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5. Comparison of the real and non-archimedean Monge-Ampère operator [05B2]

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5. Comparison of the real and non-archimedean Monge-Ampère operator

In this section we want to compare the two measures introduced in the last section. In order to make sense of this, we start with a convex function hh on a closed face of some skeleton. Then one can associate to it a metric on the trivial line bundle which will turn out to be semipositive in the interior of the closed face. Thus we can associate to hh two measures, namely the real Monge-Ampère measure and the Chambert-Loir measure, sometimes also called the non-archimedean Monge-Ampère measure. In Corollary 5.7 we will see that they are equal up to scaling. In the following KK denotes a non-archimedean non-trivially valued field.

Remark 5.1.

Let 𝔛\mathfrak{X} be a strongly nondegenerate strictly polystable formal scheme over K∘K^{\circ} of dimension n+1n+1 with associated skeleton Δ\Delta. Consider an nn-dimensional closed face τ¯\bar{\tau} of Δ\Delta with interior τ\tau and the formal open subscheme 𝔛′\mathfrak{X}^{\prime} of 𝔛\mathfrak{X} consisting of all formal open subsets 𝔘\mathfrak{U} with S⁡(𝔘)=τ¯S(\mathfrak{U})=\bar{\tau}. Let hh be a piecewise affine linear convex function (see Definition 2.10) on τ¯\bar{\tau} and 𝔇\mathfrak{D} a subdivision of τ¯\bar{\tau} such that h|Δ′h\Big|_{\Delta^{\prime}} is affine linear for all Δ′∈𝔇\Delta^{\prime}\in\mathfrak{D}. Let ι:𝔛′′→𝔛′\iota:\mathfrak{X}^{\prime\prime}\rightarrow\mathfrak{X}^{\prime} be the corresponding formal scheme (cf. Construction 2.6). We have seen in Proposition 2.11 that hh induces a Cartier divisor DD on 𝔛′′\mathfrak{X}^{\prime\prime}. We set 𝒪⁡(h∘p𝔛′):=𝒪⁡(D)\mathcal{O}(h\circ p_{\mathfrak{X}^{\prime}}):=\mathcal{O}(D) where p𝔛′:𝔛′a​n→τ¯p_{\mathfrak{X}^{\prime}}:\mathfrak{X}^{\prime an}\rightarrow\overline{\tau} is the restriction of the contraction p𝔛:𝔛an→Δp_{\mathfrak{X}}:\mathfrak{X}^{\textup{an}}\rightarrow\Delta. For a line bundle 𝔏\mathfrak{L} on a formal scheme, we will denote by c1​(𝔏)c_{1}(\mathfrak{L}) the first Chern class of the special fibre of 𝔏\mathfrak{L}.

Theorem 5.2.

In the situation of Remark 5.1 let u∈τu\in\tau be a vertex of 𝔇\mathfrak{D} with corresponding irreducible component Y⊆𝔛~′′Y\subseteq\tilde{\mathfrak{X}}^{\prime\prime} as in Corollary 2.9 (f). Let SS be the closed point in the special fibre of 𝔛\mathfrak{X} corresponding to τ\tau. Then

deg(c1(𝒪(h∘p𝔛′))n.Y)=deg(S)⋅n!⋅MA(h)(u).\Deg\left(c_{1}\left(\mathcal{O}(h\circ p_{\mathfrak{X}^{\prime}})\right)^{n}.Y\right)=\Deg(S)\cdot n!\cdot\MA(h)(u).
Proof.

Note that SS is a closed point of 𝔛~′\tilde{\mathfrak{X}}^{\prime} and hence proper over K~\tilde{K}. Therefore also YY is proper over K~\tilde{K} since it is a closed subset of ι~−1​(S)\tilde{\iota}^{-1}(S) and ι\iota is proper by [Tem00, Corollary 4.4]. Let DD be the Cartier divisor on 𝔛′′\mathfrak{X}^{\prime\prime} induced by hh as in Proposition 2.11 such that c1​(𝒪⁡(h∘p𝔛′))n.Y=Dn.Yc_{1}\left(\mathcal{O}(h\circ p_{\mathfrak{X}^{\prime}})\right)^{n}.Y=D^{n}.Y. We show by induction that for all 0≤l≤n0\leq l\leq n there is a strata cycle YlY_{l} of dimension n−ln-l whose components are contained in YY such that deg(Dn.Y)=deg(Dn−l.Yl)\Deg(D^{n}.Y)=\Deg(D^{n-l}.Y_{l}). The case l=0l=0 is clear by taking Y0:=YY_{0}:=Y. Now let l<nl<n and YlY_{l} be as claimed. Let Y′Y^{\prime} be a stratum of YlY_{l}, such that Y′Y^{\prime} is associated to an ll-dimensional open face τ′\tau^{\prime} of 𝔇\mathfrak{D}, i.e. Y′=red𝔛′′⁡(p𝔛′′−1​(τ′))Y^{\prime}=\red_{\mathfrak{X}^{\prime\prime}}(p_{\mathfrak{X}^{\prime\prime}}^{-1}(\tau^{\prime})) with u∈τ′¯u\in\overline{\tau^{\prime}} by the stratum face correspondence (Proposition 2.8). Using τ′⊆τ⊆ℝn\tau^{\prime}\subseteq\tau\subseteq\mathbb{R}^{n}, there is an affine linear function a:ℝn→ℝa:\mathbb{R}^{n}\rightarrow\mathbb{R} such that h|τ′=a|τ′h\Big|_{\tau^{\prime}}=a\Big|_{\tau^{\prime}}. Then h−a|τh-a\Big|_{\tau} defines a Cartier divisor DY′D_{Y^{\prime}} on 𝔛′′\mathfrak{X}^{\prime\prime} by Proposition 2.11 which is numerically equivalent to DD on YY by Lemma 2.13 and which is trivial on Y′Y^{\prime} because h−a|τ′=0h-a\Big|_{\tau^{\prime}}=0. Hence, as Y′¯\overline{Y^{\prime}} is a strata subset, DY′.Y′¯D_{Y^{\prime}}.\overline{Y^{\prime}} is a strata cycle. Write Yl=∑Y′mY′​Y′¯Y_{l}=\sum_{Y^{\prime}}m_{Y^{\prime}}\overline{Y^{\prime}} where the sum ranges over a finite number of n−ln-l-dimensional strata of 𝔛~′′\tilde{\mathfrak{X}}^{\prime\prime} contained in YY. Then we can calculate:

deg(Dn.Y)\displaystyle\Deg(D^{n}.Y) =deg(Dn−l.Yl)\displaystyle=\Deg\left(D^{n-l}.Y_{l}\right)
=deg(Dn−l.∑Y′mY′Y′¯)\displaystyle=\Deg\left(D^{n-l}.\sum_{Y^{\prime}}m_{Y^{\prime}}\overline{Y^{\prime}}\right)
=deg(∑Y′mY′Dn−l.Y′¯)\displaystyle=\Deg\left(\sum_{Y^{\prime}}m_{Y^{\prime}}D^{n-l}.\overline{Y^{\prime}}\right)
=deg(∑Y′mY′Dn−l−1.(DY′.Y′¯))\displaystyle=\Deg\left(\sum_{Y^{\prime}}m_{Y^{\prime}}D^{n-l-1}.(D_{Y^{\prime}}.\overline{Y^{\prime}})\right)
=deg(Dn−l−1.∑Y′mY′DY′.Y′¯)\displaystyle=\Deg\left(D^{n-l-1}.\sum_{Y^{\prime}}m_{Y^{\prime}}D_{Y^{\prime}}.\overline{Y^{\prime}}\right)

and Yl+1:=∑Y′mY′​DY′.Y′¯Y_{l+1}:=\sum_{Y^{\prime}}m_{Y^{\prime}}D_{Y^{\prime}}.\overline{Y^{\prime}} is a strata cycle as claimed. We use this for l=nl=n to see that deg(Dn.Y)=deg(Yn)\Deg(D^{n}.Y)=\Deg(Y_{n}) for a strata cycle YnY_{n} of dimension 00 contained in YY. Its components are strata points SiS_{i} of 𝔛′′\mathfrak{X}^{\prime\prime} which are mapped by ι\iota to the point SS corresponding to τ\tau. Now let 𝔘′⊆𝔛′\mathfrak{U}^{\prime}\subseteq\mathfrak{X}^{\prime} be a formal open subset with an étale morphism ψ:𝔘′→𝔛⁡(𝒏,𝒂)\psi:\mathfrak{U}^{\prime}\rightarrow\mathfrak{X}(\boldsymbol{n},\boldsymbol{a}) such that SS is the distinguished stratum of 𝔘′\mathfrak{U}^{\prime} (cf. Proposition 2.5) and define 𝔘′′:=ι−1​(𝔘′)\mathfrak{U}^{\prime\prime}:=\iota^{-1}(\mathfrak{U}^{\prime}). Note that there is no factor 𝔛⁡(m)\mathfrak{X}(m) because τ\tau is of maximal dimension. As the strata occurring in the intersection process correspond to open faces of 𝔇\mathfrak{D} with vertex uu, their intersection with 𝔘′′\mathfrak{U}^{\prime\prime} is nonempty. Hence we may calculate the multiplicities of YnY_{n} locally on 𝔘′′\mathfrak{U}^{\prime\prime}. The stratification of 𝔘~′′\tilde{\mathfrak{U}}^{\prime\prime} is obtained by the preimages of the strata of 𝔛~​(𝒏,𝒂)′\tilde{\mathfrak{X}}(\boldsymbol{n},\boldsymbol{a})^{\prime} (see proof of Proposition 2.8) with respect to the base change ψ′:𝔘′′→𝔛​(𝒏,𝒂)′\psi^{\prime}:\mathfrak{U}^{\prime\prime}\rightarrow\mathfrak{X}(\boldsymbol{n},\boldsymbol{a})^{\prime} of ψ\psi (cf. Construction 2.6). Let Yu=ψ~′​(𝔘~′′∩Y)¯Y_{u}=\overline{\tilde{\psi}^{\prime}(\tilde{\mathfrak{U}}^{\prime\prime}\cap Y)} be the irreducible component in 𝔛~​(𝒏,𝒂)′\tilde{\mathfrak{X}}(\boldsymbol{n},\boldsymbol{a})^{\prime} corresponding to uu and DuD_{u} the Cartier divisor on 𝔛​(𝒏,𝒂)′\mathfrak{X}(\boldsymbol{n},\boldsymbol{a})^{\prime} whose pullback gives the Cartier divisor DD associated to hh on 𝔘′′\mathfrak{U}^{\prime\prime} (cf. proof of Proposition 2.11). By applying the modifications of DD in the induction step also to DuD_{u} we obtain a strata cycle Ynt=∑mj​PjY_{n}^{t}=\sum m_{j}P_{j} of 𝔛~​(𝒏,𝒂)′\tilde{\mathfrak{X}}(\boldsymbol{n},\boldsymbol{a})^{\prime} whose pullback is YnY_{n} (as the intersection product is compatible with flat pullback by [Ful98, Proposition 2.3(d)]) and which has the same degree as Dun.YuD_{u}^{n}.Y_{u} using Lemma 2.13. Now let

val:(𝔾m𝒏)Kan\displaystyle\val:(\mathbb{G}_{m}^{\boldsymbol{n}})_{K}^{\textup{an}} →ℝ𝒏,\displaystyle\rightarrow\mathbb{R}^{\boldsymbol{n}},
q\displaystyle q ↦(−log⁡q⁡(x01),…,−log⁡q⁡(x0​n0),…,−log⁡q⁡(xp​1),…,−log⁡q⁡(xp​np))\displaystyle\mapsto(-\log q(x_{01}),...,-\log q(x_{0n_{0}}),...,-\log q(x_{p1}),...,-\log q(x_{pn_{p}}))

and Σ:={𝒘∈ℝ≥0𝒏|wi​1+…+wi​ni≤v(ai),0≤i≤p}\Sigma:=\left\{\boldsymbol{w}\in\mathbb{R}_{\geq 0}^{\boldsymbol{n}}\;\Big|\;w_{i1}+...+w_{in_{i}}\leq v(a_{i}),0\leq i\leq p\right\}. As we have an isomorphism

𝔛​(𝒏,𝒂)an​→~​val−1⁡(Σ)\mathfrak{X}(\boldsymbol{n},\boldsymbol{a})^{\textup{an}}\tilde{\rightarrow}\val^{-1}(\Sigma)

and using [Gub13, Corollary 6.15], we find that YuY_{u} is a toric variety with fan given by the cones generated by Δ′−u\Delta^{\prime}-u for Δ′∈𝔇\Delta^{\prime}\in\mathfrak{D} with vertex uu (in fact we identify τ\tau with Σ\Sigma by forgetting about the coordinate with index 00 for each ii). Du|YuD_{u}\Big|_{Y_{u}} is given up to multiplication by a constant by the divisor Du′D^{\prime}_{u} on YuY_{u} associated to the linear function h′:=h(⋅+u)−h(u)h^{\prime}:=h(\cdot+u)-h(u). By [Ful93, 3.4,5.3] we have

λ⁡(PDu′)=deg(D′un.Yu)n!,\lambda(P_{D^{\prime}_{u}})=\frac{\Deg({D^{\prime}}_{u}^{n}.Y_{u})}{n!},

where

PDu′={y∈ℝn|⟨z,y⟩≤ψDu′​(z)=h′​(z)​∀z∈ℝn}=∇h′​(0)P_{D^{\prime}_{u}}=\left\{y\in\mathbb{R}^{n}\;\Big|\;\langle z,y\rangle\leq\psi_{D^{\prime}_{u}}(z)=h^{\prime}(z)\;\forall z\in\mathbb{R}^{n}\right\}=\nabla h^{\prime}(0)

and λ\lambda denotes the standard Lebesgue measure. For the last term we get

∇h′​(0)\displaystyle\nabla h^{\prime}(0) ={p∈ℝn|∀x∈τ−u:h′(0)+⟨x,p⟩≤h′(x)}\displaystyle=\left\{p\in\mathbb{R}^{n}\;\Big|\;\forall x\in\tau-u\;:\;h^{\prime}(0)+\langle x,p\rangle\leq h^{\prime}(x)\right\}
={p∈ℝn|∀x∈τ−u:⟨x,p⟩≤h(x+u)−h(u)}\displaystyle=\left\{p\in\mathbb{R}^{n}\;\Big|\;\forall x\in\tau-u\;:\;\langle x,p\rangle\leq h(x+u)-h(u)\right\}
={p∈ℝn|∀x∈τ:h(u)+⟨x−u,p⟩≤h(x)}\displaystyle=\left\{p\in\mathbb{R}^{n}\;\Big|\;\forall x\in\tau\;:\;h(u)+\langle x-u,p\rangle\leq h(x)\right\}
=∇h​(u).\displaystyle=\nabla h(u).

Hence

1n!​deg⁡(Ynt)=deg(D′un.Yu)n!=λ⁡(PDu′)=λ⁡(∇h​(u))=MA⁡(h)​({u}).\frac{1}{n!}\Deg(Y_{n}^{t})=\frac{\Deg({D^{\prime}}_{u}^{n}.Y_{u})}{n!}=\lambda(P_{D^{\prime}_{u}})=\lambda(\nabla h(u))=\MA(h)(\{u\}).

With ι′\iota^{\prime} denoting the morphism 𝔛​(𝒏,𝒂)′→𝔛⁡(𝒏,𝒂)\mathfrak{X}(\boldsymbol{n},\boldsymbol{a})^{\prime}\rightarrow\mathfrak{X}(\boldsymbol{n},\boldsymbol{a}) we conclude

deg(Dn.Y)=deg(Yn)=deg(ψ′⁣∗Ynt)=deg(ι∗ψ′⁣∗∑mjPj).\Deg(D^{n}.Y)=\Deg\left(Y_{n}\right)=\Deg\left(\psi^{\prime\ast}Y_{n}^{t}\right)=\Deg\left(\iota_{\ast}\psi^{\prime\ast}\sum m_{j}P_{j}\right).

Using [Ful98, Proposition 1.7] this equals

deg(ψ∗ι∗′∑mjPj)=deg(ψ∗∑mj[Pj:{𝟎~}]⋅{𝟎~}).\Deg\left(\psi^{\ast}\iota^{\prime}_{\ast}\sum m_{j}P_{j}\right)=\Deg\left(\psi^{\ast}\sum m_{j}[P_{j}:\{\tilde{\boldsymbol{0}}\}]\cdot\{\tilde{\boldsymbol{0}}\}\right).

As ψ−1​({𝟎~})=S\psi^{-1}(\{\tilde{\boldsymbol{0}}\})=S is reduced since ψ\psi is smooth, this amounts to

deg⁡(∑mj​deg⁡(Pj)​S)=deg⁡(S)⋅deg⁡(Ynt)=deg⁡(S)⋅n!⋅MA⁡(h)​({u}).\Deg\left(\sum m_{j}\Deg(P_{j})S\right)=\Deg(S)\cdot\Deg(Y_{n}^{t})=\Deg(S)\cdot n!\cdot\MA(h)(\{u\}).

This yields the equality we wanted to prove. ∎

Remark 5.3.

Using the same arguments, one can show the following more general formula: In the situation of Theorem 5.2 instead of only one function hh consider h1,…,hnh_{1},...,h_{n} piecewise affine linear convex functions on τ¯\bar{\tau}. Refine the subdivision 𝔇\mathfrak{D} such that it suits every hih_{i}. Then

deg(⋀i=1nc1(𝒪(hi∘p𝔛′)).Y)=deg(S)⋅n!⋅MA(h1,…,hn)(u),\Deg\left(\bigwedge_{i=1}^{n}c_{1}\left(\mathcal{O}(h_{i}\circ p_{\mathfrak{X}^{\prime}})\right).Y\right)=\Deg(S)\cdot n!\cdot\MA(h_{1},...,h_{n})(u),

where

MA⁡(h1,…,hn):=1n!​∑k=1n(−1)n−k⋅∑1≤i1<…<ik≤nMA⁡(hi1+…+hik)\MA(h_{1},...,h_{n}):=\frac{1}{n!}\sum_{k=1}^{n}(-1)^{n-k}\cdot\sum_{1\leq i_{1}<...<i_{k}\leq n}\MA(h_{i_{1}}+...+h_{i_{k}})

denotes now the mixed Monge-Ampère measure of h1,…,hnh_{1},...,h_{n} (for details see [PRr04, §5]).

Remark 5.4.

In the situation of Theorem 5.2 we denote by 𝒪¯h∘p𝔛′\overline{\mathcal{O}}^{h\circ p_{\mathfrak{X}^{\prime}}} the trivial line bundle on 𝔛′an\mathfrak{X}^{\prime\textup{an}} together with the metric which is given by ∥1∥=e−h∘p𝔛′\|1\|=e^{-h\circ p_{\mathfrak{X}^{\prime}}}. After base change to the completion of an algebraic closure ℂK\mathbb{C}_{K} of KK this becomes a formally metrized line bundle by Proposition 2.11. So similarly as in Remark 4.16 we can define its non-archimedean Monge-Ampère measure by base change to ℂK\mathbb{C}_{K}.

Corollary 5.5.

We have

c1​(𝒪¯h∘p𝔛′)n=deg⁡(S)⋅n!⋅MA⁡(h)c_{1}\left(\overline{\mathcal{O}}^{h\circ p_{\mathfrak{X}^{\prime}}}\right)^{n}=\Deg(S)\cdot n!\cdot\MA(h)

on p𝔛′−1​(τ)p_{\mathfrak{X}^{\prime}}^{-1}(\tau), where MA⁡(h)\MA(h) is understood to be a measure on 𝔛′an\mathfrak{X}^{\prime\textup{an}} by pushforward with the inclusion τ↪𝔛′an\tau\hookrightarrow\mathfrak{X}^{\prime\textup{an}}.

Proof.

We already know by Theorem 5.2 that the equation holds on the set of vertices. Furthermore it is clear from the definition, that c1​(𝒪¯h∘p𝔛′)nc_{1}\left(\overline{\mathcal{O}}^{h\circ p_{\mathfrak{X}^{\prime}}}\right)^{n} is supported on the vertices of 𝔇\mathfrak{D}. What remains to show is that this also holds for MA⁡(h)\MA(h).

Let U:=τ∖{u∈τ|u​ is a vertex of ​𝔇}U:=\tau\setminus\left\{u\in\tau\;\Big|\;u\text{ is a vertex of }\mathfrak{D}\right\}. We want to show MA⁡(h)​(U)=0\MA(h)(U)=0. Let Δ1,…,Δr\Delta_{1},...,\Delta_{r} be the open faces of 𝔇\mathfrak{D} of dimension at least one. For every j∈{1,…,r}j\in\{1,...,r\} there is a vj∈ℝn∖{0}v_{j}\in\mathbb{R}^{n}\setminus\{0\} such that for all y∈Δjy\in\Delta_{j} there exists ϵ∈ℝ+\epsilon\in\mathbb{R}_{+} such that y±ϵ​vj∈Δjy\pm\epsilon v_{j}\in\Delta_{j}. Furthermore hj:=h|Δj=𝒎j​𝒙+v⁡(αj)h_{j}:=h\Big|_{\Delta_{j}}=\boldsymbol{m}_{j}\boldsymbol{x}+v(\alpha_{j}) for some 𝒎j∈ℤn\boldsymbol{m}_{j}\in\mathbb{Z}^{n} and αj∈K×\alpha_{j}\in K^{\times} and we define hjl​i​n:=𝒎j​𝒙h_{j}^{lin}:=\boldsymbol{m}_{j}\boldsymbol{x}. Now let y∈Uy\in U. Then there is an ii such that y∈Δiy\in\Delta_{i}. For p∈∇h​(y)p\in\nabla h(y) and ϵ\epsilon as above it follows

ϵ​⟨vi,p⟩\displaystyle\epsilon\langle v_{i},p\rangle =hi​(y)+⟨y+ϵ​vi−y,p⟩−hi​(y)\displaystyle=h_{i}(y)+\langle y+\epsilon v_{i}-y,p\rangle-h_{i}(y)
≤hi​(y+ϵ​vi)−hi​(y)\displaystyle\leq h_{i}(y+\epsilon v_{i})-h_{i}(y)
=hil​i​n​(ϵ​vi)\displaystyle=h_{i}^{lin}(\epsilon v_{i})
=ϵ​hil​i​n​(vi),\displaystyle=\epsilon h_{i}^{lin}(v_{i}),

hence

⟨vi,p⟩≤hil​i​n​(vi).\langle v_{i},p\rangle\leq h_{i}^{lin}(v_{i}).

A similar argument shows

−ϵ⁡⟨vi,p⟩≤−ϵ​hil​i​n​(vi)-\epsilon\langle v_{i},p\rangle\leq-\epsilon h_{i}^{lin}(v_{i})

and hence

⟨vi,p⟩≥hil​i​n​(vi).\langle v_{i},p\rangle\geq h_{i}^{lin}(v_{i}).

We conclude ⟨vi,p⟩=hil​i​n​(vi)\langle v_{i},p\rangle=h_{i}^{lin}(v_{i}) and pp lies in a hypersurface which depends on ii but not on yy. Hence ⋃y∈U∇h​(y)\bigcup_{y\in U}\nabla h(y) is contained in the union of rr hypersurfaces. Therefore

MA⁡(h)​(U)=λ⁡(⋃y∈U∇h​(y))=0,\MA(h)(U)=\lambda\left(\bigcup_{y\in U}\nabla h(y)\right)=0,

∎

In the following we consider a proper algebraic variety XX over KK of dimension nn.

Proposition 5.6.

Let 𝔛\mathfrak{X} be a strongly nondegenerate strictly polystable formal model of XanX^{\textup{an}} over K∘K^{\circ} with associated skeleton Δ\Delta, τ\tau an nn-dimensional open face of Δ\Delta and hh a rational piecewise affine linear convex function on τ\tau. Then the metric on 𝒪Xan|p𝔛−1​(τ)\mathcal{O}_{X^{\textup{an}}}\Big|_{p_{\mathfrak{X}}^{-1}(\tau)} given by ∥1∥=e−h∘p𝔛\|1\|=e^{-h\circ p_{\mathfrak{X}}} is a semipositive piecewise ℚ\mathbb{Q}-linear metric.

Proof.

Let y∈p𝔛−1​(τ)y\in p_{\mathfrak{X}}^{-1}(\tau) and x:=p𝔛​(y)∈τx:=p_{\mathfrak{X}}(y)\in\tau. There is an open neighbourhood UU of xx in τ\tau such that we can write h|U=maxi=1,…,s⁡hi|Uh\Big|_{U}=\max_{i=1,...,s}h_{i}\Big|_{U} for suitable rational affine linear functions hih_{i} on τ\tau. After passing to some multiple, each hih_{i} induces a formal metric on 𝔛′a​n\mathfrak{X}^{\prime an} by Proposition 2.11 where 𝔛′\mathfrak{X}^{\prime} is defined as in Remark 5.1. Therefore the hih_{i} induce piecewise ℚ\mathbb{Q}-linear metrics on 𝒪𝔛an|p𝔛−1​(τ)\mathcal{O}_{\mathfrak{X}^{\textup{an}}}\Big|_{p_{\mathfrak{X}}^{-1}(\tau)} since p𝔛−1​(τ)⊆𝔛′a​np_{\mathfrak{X}}^{-1}(\tau)\subseteq\mathfrak{X}^{\prime an}. Hence in the neighbourhood p𝔛−1​(U)p_{\mathfrak{X}}^{-1}(U) of yy, the metric induced by hh is given as the minimum of the metrics corresponding to the hih_{i}, which are semipositive at yy by Lemma 2.13. Indeed let (𝔛i′′,𝔏i)(\mathfrak{X}^{\prime\prime}_{i},\mathfrak{L}_{i}) be a formal model of the trivial bundle associated to hih_{i} as obtained by Proposition 2.11. Then by [GK19, Proposition 6.5] (the proof of the implication we need does neither use that KK is algebraically closed nor that the generic fibre is algebraic) it is enough to show that deg𝔏i⁡(Y)≥0\Deg_{\mathfrak{L}_{i}}(Y)\geq 0 for any closed curve YY in 𝔛~i′′\tilde{\mathfrak{X}}^{\prime\prime}_{i} with Y⊆red⁡(p𝔛i′′−1​(τ))Y\subseteq\red(p_{\mathfrak{X}^{\prime\prime}_{i}}^{-1}(\tau)) but by Lemma 2.13 we even have equality. Now we extend the metrics induced by the hih_{i} from a compact strictly KK-analytic neighbourhood of yy to XanX^{\textup{an}} by [GM19, Proposition 2.7] and then it follows from Proposition 3.11 that ||⋅||||\cdot|| is semipositive at yy. ∎

Corollary 5.7.

Let 𝔛\mathfrak{X} be a strongly nondegenerate strictly polystable formal model of XanX^{\textup{an}} over K∘K^{\circ} with associated skeleton Δ\Delta. Let τ\tau be an nn-dimensional open face of Δ\Delta and hh a convex function on τ\tau. Denote by 𝒪¯h∘p𝔛\overline{\mathcal{O}}^{h\circ p_{\mathfrak{X}}} the trivial bundle on p𝔛−1​(τ)p_{\mathfrak{X}}^{-1}(\tau) endowed with the metric given by ∥1∥=e−h∘p𝔛\|1\|=e^{-h\circ p_{\mathfrak{X}}}. Then the latter is locally a semipositive metric (Definition 4.17) and

c1​(𝒪¯h∘p𝔛)n=deg⁡(S)⋅n!⋅MA⁡(h)c_{1}\left(\overline{\mathcal{O}}^{h\circ p_{\mathfrak{X}}}\right)^{n}=\Deg(S)\cdot n!\cdot\MA(h)

on p𝔛−1​(τ)p_{\mathfrak{X}}^{-1}(\tau) where SS is the point in the special fibre of 𝔛\mathfrak{X} corresponding to τ\tau.

Proof.

We can cover τ\tau by polytopes (Δm)m∈ℕ(\Delta_{m})_{m\in\mathbb{N}} such that Δm−1⊆Δm\Delta_{m-1}\subseteq\Delta_{m}. By [BPS14, Proposition 2.5.24] for each mm there is a family of rational piecewise affine linear convex functions (him)i∈ℕ(h_{i}^{m})_{i\in\mathbb{N}} on Δm\Delta_{m} converging uniformly to h|Δmh\Big|_{\Delta_{m}} (note that after normalization we can assume that ℤ\mathbb{Z} is contained in the value group of KK). We extend these functions to rational piecewise affine linear convex functions on τ¯\overline{\tau}. Then by Proposition 5.6 the metrics induced by the himh_{i}^{m} are semipositive piecewise ℚ\mathbb{Q}-linear metrics on p𝔛−1​(τ)p_{\mathfrak{X}}^{-1}(\tau) which implies that the metric induced by h|Δmh\Big|_{\Delta_{m}} is semipositive. By Corollary 5.5 we have

c1​(𝒪¯him∘p𝔛)n=deg⁡(S)⋅n!⋅MA⁡(him)c_{1}\left(\overline{\mathcal{O}}^{h_{i}^{m}\circ p_{\mathfrak{X}}}\right)^{n}=\Deg(S)\cdot n!\cdot\MA(h_{i}^{m})

for every m,i∈ℕm,i\in\mathbb{N}. Denoting the interior of Δm\Delta_{m} by Δm∘\Delta_{m}^{\circ} and using Proposition 4.13 we find that for fixed mm the left hand side converges to c1​(𝒪¯h∘p𝔛)nc_{1}\left(\overline{\mathcal{O}}^{h\circ p_{\mathfrak{X}}}\right)^{n} on p𝔛−1​(Δm∘)p_{\mathfrak{X}}^{-1}(\Delta_{m}^{\circ}). The right hand side converges to deg⁡(S)⋅n!⋅MA⁡(h)\Deg(S)\cdot n!\cdot\MA(h) on Δm∘\Delta_{m}^{\circ} by continuity of the real Monge-Ampère operator. As this holds for any mm and the Δm\Delta_{m} cover τ\tau this proves the corollary. ∎

Definition 5.8.

Let 𝔛\mathfrak{X} be a strongly nondegenerate polystable formal scheme with associated skeleton Δ\Delta and τ\tau an open face of Δ\Delta. A function h:τ→ℝh:\tau\rightarrow\mathbb{R} is called convex if there exists a surjective étale morphism φ:𝔛′→𝔛\varphi:\mathfrak{X}^{\prime}\rightarrow\mathfrak{X} with a strongly nondegenerate strictly polystable formal scheme 𝔛′\mathfrak{X}^{\prime} and an open face τ′\tau^{\prime} of the skeleton associated to 𝔛′\mathfrak{X}^{\prime} with φan​(τ′)=τ\varphi^{\textup{an}}(\tau^{\prime})=\tau such that h∘φan:τ′→ℝh\circ\varphi^{\textup{an}}:\tau^{\prime}\rightarrow\mathbb{R} is convex. For such a convex function hh on τ\tau we define MA⁡(h):=(φan|p𝔛′−1​(τ′))∗​MA⁡(h∘φan|τ′)\MA(h):=\left(\varphi^{\textup{an}}\Big|_{p_{\mathfrak{X}^{\prime}}^{-1}(\tau^{\prime})}\right)_{\ast}\MA\left(h\circ\varphi^{\textup{an}}\Big|_{\tau^{\prime}}\right). It will follow from Corollary 5.10 that this is independent of the choices.

Proposition 5.9.

Let KK be algebraically closed, 𝔛\mathfrak{X} a strongly nondegenerate polystable formal model of XanX^{\textup{an}} over K∘K^{\circ} with associated skeleton Δ\Delta, τ\tau an nn-dimensional open face of Δ\Delta and hh a rational piecewise affine linear convex function on τ\tau. Then the metric on 𝒪Xan|p𝔛−1​(τ)\mathcal{O}_{X^{\textup{an}}}\Big|_{p_{\mathfrak{X}}^{-1}(\tau)} which is given by ∥1∥=e−h∘p𝔛\|1\|=e^{-h\circ p_{\mathfrak{X}}} is a semipositive piecewise ℚ\mathbb{Q}-linear metric.

Proof.

Let 𝔛′\mathfrak{X}^{\prime} be a strongly nondegenerate strictly polystable formal scheme such that there is a surjective étale morphism φ:𝔛′→𝔛\varphi:\mathfrak{X}^{\prime}\rightarrow\mathfrak{X}. Let q∈𝔛~q\in\tilde{\mathfrak{X}} be the closed point corresponding to τ\tau. By Proposition 2.4 we have red𝔛−1⁡(q)=p𝔛−1​(τ)\red_{\mathfrak{X}}^{-1}(q)=p_{\mathfrak{X}}^{-1}(\tau). Choose q′∈𝔛′~q^{\prime}\in\tilde{\mathfrak{X}^{\prime}} with φ⁡(q′)=q\varphi(q^{\prime})=q. By [Gub07, Proposition 2.9] we have that φ\varphi induces an isomorphism red𝔛′−1⁡(q′)​→~​p𝔛−1​(τ)\red_{\mathfrak{X}^{\prime}}^{-1}(q^{\prime})\tilde{\rightarrow}p_{\mathfrak{X}}^{-1}(\tau). Hence the pullback of (𝒪Xan|p𝔛−1​(τ),∥⋅∥)\left(\mathcal{O}_{X^{\textup{an}}}\Big|_{p_{\mathfrak{X}}^{-1}(\tau)},\|\cdot\|\right) is the trivial bundle on red𝔛′−1⁡(q′)\red_{\mathfrak{X}^{\prime}}^{-1}(q^{\prime}) endowed with the metric ∥1∥′=e−h∘p𝔛∘φ\|1\|^{\prime}=e^{-h\circ p_{\mathfrak{X}}\circ\varphi}. Since p𝔛∘φ=φ∘p𝔛′p_{\mathfrak{X}}\circ\varphi=\varphi\circ p_{\mathfrak{X}^{\prime}} it follows that ∥⋅∥′\|\cdot\|^{\prime} is the metric associated to the function h∘φh\circ\varphi on τ′:=p𝔛′​(red𝔛′−1⁡(q′))\tau^{\prime}:=p_{\mathfrak{X}^{\prime}}(\red_{\mathfrak{X}^{\prime}}^{-1}(q^{\prime})) which is again rational piecewise affine linear by [Ber04, Theorem 6.1.1] and we may assume it is convex by definition. Let y∈p𝔛−1​(τ)y\in p_{\mathfrak{X}}^{-1}(\tau). In a neighbourhood of p𝔛′​(y′)p_{\mathfrak{X}^{\prime}}(y^{\prime}) where y′∈p𝔛′−1​(τ′)y^{\prime}\in p_{\mathfrak{X}^{\prime}}^{-1}(\tau^{\prime}) with φan​(y′)=y\varphi^{\textup{an}}(y^{\prime})=y we can write h∘φan=maxi=1,…,s⁡hi′h\circ\varphi^{\textup{an}}=\max_{i=1,...,s}h^{\prime}_{i} for suitable affine linear functions hi′h^{\prime}_{i} on τ′\tau^{\prime}. Now as φan:p𝔛′−1​(τ′)→p𝔛−1​(τ)\varphi^{\textup{an}}:p_{\mathfrak{X}^{\prime}}^{-1}(\tau^{\prime})\rightarrow p_{\mathfrak{X}}^{-1}(\tau) is an isomorphism we have h=maxi=1,…,s⁡hih=\max_{i=1,...,s}h_{i} where hih_{i} are the piecewise affine linear functions on τ\tau satisfying hi′=hi∘φanh^{\prime}_{i}=h_{i}\circ\varphi^{\textup{an}}. Now the metrics associated to the hi′h^{\prime}_{i} are piecewise ℚ\mathbb{Q}-linear and semipositive in y′y^{\prime} by the same argument as in the proof of Proposition 5.6. Hence the piecewise ℚ\mathbb{Q}-linear metrics associated to the hih_{i} extend from a compact strictly KK-analytic neighbourhood of yy to global metrics by [GM19, Proposition 2.7] which are semipositive in yy. Now as ∥⋅∥\|\cdot\| is locally around yy given as the minimum of these metrics, also ∥⋅∥\|\cdot\| is a piecewise ℚ\mathbb{Q}-linear metric which is semipositive in yy by Proposition 3.11. ∎

Corollary 5.10.

Let 𝔛\mathfrak{X} be a strongly nondegenerate polystable formal model of XanX^{\textup{an}} over K∘K^{\circ} with associated skeleton Δ\Delta. Let τ\tau be an nn-dimensional open face of Δ\Delta with corresponding point SS in the special fibre of 𝔛\mathfrak{X} and hh a convex function on τ\tau. Denote by 𝒪¯h∘p𝔛\overline{\mathcal{O}}^{h\circ p_{\mathfrak{X}}} the trivial bundle on p𝔛−1​(τ)p_{\mathfrak{X}}^{-1}(\tau) endowed with the metric given by ∥1∥=e−h∘p𝔛\|1\|=e^{-h\circ p_{\mathfrak{X}}}. Then ∥⋅∥\|\cdot\| is locally a potentially semipositive metric and

c1​(𝒪¯h∘p𝔛)n=deg⁡(S)⋅n!⋅MA⁡(h)c_{1}\left(\overline{\mathcal{O}}^{h\circ p_{\mathfrak{X}}}\right)^{n}=\Deg(S)\cdot n!\cdot\MA(h)

on p𝔛−1​(τ)p_{\mathfrak{X}}^{-1}(\tau).

Proof.

Let ℂK\mathbb{C}_{K} be the completion of an algebraic closure of KK. Then there are exactly deg⁡(S)\Deg(S) points in the special fibre of 𝔛ℂK\mathfrak{X}_{\mathbb{C}_{K}} mapping to SS, hence there are precisely deg⁡(S)\Deg(S) open faces in the skeleton associated to 𝔛ℂK\mathfrak{X}_{\mathbb{C}_{K}} lying over τ\tau. As the base change induces an isomorphism of each of these faces with τ\tau, we have ι∗​MA⁡(ι∗​h)=deg⁡(S)​MA⁡(h)\iota_{\ast}\MA(\iota^{\ast}h)=\Deg(S)\MA(h). Using this and the invariance of the non-archimedean Monge-Ampère measure under base change we may assume K=ℂKK=\mathbb{C}_{K}. As in the proof of Proposition 5.9 we choose a strongly nondegenerate strictly polystable formal scheme 𝔛′\mathfrak{X}^{\prime} and a surjective étale morphism φ:𝔛′→𝔛\varphi:\mathfrak{X}^{\prime}\rightarrow\mathfrak{X}. Let τ′\tau^{\prime} be an open face of the skeleton associated to 𝔛′\mathfrak{X}^{\prime} lying over τ\tau. As we have seen, φ\varphi induces an isomorphism p𝔛′−1​(τ′)​→~​p𝔛−1​(τ)p_{\mathfrak{X}^{\prime}}^{-1}(\tau^{\prime})\tilde{\rightarrow}p_{\mathfrak{X}}^{-1}(\tau). As in the proof of Corollary 5.7 there is a sequence of rational piecewise affine linear convex functions (hi′)i∈ℕ(h^{\prime}_{i})_{i\in\mathbb{N}} on τ′\tau^{\prime} converging locally uniformly to h∘φanh\circ\varphi^{\textup{an}}. Let hih_{i} be the piecewise affine linear functions on τ\tau such that hi∘φan=hi′h_{i}\circ\varphi^{\textup{an}}=h^{\prime}_{i}. By Proposition 5.9 the metrics induced by the hih_{i} are semipositive piecewise ℚ\mathbb{Q}-linear metrics on p𝔛−1​(τ)p_{\mathfrak{X}}^{-1}(\tau) which implies that the metric induced by hh is locally semipositive. As the restriction of φ\varphi to p𝔛′−1​(τ′)p_{\mathfrak{X}^{\prime}}^{-1}(\tau^{\prime}) is an isomorphism onto p𝔛−1​(τ)p_{\mathfrak{X}}^{-1}(\tau) we have

c1​(𝒪¯hi∘p𝔛)n=(φ|p𝔛′−1​(τ′))∗​c1​((φ|p𝔛′−1​(τ′))∗​𝒪¯hi∘p𝔛)nc_{1}\left(\overline{\mathcal{O}}^{h_{i}\circ p_{\mathfrak{X}}}\right)^{n}=\left(\varphi\Big|_{p_{\mathfrak{X}^{\prime}}^{-1}(\tau^{\prime})}\right)_{\ast}c_{1}\left(\left(\varphi\Big|_{p_{\mathfrak{X}^{\prime}}^{-1}(\tau^{\prime})}\right)^{\ast}\overline{\mathcal{O}}^{h_{i}\circ p_{\mathfrak{X}}}\right)^{n}

By Corollary 5.5 we have

c1​((φ|p𝔛′−1​(τ′))∗​𝒪¯hi∘p𝔛)n=c1​(𝒪¯hi∘φan∘p𝔛′)n=n!⋅MA⁡(hi∘φan|τ′).c_{1}\left(\left(\varphi\Big|_{p_{\mathfrak{X}^{\prime}}^{-1}(\tau^{\prime})}\right)^{\ast}\overline{\mathcal{O}}^{h_{i}\circ p_{\mathfrak{X}}}\right)^{n}=c_{1}\left(\overline{\mathcal{O}}^{h_{i}\circ\varphi^{\textup{an}}\circ p_{\mathfrak{X}^{\prime}}}\right)^{n}=n!\cdot\MA\left(h_{i}\circ\varphi^{\textup{an}}\Big|_{\tau^{\prime}}\right).

Hence

c1​(𝒪¯hi∘p𝔛)n=(φ|p𝔛′−1​(τ′))∗​(n!⋅MA⁡(hi∘φan|τ′))=n!⋅MA⁡(hi).c_{1}\left(\overline{\mathcal{O}}^{h_{i}\circ p_{\mathfrak{X}}}\right)^{n}=\left(\varphi\Big|_{p_{\mathfrak{X}^{\prime}}^{-1}(\tau^{\prime})}\right)_{\ast}\left(n!\cdot\MA\left(h_{i}\circ\varphi^{\textup{an}}\Big|_{\tau^{\prime}}\right)\right)=n!\cdot\MA(h_{i}).

It is easily seen that in Proposition 4.13 we can replace uniform convergence by locally uniform convergence. The claim follows from this fact and continuity of the real Monge-Ampère operator. ∎

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