\wedge c_{1}\left(\overline{L}_{s}\right)\wedge c_{1}\left(\overline{L}_{i+1,k^{\prime}}\right)\wedge...\wedge c_{1}\left(\overline{L}_{n,k^{\prime}}\right) are positive measures on VV which converge by the induction hypothesis weakly to a positive Radon measure which implies that their mass of supp⁡(f)\supp(f) is bounded with respect to k,k′k,k^{\prime}. To go into more detail, let gg be a continuous non-negative function on VV with compact support such that g⁡(x)>1g(x)>1 for all x∈supp⁡(f)x\in\supp(f). The existence of such a function follows for example from a partition of unity argument ([Flo03, 1.5.1]) applied to the open cover {V¯∖supp⁡(f),V}\{\overline{V}\setminus\supp(f),V\} of the closure V¯\overline{V} of VV (note that V¯\overline{V} is compact as XX is proper over KK). Then

μm\displaystyle\mu_{m} ∧c1​(L¯m+1,k)∧…∧c1​(L¯i−1,k)∧c1​(L¯s)∧c1​(L¯i+1,k′)∧…∧c1​(L¯n,k′)​(supp⁡(f))\displaystyle\wedge c_{1}\left(\overline{L}_{m+1,k}\right)\wedge...\wedge c_{1}\left(\overline{L}_{i-1,k}\right)\wedge c_{1}\left(\overline{L}_{s}\right)\wedge c_{1}\left(\overline{L}_{i+1,k^{\prime}}\right)\wedge...\wedge c_{1}\left(\overline{L}_{n,k^{\prime}}\right)(\supp(f))
≤∫g​μm∧c1​(L¯m+1,k)∧…∧c1​(L¯i−1,k)∧c1​(L¯s)∧c1​(L¯i+1,k′)∧…∧c1​(L¯n,k′)\displaystyle\leq\int g\;\mu_{m}\wedge c_{1}\left(\overline{L}_{m+1,k}\right)\wedge...\wedge c_{1}\left(\overline{L}_{i-1,k}\right)\wedge c_{1}\left(\overline{L}_{s}\right)\wedge c_{1}\left(\overline{L}_{i+1,k^{\prime}}\right)\wedge...\wedge c_{1}\left(\overline{L}_{n,k^{\prime}}\right)

where the last term converges for k,k′→∞k,k^{\prime}\rightarrow\infty and is hence bounded with respect to k,k′k,k^{\prime}.
We now define a positive linear functional on the space of continuous functions with compact support in VV by

Cc​(V)\displaystyle C_{c}(V) →ℝ≥0,\displaystyle\rightarrow\mathbb{R}_{\geq 0},
f\displaystyle f ↦limk→∞∫Vf​μm,k.\displaystyle\mapsto\lim_{k\rightarrow\infty}\int_{V}f\;\mu_{m,k}.

By the Riesz Representation Theorem ([Rud87, Theorem 2.14]) this corresponds to a positive Radon measure μ\mu on VV and we have μm,k→μ\mu_{m,k}\rightarrow\mu weakly for k→∞k\rightarrow\infty.
It remains to show that μm,k​(Z)\mu_{m,k}(Z) is bounded with respect to kk for every compact subset Z⊆VZ\subseteq V. So let Z⊆VZ\subseteq V be compact and ff a continuous non-negative function on VV with compact support such that f⁡(x)>1f(x)>1 for all x∈Zx\in Z. As above the existence of such a function follows from a partition of unity argument ([Flo03, 1.5.1]) applied to the open cover {V¯∖Z,V}\{\overline{V}\setminus Z,V\} of the closure V¯\overline{V} of VV. Again we may assume that ff is a model function, i.e. of the from −log∥⋅∥-\log\|\cdot\| for a piecewise ℚ\mathbb{Q}-linear metric ∥⋅∥\|\cdot\| on 𝒪Xan\mathcal{O}_{X^{\textup{an}}} (we can even assume that ∥⋅∥\|\cdot\| is a formal metric) and we use the same notation as above. To be more precise, let ϵ>0\epsilon>0 such that f⁡(x)>1+ϵf(x)>1+\epsilon for all x∈Zx\in Z. First extend ff to XanX^{\textup{an}} by zero and then define a new function f~\tilde{f} by f~​(x)=f​(x)−ϵ/2\tilde{f}(x)=f(x)-\epsilon/2. By Proposition 3.13 we may approximate f~\tilde{f} by a model function ϕ\phi such that |ϕ⁡(x)−f~​(x)|<ϵ/2|\phi(x)-\tilde{f}(x)|<\epsilon/2 for all x∈Xanx\in X^{\textup{an}}. Then by [GM19, Proposition 2.12 (d)], max⁡{0,ϕ}\max\{0,\phi\} is a model function on XanX^{\textup{an}} with compact support in VV which is greater than one at ZZ. We have

supk∈ℕμm,k​(Z)\displaystyle\sup_{k\in\mathbb{N}}\mu_{m,k}(Z) ≤supk∈ℕ∫Vf​μm,k\displaystyle\leq\sup_{k\in\mathbb{N}}\int_{V}f\;\mu_{m,k}
=supk∈ℕ∫supp⁡(f)f​μm∧c1​(L¯m+1,k)∧…∧c1​(L¯n,k)\displaystyle=\sup_{k\in\mathbb{N}}\int_{\supp(f)}f\;\mu_{m}\wedge c_{1}(\overline{L}_{m+1,k})\wedge...\wedge c_{1}(\overline{L}_{n,k})
=supk∈ℕ∫supp⁡(f)f​μm∧c1​(L¯m+1,1⊗𝒪¯m+1,k)∧c1​(L¯m+2,k)∧…∧c1​(L¯n,k)\displaystyle=\sup_{k\in\mathbb{N}}\int_{\supp(f)}f\;\mu_{m}\wedge c_{1}(\overline{L}_{m+1,1}\otimes\overline{\mathcal{O}}_{m+1,k})\wedge c_{1}(\overline{L}_{m+2,k})\wedge...\wedge c_{1}(\overline{L}_{n,k})
=supk∈ℕ∫supp⁡(f)f​μm∧c1​(L¯m+1,1)∧c1​(L¯m+2,k)∧…∧c1​(L¯n,k)\displaystyle=\sup_{k\in\mathbb{N}}\int_{\supp(f)}f\;\mu_{m}\wedge c_{1}(\overline{L}_{m+1,1})\wedge c_{1}(\overline{L}_{m+2,k})\wedge...\wedge c_{1}(\overline{L}_{n,k})
+∫supp⁡(f)fμm∧c1(𝒪¯m+1,k)∧c1(L¯m+2,k)∧…∧c1(L¯n,k)\displaystyle\phantom{\lim_{k\rightarrow\infty}}+\int_{\supp(f)}f\;\mu_{m}\wedge c_{1}(\overline{\mathcal{O}}_{m+1,k})\wedge c_{1}(\overline{L}_{m+2,k})\wedge...\wedge c_{1}(\overline{L}_{n,k})

Again using Lemma 4.6 and the same argumentation as above for the second summand this amounts to

supk∈ℕ\displaystyle\sup_{k\in\mathbb{N}} ∫supp⁡(f)f​μm∧c1​(L¯m+1,1)∧c1​(L¯m+2,k)∧…∧c1​(L¯n,k)\displaystyle\int_{\supp(f)}f\;\mu_{m}\wedge c_{1}(\overline{L}_{m+1,1})\wedge c_{1}(\overline{L}_{m+2,k})\wedge...\wedge c_{1}(\overline{L}_{n,k})
+∫supp⁡(f)−log∥1∥m+1,k′μm∧c1(𝒪¯Xf)∧c1(L¯m+2,k)∧…∧c1(L¯n,k)\displaystyle\phantom{\lim_{k\rightarrow\infty}}+\int_{\supp(f)}-\log\|1\|^{\prime}_{m+1,k}\;\mu_{m}\wedge c_{1}(\overline{\mathcal{O}}_{X}^{f})\wedge c_{1}(\overline{L}_{m+2,k})\wedge...\wedge c_{1}(\overline{L}_{n,k})
≤supk∈ℕsupx∈Vf⁡(x)⋅μm∧c1​(L¯m+1,1)∧c1​(L¯m+2,k)∧…∧c1​(L¯n,k)​(supp⁡(f))\displaystyle\leq\sup_{k\in\mathbb{N}}\sup_{x\in V}f(x)\cdot\mu_{m}\wedge c_{1}(\overline{L}_{m+1,1})\wedge c_{1}(\overline{L}_{m+2,k})\wedge...\wedge c_{1}(\overline{L}_{n,k})(\supp(f))
+supk∈ℕsupx∈supp⁡(f)|log(∥1∥m+1,k′(x))|\displaystyle\phantom{\lim_{k\rightarrow\infty}}+\sup_{k\in\mathbb{N}}\sup_{x\in\supp(f)}\left|\log\Big(\|1\|^{\prime}_{m+1,k}(x)\Big)\right|
⋅2⋅maxs∈{+,−}⁡μm∧c1​(L¯s)∧c1​(L¯m+2,k)∧…∧c1​(L¯n,k)​(supp⁡(f))\displaystyle\phantom{\lim_{k\rightarrow\infty}+}\cdot 2\cdot\max_{s\in\{+,-\}}\mu_{m}\wedge c_{1}(\overline{L}_{s})\wedge c_{1}(\overline{L}_{m+2,k})\wedge...\wedge c_{1}(\overline{L}_{n,k})(\supp(f))

By the induction hypothesis all measures appearing in this last term converge for k→∞k\rightarrow\infty. Hence the measure of supp⁡(f)\supp(f) is bounded with respect to kk. Furthermore supx∈Vf⁡(x)<∞\sup_{x\in V}f(x)<\infty as ff has compact support in VV and supx∈supp⁡(f)|log⁡(‖1‖m+1,k′​(x))|\sup_{x\in\supp(f)}\left|\log\Big(\|1\|^{\prime}_{m+1,k}(x)\Big)\right| is bounded with respect to kk by uniform convergence of (∥⋅∥m+1,k′)k∈ℕ\Big(\|\cdot\|^{\prime}_{m+1,k}\Big)_{k\in\mathbb{N}} and compactness of supp⁡(f)\supp(f). We conclude that the last term is bounded with respect to kk. This proves the induction step. The claim is then the case m=0m=0. ∎

Remark 4.14.

In the situation of Proposition 4.13, the limit depends only on the metrics ∥⋅∥i\|\cdot\|_{i} but not on the sequences (∥⋅∥i,k)k∈ℕ(\|\cdot\|_{i,k})_{k\in\mathbb{N}}. Namely, if (∥⋅∥i,k′)k∈ℕ(\|\cdot\|^{\prime}_{i,k})_{k\in\mathbb{N}} are other sequences converging uniformly to ∥⋅∥i\|\cdot\|_{i} then the sequences (∥⋅∥i,k′′)k∈ℕ(\|\cdot\|^{\prime\prime}_{i,k})_{k\in\mathbb{N}} defined by

∥⋅∥′′i,k:={∥⋅∥i,k2,k even∥⋅∥′i,k−12,k odd\|\cdot\|^{\prime\prime}_{i,k}:=\begin{cases}\|\cdot\|_{i,\frac{k}{2}},\;k\text{ even}\\ \|\cdot\|^{\prime}_{i,\frac{k-1}{2}},\;k\text{ odd}\end{cases}

converge uniformly to ∥⋅∥i\|\cdot\|_{i}. As (∥⋅∥i,k)k∈ℕ(\|\cdot\|_{i,k})_{k\in\mathbb{N}} and (∥⋅∥i,k′)k∈ℕ(\|\cdot\|^{\prime}_{i,k})_{k\in\mathbb{N}} are subsequences of ∥⋅∥′′i,k\|\cdot\|^{\prime\prime}_{i,k} the limit of the measures is the same. We denote the measure corresponding to the metrics ∥⋅∥i\|\cdot\|_{i} by c1​(L¯1)∧…∧c1​(L¯n)c_{1}(\overline{L}_{1})\wedge...\wedge c_{1}(\overline{L}_{n}).

Corollary 4.15.

Let XX be a separated scheme of finite type over KK of dimension nn. Let M1,…,MnM_{1},...,M_{n} be line bundles on XanX^{\textup{an}}, VV an open subset of XanX^{\textup{an}} and for i∈{1,…,n}i\in\{1,...,n\} let (∥⋅∥i,k)k∈ℕ(\|\cdot\|_{i,k})_{k\in\mathbb{N}} be piecewise ℚ\mathbb{Q}-linear metrics on M1|V,…,Mn|VM_{1}\Big|_{V},...,M_{n}\Big|_{V} converging uniformly to a continuous metric ∥⋅∥i\|\cdot\|_{i} on Mi|VM_{i}\Big|_{V}. Suppose that all ∥⋅∥i,k\|\cdot\|_{i,k} are semipositive in VV. Write M¯i,k:=(Mi,∥⋅∥i,k)\overline{M}_{i,k}:=\left(M_{i},\|\cdot\|_{i,k}\right) and let L¯1,…,L¯n\overline{L}_{1},...,\overline{L}_{n} be line bundles on XanX^{\textup{an}} endowed with piecewise ℚ\mathbb{Q}-linear metrics on VV. Then the measures c1​(L¯1⊗M¯1,k)∧…∧c1​(L¯n⊗M¯n,k)c_{1}\left(\overline{L}_{1}\otimes\overline{M}_{1,k}\right)\wedge...\wedge c_{1}\left(\overline{L}_{n}\otimes\overline{M}_{n,k}\right) converge weakly to a Radon measure on VV denoted by c1(L¯1⊗(M1,∥⋅∥1))∧…∧c1(L¯n⊗(Mn,∥⋅∥n))c_{1}(\overline{L}_{1}\otimes(M_{1},\|\cdot\|_{1}))\wedge...\wedge c_{1}(\overline{L}_{n}\otimes(M_{n},\|\cdot\|_{n})) (as above this measure does not depend on the choice of the ∥⋅∥i,k\|\cdot\|_{i,k}).

Proof.

As in the proof of Proposition 4.13, we may assume that XX is projective and write the metrics of the LiL_{i} as a quotient of two semipositive metrics. Using multilinearity, this is now a direct consequence of Proposition 4.13. ∎

Remark 4.16.

To extend the theory to the case where KK is not algebraically closed, choose an algebraic closure of KK and denote its completion by ℂK\mathbb{C}_{K}. Then we define the Monge-Ampère measure as the push-forward of the previously defined Monge-Ampère measure on the base change to ℂK\mathbb{C}_{K}. We explain it here in the situation of Definition 4.11. Let VV be a strictly KK-analytic Hausdorff space of dimension nn, L¯1,…,L¯n\overline{L}_{1},...,\overline{L}_{n} potentially semipositive piecewise linear metrized line bundles on VV (i.e. metrized line bundles on VV which become semipositive piecewise linear metrized line bundles after base change to ℂK\mathbb{C}_{K}) and π:VℂK→V\pi:V_{\mathbb{C}_{K}}\rightarrow V the base change. We can then define a measure on VℂKV_{\mathbb{C}_{K}} with respect to the pull-backs of the line bundles L¯1,…,L¯n\overline{L}_{1},...,\overline{L}_{n} by Definition 4.11 and push the resulting measure forward to VV via π\pi. To make this well defined we show that π\pi is a proper map of topological spaces. So let C⊆VC\subseteq V be compact. Then we can cover CC by finitely many affinoid subdomains U1,…,UrU_{1},...,U_{r}. Then π−1​(C)=π−1​(⋃C∩Ui)=⋃π−1​(C∩Ui)\pi^{-1}(C)=\pi^{-1}\left(\bigcup C\cap U_{i}\right)=\bigcup\pi^{-1}(C\cap U_{i}) and it is enough to show that π−1​(C∩Ui)\pi^{-1}(C\cap U_{i}) is compact for any ii so we may assume that VV is affinoid. But then π\pi is a continuous map between compact Hausdorff spaces and hence proper which yields the claim. We denote this measure again by c1​(L¯1)∧…∧c1​(L¯n)c_{1}(\overline{L}_{1})\wedge...\wedge c_{1}(\overline{L}_{n}). One can check that all the results of this section remain true in this more general situation.

Definition 4.17.

Let KK be a complete, non-archimedean, non-trivially valued field, VV a strictly KK-analytic space and LL a line bundle on VV. A continuous metric ∥⋅∥\|\cdot\| on LL is called locally semipositive if for any x∈Vx\in V there is an open neighbourhood UU of xx such that ∥⋅∥|U\|\cdot\|\Big|_{U} is a uniform limit of semipositive piecewise ℚ\mathbb{Q}-linear metrics on L|UL\Big|_{U}. It is called locally potentially semipositive if its base change to the completion of an algebraic closure of KK is locally semipositive. If VV is an open subset of XanX^{\textup{an}} for a separated scheme XX of finite type over KK then using the Remarks 4.14 and 4.16 we define the Monge-Ampère measure c1​(L¯1)∧…∧c1​(L¯n)c_{1}(\overline{L}_{1})\wedge...\wedge c_{1}(\overline{L}_{n}) for locally potentially semipositive metrized line bundles L¯1,…,L¯n\overline{L}_{1},...,\overline{L}_{n} on VV.

Remark 4.18.

The measures defined in this section are invariant under base change. In the spirit of Remark 4.16 this allows to define them in the trivially valued case for line bundles which become semipositive after base change to a non-trivially valued field. Such metrics and their measures are important for example in [BJ].

Original mathematics by the credited authors. Source-backed reader collection; mathematical self-containment is not assessed.