3 The domain of definition of the complex Monge-Ampère operator.
We start with a few definitions.
Definition 1
Let be a compact complex manifold of complex dimension , let
be a pseudoeffective class, let be a hermitian form and let be a finite covering of coordinate starshaped open sets. We denote by the set of closed positive -currents such that
|
|
|
with , and over , for all .
It is clear by the definition that the closed positive currents , for are globally well defined. Consider now be a closed positive -current with continuous local potentials. We define
|
|
|
Let with zero Lelong numbers. It is well known from the first author work (which becomes drastically simple in this particular case) the existence of a family , , such that as . In the case
the convergence of is also uniform. We have the following crucial result.
Theorem 5
(Degenerate monotone convergence result).
Let be a polarized compact Kähler manifold of complex dimension and let , be closed positive -currents with continuous local potentials. Then the following statements hold true.
A) For all , and , ,
|
|
|
B) Let , with zero Lelong numbers and , such that as
. Then for all , ,
|
|
|
(3.1) |
|
|
|
|
|
|
(3.2) |
weakly as . Moreover for all and , .
As follows immediately from the proof, the statement of this theorem still holds if we replace with a product , where the currents have the same properties as . As a matter of fact, we wrote the statement in the previous special case only for the sake of notation simplicity. However during the proof it is useful to consider that statements concerning terms involving are still valid if we replace with .
Proof. Statement (3.2) follows from (3.1) by using the weak continuity of the operator and an induction on (3.2). We remark that claim 2 asserts statement A) in full generality for .
We denote by the assertion A) in the statement of the theorem for the relative indices . For all and we define the following statement : for all
|
|
|
(3.3) |
|
|
|
|
|
|
(3.4) |
|
|
|
|
|
|
(3.5) |
|
|
|
|
|
|
(3.6) |
weakly as and
|
|
|
(3.7) |
We remark that (3.4) follows from (3.3) by the weak continuity of the operator. By combining (3.4) with the weak continuity of the operator we obtain
|
|
|
weakly as .
On the other hand implies
|
|
|
|
|
|
|
|
|
|
In this way we deduce (3.5).
The symmetry identity (3.7) follows from (3.5) for and from the fact that
|
|
|
weakly as . This last convergence statement follows by combining (3.5) for with an induction on by means of the weak continuity of the operator.
We now prove simultaneously the statements and , by using an induction on . For the moment we assume that the potential in statement A) also has zero Lelong numbers, but we will get rid of this hypothesis at the end.
Statements and are true by claim 2 and its proof. So we assume that these statements hold for and we prove them for .
The induction process is divided in two main steps.
Step I. This step consists in proving the
Claim 5
.
If and hold true for all , then implies , with .
As pointed out before in order to prove is sufficient to show (3.3) and (3.6). The proof of (3.6) is quite similar to the proof of (3.3) that we now explain. We first prove by induction on the inequality
|
|
|
|
|
(3.8) |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Inequality (3.8) is obviously true for . (Here we adopt the usual convention of neglecting a sum when it runs over an empty set of indices.) Before procedding to the proof of inequality (3.8), we need to point out two useful remarks.
1) Let be a smooth closed real -form, e a closed positive -current, be a measurable function such that . This implies that the currents and
are well defined.
Then the Leibnitz formula implies
|
|
|
(3.9) |
2) Thanks to the inductive hypothesis , we have
|
|
|
for all . By (3.9) this implies
|
|
|
so the current
|
|
|
is well defined and we can define the current
|
|
|
Then the integration by parts formula
|
|
|
writes explicitly as
|
|
|
|
|
(3.10) |
|
|
|
|
|
We suppose now inequality (3.8) true for and we prove it for . We start by expanding, thanks to formula (3.9), the integral
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
By applying the integration by parts formula (3.10) to the last integral we deduce
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
By combining the main -inductive hypothesis
, in , for
with formula (3.9) we get
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
By plugging this into the previous expression of we obtain
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
which implies inequality (3.8) for . The inequality (3.8) for rewrites as
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
By using the convergence inductive hypothesis , in for we deduce
|
|
|
|
|
|
|
|
|
(3.11) |
since we suppose true. (We can always arrange for all by changing into .) Thus by weak compactness of the mass there exists a sequence , and a current of order zero such that
|
|
|
weakly as . So for any strongly positive -form , we have
|
|
|
weakly as . The fact that and
|
|
|
weakly as , by the convergence inductive hypothesis , implies
|
|
|
thanks to lemma (3.9), page 189 in [Dem1]. Thus . Combining this with the inequality (3.11) we obtain
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
We deduce , which implies since . This proves statement .
Step II. This step consists in proving
Claim 6
.
If and hold true for all , then hold also true for all .
We prove this claim by induction on . For the conclusion follows from the hypothesis . So we assume and we prove . For this purpose set , with smooth and continuous and expand the integral
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Hypothesis implies by step I
|
|
|
weakly as . Thus by taking the limit as in the previous identity and by combining the inductive hypothesis for with the weak continuity of the operator, we deduce
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
By weak compactness of the mass we infer the existence of a sequence , and a current of order zero such that
|
|
|
weakly as . In particular
|
|
|
weakly as . The fact that and
|
|
|
weakly as , by the convergence -inductive hypothesis , in the statement , implies
|
|
|
thanks to lemma (3.9), page 189 in [Dem1]. We conclude
|
|
|
End of the proof. In the case the Lelong numbers of are not zero we replace in the previous computations with and with . Here is chosen sufficiently big such that for all and as . Then the previous arguments still work and statement A) rewrites as
|
|
|
Statement B) of the theorem rewrites as
|
|
|
(3.12) |
|
|
|
|
|
|
(3.13) |
weakly as
and for the relative indices . The last inequality implies . In fact this follows by expanding by linearity the term and using an induction by means of formula (3.9). The base of the induction follows from claim 2.
We consider also the subset .
Without changes in the proof of theorem 5 we get the following corollary.
Corollary 2
For all the assertions A, B and (3.12), (3.13) of theorem 5 hold for all .
Let now be a closed positive -current and consider the -space
|
|
|
equipped with the hermitian product , which is well defined by the polarization identity. The -almost everywhere equality relation is defined by : iff
|
|
|
Let . We say that the sequence converges -weakly to if
|
|
|
for all . Let such that . Then one can define . We write if there exists such that in the sense of currents. In this case we write
|
|
|
With this notations we have the following corolary of theorem 5.
Corollary 3
.
Let be a polarized compact Kähler manifold of complex dimension and let , be closed positive -currents with continuous local potentials, let be a closed positive -current and consider , , , . Then for all , ,
|
|
|
(3.14) |
|
|
|
|
|
|
(3.15) |
Moreover let , ,
, such that , as . Then
|
|
|
(3.16) |
|
|
|
|
|
|
(3.17) |
Proof. By integrating by parts we obtain
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
By the proof of theorem 5 we can take the limit, so
|
|
|
|
|
(3.18) |
|
|
|
|
|
On the other hand the weak convergence of the sequence
|
|
|
combined with the weak continuity of the operator implies
|
|
|
weakly as . Then the -weak compactness implies (3.14) and
the -weak convergence as , which implies
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
by identity (3.18). This implies (3.16) by elementary facts about Hilbert spaces. The proof of (3.15) and (3.17) is quite similar.
The conclusion of the corollary 3 still holds true if we replace the current with a sum of currents
|
|
|
where such that . We infer the linearity formula
|
|
|