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 -analytic spaces. Chambert-Loir introduced these measures in [Cha06] on the analytification of a proper variety over under the assumption that 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 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 be bounded, open and convex and denote by the standard Lebesgue measure on and by the standard scalar product on . Let be a convex function on and . We define the gradient image of under to be
and for
Note that if is a Borel set, the same is true for . Finally we define the Monge-Ampère measure associated to by
for all Borel sets . It is indeed a measure on the Borel -algebra, for details see [RT77, Section 2]. The real Monge-Ampère operator is continuous in the sense that if is a sequence of convex functions on converging pointwise to a convex function then converges weakly to . If is two times continuously differentiable then .
Definition 4.2.
In [Con99, Definition 2.2.2] Conrad defined the notion of irreducibility for analytic spaces which we recall here. Let be a paracompact strictly -analytic space and the normalization of ([Con99, 2.1]). Then the irreducible components of are defined to be the sets where are the connected components of . The space is said to be irreducible if it has a unique irreducible component. By [Con99, Lemma 2.2.3] is irreducible if and only if it can not non trivially be written as a union of two closed strictly -analytic subsets.
Let be an irreducible component of and an affinoid domain with . Then by [Con99, Corollary 2.2.9] there is an irreducible component of which is contained in . Then corresponds to a minimal prime ideal of and hence to an irreducible component of . We define the multiplicity of to be the multiplicity of this component. Note that this does not depend on the choice of and : If and is a minimal prime ideal of lying over then is reduced by [BGR84, Corollary 7.3.2/10] as it induces an affinoid domain in which is reduced. Hence also is reduced and since is a local ring of dimension 0, this implies . Hence by [Ful98, Lemma A.4.1] the multiplicity of the irreducible component corresponding to is equal to that of the irreducible component corresponding to .
Let be a proper surjective morphism of irreducible and reduced strictly -analytic spaces. If we set . Otherwise is a finite morphism outside a lower dimensional analytic subset of . Let be an affinoid domain in , an irreducible component of and then is finite and we define to be the sum of the degrees of the irreducible components of over . 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 be an admissible formal scheme over of dimension with generic fibre . Our goal is to introduce a Monge-Ampère measure on for formal line bundles on . We assume first that is irreducible and reduced and that the special fibre of is reduced. Then the non-archimedean Monge-Ampère measure on with respect to these metrized line bundles is defined as
where denotes the Dirac-measure at the unique point which is mapped to the generic point of the proper irreducible component under the reduction map (cf. [Ber90, Proposition 2.4.4]).
If has irreducible and reduced generic fibre but no longer reduced special fibre, there is a canonical admissible formal model of with reduced special fibre together with a finite morphism which restricts to the identity on which can be constructed as follows (cf. [Gub98, Definition 3.10]). Choose a cover of by affine formal subschemes. Define . If is a formal open subscheme for some then induces a morphism for . Hence by standard arguments we can glue the to obtain and the canonical morphisms induce the morphism . We then define
In the general case, let be the decomposition of the generic fibre into prime cycles. By [Gub98, Proposition 3.3] the closure of in is an admissible formal scheme with irreducible and reduced generic fibre . We define
as a measure on .
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 has irreducible, reduced and boundaryless generic fibre and reduced special fibre. In addition to let be a formal line bundle on which is trivial on the generic fibre and set where is the formal metric induced by . Suppose that has compact support and let be the Cartier divisor on induced by as in [Gub98, Remark 3.1]. We examine the Weil divisor associated to as defined in [Gub98, §3]. Since is trivial on the generic fibre, the horizontal part of is zero while the vertical part is by definition ([Gub98, 3.8]) given by . Now since has no boundary, every irreducible component of is proper by Corollary A.4 and together with the definition of the intersection product ([Gub98, §4]) we obtain
Proposition 4.5.
The measure defined above has the following properties:
- i)
is a discrete measure (i.e. of the form with a closed discrete subset, and the Dirac-measure at ) whose support is contained in the relative interior of over (in the sense of [Ber93, 1.5]).
- ii)
is multilinear and symmetric in .
- iii)
Let be a proper morphism of admissible formal schemes over with irreducible and reduced generic fibres of dimension such that the induced morphism on the generic fibres is surjective. Then for formal line bundles on we have
Proof.
ii) follows from symmetry and multilinearity of the intersection product ([Ful98, Proposition 2.5]). For iii) we reduce first to the case where and have reduced special fibre. Let respectively be the canonical formal models with reduced special fibre as in 4. This construction is functorial and we obtain a commutative diagram
Assuming that we know the claim for reduced special fibres we obtain
So from now on assume that and have reduced special fibre. Let be an irreducible component of with corresponding Shilov point . Let be the preimages of under with corresponding irreducible components of . If is proper then clearly all the are proper. If on the other hand one of the is proper then is proper by [GW10, Proposition 12.59]. In this case we can use the projection formula to calculate:
As already mentioned in Definition 4.2, 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 .
On the other hand, if is an irreducible component of whose image is not an irreducible component of then its degree with respect to the line bundles is by the projection formula, as the image is of lower dimension. This proves iii).
For i) let be the decomposition into prime cycles. It is then enough to prove the claim for each and by definition of the measure we may hence assume that has irreducible and reduced generic fibre and reduced special fibre. Let be the set of all where is a proper irreducible component of with . Then is discrete as is an open neighbourhood of which does not contain any other points of . Furthermore is the union of all where runs over all irreducible components of and as all of these sets contain at most one point of and by paracompactness of , every has an open neighbourhood which does not intersect and hence is closed. By definition is of the desired form and its support is contained in the relative interior of over by Corollary A.4.
∎
Lemma 4.6.
Let be an admissible formal scheme over of dimension with boundaryless generic fibre and line bundles on endowed with formal metrics corresponding to the models on . Suppose that , denote by and the metrics on respectively and set , . Suppose that and have compact support. Then
Proof.
Let be the decomposition into prime cycles. It is enough to prove the claim for the closures of in . We may hence assume that is irreducible and reduced. Furthermore by passing to a dominating model as in 4, we may assume that the special fibre of is reduced. As has no boundary, every irreducible component of is proper by Corollary A.4 and hence using commutativity of the intersection product ([Gub98, Theorem 5.9]) we obtain
∎
Definition 4.7.
Let be an -dimensional paracompact strictly -analytic space and formally metrized line bundles on . Let be a formal model of on which there exist formal models of . The existence of such a formal model follows from Remark 3.2. We then define
Note that this definition is independent of the choice of and by the projection formula. If the metrics on are semipositive then is a positive measure.
Lemma 4.8.
Let be a paracompact strictly -analytic space of dimension and a paracompact strictly -analytic subdomain of . Then for formally metrized line bundles on , we have in the topological interior of in .
Proof.
Let be the decomposition of into prime cycles and for each let be the irreducible components of with . Then is the decomposition of into prime cycles. Furthermore, the intersection of any two irreducible components of 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 and are irreducible and reduced. Let be a formal model of with reduced special fibre on which there exist formal models of . Let be a formal model of which exists by paracompactness of , see Remark 3.2. After possibly blowing up, the inclusion induces a morphism ([Bos14, Theorem 8.4.3]). Let . As both measures are discrete it is enough to show that they have the same mass at . Let denote the relative interior of over in the sense of [Ber93, 1.5]. If then by [Ber93, Proposition 1.5.5 (ii)]. Conversely if then there exists an affinoid neighbourhood of in such that is in the relative interior of over . But is also a neighbourhood of in as and therefore . Hence if and only if . If this is not the case then by definition of the measures and Corollary A.4, both of them are zero at . So assume that . Choose a locally finite cover of by open affine formal subschemes and let be the union of all which contain . Then is an open and quasi-compact formal subscheme of . Analogously choose a cover of by open affine formal subschemes. As is quasi-compact, there is a finite subcover of it. Let be the union of the sets in this subcover and add all with . Then also is an open and quasi-compact formal subscheme of and induces a morphism . By [BL93, Corollary 5.4] there is an admissible formal blowing up such that the induced morphism is an open immersion.
Let be an irreducible component of with corresponding divisorial point . Then by definition and hence we may calculate the mass of at using . By Proposition 4.5 iii) we may also use . So let be the irreducible component of corresponding to . Since we see that is an irreducible component of . Additionally, by Corollary A.4, and are proper and hence and it is an irreducible component of . It’s image in is a proper irreducible component of and hence also an irreducible component of . By the same argumentation as above we may use instead of to calculate the mass of at . This shows that the mass of the two measures is equal at in this case.
Conversely, if is an irreducible component of with corresponding divisorial point then by definition. Again we may use to calculate the mass at and we denote the corresponding irreducible component by . Then the closure of in is an irreducible component of with corresponding divisorial point and hence by the above . Therefore and coincide at .
∎
Definition 4.9.
Let be a Hausdorff topological space. A measure on the -algebra of Borel sets of is called a Radon measure if
- i)
for every there exists an open neighbourhood of with ,
- ii)
for every open set we have ,
- iii)
for every Borel set of we have .
Definition 4.11.
Let be a strictly -analytic Hausdorff space of dimension and semipositive piecewise -linear metrized line bundles on . The assignment
where is a compact strictly -analytic domain with and are non-zero integers such that is a formally metrized line bundle, yields a positive linear functional on the space of continuous functions with compact support in and hence by the Riesz Representation Theorem (see [Rud87, Theorem 2.14]) a positive Radon measure on which we again denote by . Note that the integral does neither depend on the choice of by Lemma 4.8 nor on the choice of the by Proposition 4.5 and that we can always find such a together with the by choosing for every point in a compact strictly -analytic neighbourhood where some powers of the are formally metrized and using compactness of .
Remark 4.12.
Proposition 4.13.
Let be a separated scheme of finite type over of dimension with line bundles on . Let be an open subset of and a continuous metric on for each . Denote by the line bundles , endowed with these metrics. For let be piecewise -linear metrics on converging uniformly to the continuous metric on . Suppose that all are semipositive in . Denote by the line bundle endowed with the metric . Then the measures converge weakly to a positive Radon measure on .
Proof.
By Vojta’s version of Nagata’s compactification theorem ([Voj, Theorem 5.7]) we may assume that is proper. We show by reverse induction over that the claim holds when for some choice of pairwise different the sequences are constant with respect to . The case is clear. So let and assume that the claim holds for . For we can write for a sequence of piecewise -linear metrics on converging uniformly to a continuous metric on . Denote by the line bundle endowed with the metric . We show that
is a Cauchy sequence with respect to the weak topology on the space of Borel-measures on . Thus we have to show that for all continuous functions on with compact support in :
Let be a compact strictly -analytic domain with and . By [GM19, Proposition 2.7] we may extend the metrics from to and hence assume that they are defined on the whole space. Hence by Chow’s lemma and the projection formula we may assume that is projective. Then by [Gub03, Proposition 10.5] any formal model of is dominated by a projective model. Any formal line bundle on this model becomes semipositive after tensoring with for 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 as a quotient of two semipositive formal metrics (on possibly different line bundles). We will see below, that is bounded with respect to for every compact subset . Hence, as the set of piecewise -linear metrics is dense in the space of continuous metrics on with respect to uniform convergence (Proposition 3.13), we may assume that for a formal metric on . Then we can write for two semipositive formal metrics on some line bundles respectively on . In fact but we will use the notation and to distinguish between the two metrics. Write for the line bundle endowed with the metric and to shorten notation which is a purely formal notation. Furthermore without loss of generality assume . We have
Since the support of is contained in and by Lemma 4.8 these last integrals depend only on the restrictions of the metrics to . Hence we may instead consider them as integrals over which allows us to use Lemma 4.6 as has no boundary ([Ber90, Theorem 3.4.1]). In combination with an index shift, the last term amounts to
As any point in has a strictly -analytic neighbourhood on which vanishes, the support of is contained in by Lemma 4.8. So the last display equals
Here the last term converges to zero as tends to zero by uniform convergence of and compactness of and are positive measures on which converge by the induction hypothesis weakly to a positive Radon measure which implies that their mass of is bounded with respect to . To go into more detail, let be a continuous non-negative function on with compact support such that for all . The existence of such a function follows for example from a partition of unity argument ([Flo03, 1.5.1]) applied to the open cover of the closure of (note that is compact as is proper over ). Then
where the last term converges for and is hence bounded with respect to .
We now define a positive linear functional on the space of continuous functions with compact support in by
By the Riesz Representation Theorem ([Rud87, Theorem 2.14]) this corresponds to a positive Radon measure on and we have weakly for .
It remains to show that is bounded with respect to for every compact subset . So let be compact and a continuous non-negative function on with compact support such that for all . As above the existence of such a function follows from a partition of unity argument ([Flo03, 1.5.1]) applied to the open cover of the closure of . Again we may assume that is a model function, i.e. of the from for a piecewise -linear metric on (we can even assume that is a formal metric) and we use the same notation as above. To be more precise, let such that for all . First extend to by zero and then define a new function by . By Proposition 3.13 we may approximate by a model function such that for all . Then by [GM19, Proposition 2.12 (d)], is a model function on with compact support in which is greater than one at . We have
Again using Lemma 4.6 and the same argumentation as above for the second summand this amounts to
By the induction hypothesis all measures appearing in this last term converge for . Hence the measure of is bounded with respect to . Furthermore as has compact support in and is bounded with respect to by uniform convergence of and compactness of . We conclude that the last term is bounded with respect to . This proves the induction step. The claim is then the case . ∎
Remark 4.14.
In the situation of Proposition 4.13, the limit depends only on the metrics but not on the sequences . Namely, if are other sequences converging uniformly to then the sequences defined by
converge uniformly to . As and are subsequences of the limit of the measures is the same. We denote the measure corresponding to the metrics by .
Corollary 4.15.
Let be a separated scheme of finite type over of dimension . Let be line bundles on , an open subset of and for let be piecewise -linear metrics on converging uniformly to a continuous metric on . Suppose that all are semipositive in . Write and let be line bundles on endowed with piecewise -linear metrics on . Then the measures converge weakly to a Radon measure on denoted by (as above this measure does not depend on the choice of the ).
Proof.
Remark 4.16.
To extend the theory to the case where is not algebraically closed, choose an algebraic closure of and denote its completion by . Then we define the Monge-Ampère measure as the push-forward of the previously defined Monge-Ampère measure on the base change to . We explain it here in the situation of Definition 4.11. Let be a strictly -analytic Hausdorff space of dimension , potentially semipositive piecewise linear metrized line bundles on (i.e. metrized line bundles on which become semipositive piecewise linear metrized line bundles after base change to ) and the base change. We can then define a measure on with respect to the pull-backs of the line bundles by Definition 4.11 and push the resulting measure forward to via . To make this well defined we show that is a proper map of topological spaces. So let be compact. Then we can cover by finitely many affinoid subdomains . Then and it is enough to show that is compact for any so we may assume that is affinoid. But then is a continuous map between compact Hausdorff spaces and hence proper which yields the claim. We denote this measure again by . One can check that all the results of this section remain true in this more general situation.
Definition 4.17.
Let be a complete, non-archimedean, non-trivially valued field, a strictly -analytic space and a line bundle on . A continuous metric on is called locally semipositive if for any there is an open neighbourhood of such that is a uniform limit of semipositive piecewise -linear metrics on . It is called locally potentially semipositive if its base change to the completion of an algebraic closure of is locally semipositive. If is an open subset of for a separated scheme of finite type over then using the Remarks 4.14 and 4.16 we define the Monge-Ampère measure for locally potentially semipositive metrized line bundles on .
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].