5. Comparison of the real and non-archimedean Monge-Ampère operator [05B2]
Original official author HTML, exact retained edition. Historical TeX conversion verdicts remain unchanged. Cited-edition alignment and mathematical self-containment are not assessed.
Complete original source context · Original author HTML
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 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 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 denotes a non-archimedean non-trivially valued field.
Remark 5.1.
Let be a strongly nondegenerate strictly polystable formal scheme over of dimension with associated skeleton . Consider an -dimensional closed face of with interior and the formal open subscheme of consisting of all formal open subsets with . Let be a piecewise affine linear convex function (see Definition 2.10) on and a subdivision of such that is affine linear for all . Let be the corresponding formal scheme (cf. Construction 2.6). We have seen in Proposition 2.11 that induces a Cartier divisor on . We set where is the restriction of the contraction . For a line bundle on a formal scheme, we will denote by the first Chern class of the special fibre of .
Theorem 5.2.
Proof.
Note that is a closed point of and hence proper over . Therefore also is proper over since it is a closed subset of and is proper by [Tem00, Corollary 4.4]. Let be the Cartier divisor on induced by as in Proposition 2.11 such that . We show by induction that for all there is a strata cycle of dimension whose components are contained in such that . The case is clear by taking . Now let and be as claimed. Let be a stratum of , such that is associated to an -dimensional open face of , i.e. with by the stratum face correspondence (Proposition 2.8). Using , there is an affine linear function such that . Then defines a Cartier divisor on by Proposition 2.11 which is numerically equivalent to on by Lemma 2.13 and which is trivial on because . Hence, as is a strata subset, is a strata cycle. Write where the sum ranges over a finite number of -dimensional strata of contained in . Then we can calculate:
and is a strata cycle as claimed. We use this for to see that for a strata cycle of dimension contained in . Its components are strata points of which are mapped by to the point corresponding to . Now let be a formal open subset with an étale morphism such that is the distinguished stratum of (cf. Proposition 2.5) and define . Note that there is no factor because is of maximal dimension. As the strata occurring in the intersection process correspond to open faces of with vertex , their intersection with is nonempty. Hence we may calculate the multiplicities of locally on . The stratification of is obtained by the preimages of the strata of (see proof of Proposition 2.8) with respect to the base change of (cf. Construction 2.6). Let be the irreducible component in corresponding to and the Cartier divisor on whose pullback gives the Cartier divisor associated to on (cf. proof of Proposition 2.11). By applying the modifications of in the induction step also to we obtain a strata cycle of whose pullback is (as the intersection product is compatible with flat pullback by [Ful98, Proposition 2.3(d)]) and which has the same degree as using Lemma 2.13. Now let
and . As we have an isomorphism
and using [Gub13, Corollary 6.15], we find that is a toric variety with fan given by the cones generated by for with vertex (in fact we identify with by forgetting about the coordinate with index for each ). is given up to multiplication by a constant by the divisor on associated to the linear function . By [Ful93, 3.4,5.3] we have
where
and denotes the standard Lebesgue measure. For the last term we get
Hence
With denoting the morphism we conclude
Using [Ful98, Proposition 1.7] this equals
As is reduced since is smooth, this amounts to
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 consider piecewise affine linear convex functions on . Refine the subdivision such that it suits every . Then
where
denotes now the mixed Monge-Ampère measure of (for details see [PRr04, §5]).
Remark 5.4.
In the situation of Theorem 5.2 we denote by the trivial line bundle on together with the metric which is given by . After base change to the completion of an algebraic closure of 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 .
Corollary 5.5.
We have
on , where is understood to be a measure on by pushforward with the inclusion .
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 is supported on the vertices of . What remains to show is that this also holds for .
Let . We want to show . Let be the open faces of of dimension at least one. For every there is a such that for all there exists such that . Furthermore for some and and we define . Now let . Then there is an such that . For and as above it follows
hence
A similar argument shows
and hence
We conclude and lies in a hypersurface which depends on but not on . Hence is contained in the union of hypersurfaces. Therefore
∎
In the following we consider a proper algebraic variety over of dimension .
Proposition 5.6.
Let be a strongly nondegenerate strictly polystable formal model of over with associated skeleton , an -dimensional open face of and a rational piecewise affine linear convex function on . Then the metric on given by is a semipositive piecewise -linear metric.
Proof.
Let and . There is an open neighbourhood of in such that we can write for suitable rational affine linear functions on . After passing to some multiple, each induces a formal metric on by Proposition 2.11 where is defined as in Remark 5.1. Therefore the induce piecewise -linear metrics on since . Hence in the neighbourhood of , the metric induced by is given as the minimum of the metrics corresponding to the , which are semipositive at by Lemma 2.13. Indeed let be a formal model of the trivial bundle associated to as obtained by Proposition 2.11. Then by [GK19, Proposition 6.5] (the proof of the implication we need does neither use that is algebraically closed nor that the generic fibre is algebraic) it is enough to show that for any closed curve in with but by Lemma 2.13 we even have equality. Now we extend the metrics induced by the from a compact strictly -analytic neighbourhood of to by [GM19, Proposition 2.7] and then it follows from Proposition 3.11 that is semipositive at . ∎
Corollary 5.7.
Let be a strongly nondegenerate strictly polystable formal model of over with associated skeleton . Let be an -dimensional open face of and a convex function on . Denote by the trivial bundle on endowed with the metric given by . Then the latter is locally a semipositive metric (Definition 4.17) and
on where is the point in the special fibre of corresponding to .
Proof.
We can cover by polytopes such that . By [BPS14, Proposition 2.5.24] for each there is a family of rational piecewise affine linear convex functions on converging uniformly to (note that after normalization we can assume that is contained in the value group of ). We extend these functions to rational piecewise affine linear convex functions on . Then by Proposition 5.6 the metrics induced by the are semipositive piecewise -linear metrics on which implies that the metric induced by is semipositive. By Corollary 5.5 we have
for every . Denoting the interior of by and using Proposition 4.13 we find that for fixed the left hand side converges to on . The right hand side converges to on by continuity of the real Monge-Ampère operator. As this holds for any and the cover this proves the corollary. ∎
Definition 5.8.
Let be a strongly nondegenerate polystable formal scheme with associated skeleton and an open face of . A function is called convex if there exists a surjective étale morphism with a strongly nondegenerate strictly polystable formal scheme and an open face of the skeleton associated to with such that is convex. For such a convex function on we define . It will follow from Corollary 5.10 that this is independent of the choices.
Proposition 5.9.
Let be algebraically closed, a strongly nondegenerate polystable formal model of over with associated skeleton , an -dimensional open face of and a rational piecewise affine linear convex function on . Then the metric on which is given by is a semipositive piecewise -linear metric.
Proof.
Let be a strongly nondegenerate strictly polystable formal scheme such that there is a surjective étale morphism . Let be the closed point corresponding to . By Proposition 2.4 we have . Choose with . By [Gub07, Proposition 2.9] we have that induces an isomorphism . Hence the pullback of is the trivial bundle on endowed with the metric . Since it follows that is the metric associated to the function on which is again rational piecewise affine linear by [Ber04, Theorem 6.1.1] and we may assume it is convex by definition. Let . In a neighbourhood of where with we can write for suitable affine linear functions on . Now as is an isomorphism we have where are the piecewise affine linear functions on satisfying . Now the metrics associated to the are piecewise -linear and semipositive in by the same argument as in the proof of Proposition 5.6. Hence the piecewise -linear metrics associated to the extend from a compact strictly -analytic neighbourhood of to global metrics by [GM19, Proposition 2.7] which are semipositive in . Now as is locally around given as the minimum of these metrics, also is a piecewise -linear metric which is semipositive in by Proposition 3.11. ∎
Corollary 5.10.
Let be a strongly nondegenerate polystable formal model of over with associated skeleton . Let be an -dimensional open face of with corresponding point in the special fibre of and a convex function on . Denote by the trivial bundle on endowed with the metric given by . Then is locally a potentially semipositive metric and
on .
Proof.
Let be the completion of an algebraic closure of . Then there are exactly points in the special fibre of mapping to , hence there are precisely open faces in the skeleton associated to lying over . As the base change induces an isomorphism of each of these faces with , we have . Using this and the invariance of the non-archimedean Monge-Ampère measure under base change we may assume . As in the proof of Proposition 5.9 we choose a strongly nondegenerate strictly polystable formal scheme and a surjective étale morphism . Let be an open face of the skeleton associated to lying over . As we have seen, induces an isomorphism . As in the proof of Corollary 5.7 there is a sequence of rational piecewise affine linear convex functions on converging locally uniformly to . Let be the piecewise affine linear functions on such that . By Proposition 5.9 the metrics induced by the are semipositive piecewise -linear metrics on which implies that the metric induced by is locally semipositive. As the restriction of to is an isomorphism onto we have
By Corollary 5.5 we have
Hence
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. ∎