4 Existence and uniqueness of the solutions.
Theorem 6
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Let be a compact Kähler manifold of complex dimension , let be a smooth volume form, let be a continuous closed positive -form such that is a set of measure , let be a closed positive -current with continuous local potentials such that , . Let also , such that and be a real number. Then there exists a unique solution of the degenerate complex Monge-Ampère equation
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which in the case is normalized by .
The solution is continuous and satisfies the -estimate
, with
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Moreover the constant stays bounded for perturbations of as in the statement (C) of theorem 3.
Proof. Let be a Kähler metric and write whith smoth and continuous. There exist a sequence , whith , such that uniformly and whith . We consider also a regularizing family , of in . We can assume
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otherwise we multiply by a constant which converges to by the normalising condition . We distinguish two cases.
Case .
By Yau’s solution of the Calabi conjecture there exists a unique family ,
of smooth solutions of the complex Monge-Ampère equations
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The hypothesis (C1) and (C2a) of statement (C) of theorem 3 are obviously satisfied for the family .
We deduce that the constant in the statement of theorem 3,A does not blow up as . Moreover the uniform estimate
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(4.1) |
holds for all .
(See [Ra-Re] page 364 or [Iw-Ma], theorem 4.12.2, page 79.) Thus by theorem 3, A we obtain the uniform estimate . On the other hand we have a priori a uniform estimate since the local potentials of the family stay uniformly bounded. Thus, by elementary properties of plurisubharmonic functions, (see [Dem1], chapter 1) there exists a -convergent subsequence (which by abuse of notations we denote in the same way).
We can apply theorem 3, B to the complex Monge-Ampère equation in consideration
since we dispose of the estimate (4.1) and
the constant
in the relative statement is uniformly bounded in thanks to the same considerations concerning the constant .
We infer that the sequence is a Cauchy sequence in the -topology, thus convergent to some
. This implies the convergence of the weak limits
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thus is the required solution of our degenerate complex Monge-Ampère equation.
We normalise the solution whith the condition .
We prove now the uniqueness of the solutions in the case . Let be another solution. Then the identity implies by claim 2 in the proof of theorem 5.
Let , be as in the statement of corollary 3 and set , . Let us also recall the formula
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From there we deduce
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(4.2) |
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since in by corollary 3.
Inspired by an idea of S. Blocki [Blo1], we will prove by induction on that
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(4.3) |
for all , . For this follows from (4.2). So we assume (4.3) for and we prove it for . In fact consider the identity
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By applying several times corollary 3 and by integrating by parts we derive
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(4.4) |
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Set or . Then the Cauchy-Schwarz inequality implies
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by the inductive hypothesis. This combined with (4.4) implies (4.3) for . So at the end of the induction we get
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which implies by elementary properties of plurisubharmonic functions.
Case . We start by proving the following lemma, which is a particular case of a more general result due to Yau. (See [Yau], sect. 6, page 376).
Lemma 6
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Let be a compact Kähler manifold of complex dimension , let be a smooth function such that and a solution of the complex Monge-Ampère equation
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(4.5) |
. Consider also two solutions of the complex Monge-Ampère equation such that . Then .
Proof. The argument is a simplification, in our particular case, of Yau’s original argument for the proof of thm. 4, sect. 6 in [Yau]. Set and consider the solutions of the complex Monge-Ampère equations given by the iteration
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(4.6) |
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(4.7) |
Notice that we can solve this equations even if the terms , are not normalized, see lem. 2 page 378 in [Yau]. Set and consider
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At a maximum point of we have the inequality
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By plugging this into the previous one, we deduce . We now prove by induction the inequality . In fact by dividing with we get
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At a maximum point of we again find the inequality
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Combining this with the previous one we deduce . We also prove by induction the inequality , which is true by definition in the case . By dividing with we get
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by the induction hypothesis . At a minimum point of we get
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hence . As a conclusion, we have proved the sequence of inequalities
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(4.8) |
These inequalities imply , where satisfies the uniform estimate
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(4.9) |
and , which are obtained by applying the maximum principle in a way similar to Yau’s proof of the second order estimate for the solution of the Calabi conjecture [Yau]. (In the case the uniform estimate follows immediately from the inequalities (4.8).) Fix now a constant such that the inequality
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hold for all .
This implies by (4.9) the estimate
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thus
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by iteration. By taking the derivate in the Green Formula (see [Aub], Th. 4.13 page 108) we get the identity
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which implies the estimate . By applying the complex version of the Evans-Krylov theory [Ti2] we deduce the uniform estimate . This implies that the sequence converges in the -topology to the unique solution of the complex Monge-Ampère equation (4.5). Then the conclusion follows from inequalities (4.8).
We
consider now the unique family ,
of smooth solutions of the complex Monge-Ampère equations
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given by the Aubin-Yau’s solution of the Calabi conjecture.
Consider also the solutions , , of the complex Monge-Ampère equation
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By applying lemma 6 we deduce for all . By the same argument in the case , we deduce , thus and so
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This fact allows us to apply theorem 3, B as in the case in order to get a sequence of solutions convergent in the uniform topology to some . This implies the convergence of the weak limits
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The integral estimate in the statement of theorem 6 follows immediately from theorem 3, A and from the inequalities . We prove now the uniqueness of the solutions. Let be another solution. The fact that implies
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which allows us to solve the degenerate complex Monge-Ampère equation
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with . By the uniqueness result in the case we deduce , thus
. By applying the comparison principle we get
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which implies since . This implies -almost everywhere, thus
-almost everywhere. By symmetry we also deduce -almost everywhere. The fact that are solutions of our complex Monge-Ampère equation implies that a property holds -almost everywhere if and only if it holds -almost everywhere and the same for . We thus infer , which implies by the expression of the Monge-Ampère equation.
The following lemma gives us an important class of functions for the right hand side of the degenerate complex Monge-Ampère equation.
Lemma 7
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Let be a compact complex manifold, let be a smooth volume form and let , , , be, non identically zero, holomorphic sections of some holomorphic vector bundles over such that the integral condition
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holds for some real numbers . Then the integrand function belongs to some space, and the family of functions
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, , converges in -norm to function when .
Proof.
We define the coherent complex analytic sheaves and ,
with , for some local holomorphic trivializations of the vector bundles , . Clearly the definition is independent of the local trivialization, thus this sheaves are globally well defined. By the Hironaka desingularization theorem [Hir] we can find a proper bimeromorphic morphism
of compact complex manifolds such that there exists a family , of smooth hypersurfaces with normal crossing in such that
,
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and
, , , for all .
The fact that is a holomorphic map implies , with . Thus the divisor of the Jacobian of is by definition . On the other hand the invariance of the integral by orientation preserving diffeomorphisms implies
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For any open set we denote by . For any point in one can find a coordinate neighborhood such that , . With respect to this coordinates, we have
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, and , , . Then modulo factors that are bounded away from and , we find
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with , . The latter nonvanishing property follows from the fact that the terms ,
, correspond to local generators of the sheaf and are local generators of the sheaf . We infer
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with , , , and . Set also . The hypothesis implies for all . Thus there exists such that for all . If is a finite covering of , with as and then . This proves the first claim in the statement of lemma 7. In order to prove the convergence in the norm of the functions we distinguish two cases. In the case where for all , the claim follows imediately from the monotone convergence theorem. The other possible case is for all . In this case we set . Then our setting implies . For all consider the sequence of inequalities
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where is a constant uniform in . Thus by letting and by applying the increasing monotone convergence theorem to the first integral in the previous inequalities, we obtain
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Then the conclusion follows by letting and by the fact that converges pointwise almost everywhere to as .