Now we prove the equality part of Theorem2.1in the general case by induction on n=dimX. The casen=1 is proved, so we assumen≥2.
LetM,L1, . . . ,Ln−1be integrable line bundles onX such that the following con-ditions hold:
(1) Mis trivial onX;
(2) MisLi-bounded for everyi; (3) M2·L1· · ·Ln−1=0.
There are two key lemmas in our proof. The first one is inspired by Błocki [3].
Lemma 2.5 For any semipositive integrable line bundles L0i on X with underlying bundle L0i equal to Li, and any integrable integrable line bundle M with trivial underlying line bundle M, the following identities hold:
M·M·L01· · ·L0n−1=0, M2·M·L01· · ·L0n−2=0.
Proof The second equality follows from the first one by takingL¯0n−1= ¯Ln−1±M¯ with small. So it suffices to prove the first inequality. Note that condition (2) implies
that every Li is semipositive. ReplacingLi by a large multiple if necessary, we can assume that bothLi±Mare semipositive for eachi. DenoteL±i =Li ±M.
First, we have the following equality
M2·L(11)· · ·L(n−n1−1) =0
for any sign function : {1, . . . ,n−1} → {+,−}. In fact, sinceMis Li-bounded, there is a constantt >0 such that
L±i t :=Li −t L±i =(1−t)Li∓t M is semipositive for anyi. Write
M2·L1· · ·Ln−1=M2·t L(11)· · ·t L(n−n1−1)+
n−1
=1
M2·
i≤
L(i ti)·
j>
L(i i).
Then the inequality part of Theorem2.1implies that every term on the right hand side is non-positive. It forces
M2·L(11)· · ·L(n−n1−1)=0.
Second, we claim that for anyand any sign functionon{, . . . ,n−1}, M2·L01· · ·L0−1·L() · · ·L(n−n1−1)=0. (1) When=n, it gives
M2·L01· · ·L0n−1=0.
By the Cauchy–Schwarz inequality in Corollary2.4, it follows that M·M·L01· · ·L0n−1=0.
We now prove the claim by induction on . We already have the case = 1.
Assume the equality (1) forand for all sign functions on{, . . . ,n−1}. Letbe a sign function on{+1, . . . ,n−1}which has two extensions+, −on{, . . . ,n−1} by±()= ±. Applying Corollary2.4again toM,M± =L± −L0, and semipositive bundlesL¯01, . . . ,L¯0−1,L¯±(), . . . ,L¯(n−n1−1), we have four equalities:
M·Mδ1 ·L01· · ·L0−1·Lδ2· · ·L(n−n1−1)=0, δ1= ±, δ2= ±.
Take the difference for two equations with fixedδ1to obtain M·Mδ1·L01· · ·L0−1·M·L(++11)· · ·L(n−n1−1)=0.
It is just
M2·L01· · ·L0−1·(Lδ1 −L0)·L(++11)· · ·L(n−n−11)=0.
The left-hand side splits to a difference of two terms. One term is zero by the induction assumption for. It follows that the other term is also zero, which gives
M2·L01· · ·L0−1·L0·L(++11)· · ·L(n−n1−1)=0.
It is exactly the case+1. The proof is complete.
The second key lemma is the following induction result.
Lemma 2.6 Assume that Ln−1is ample. For any closed subvariety Y of codimension one in X , we have
M|2Y ·L1|Y· · ·Ln−2|Y =0.
Proof ReplacingLn−1by a multiple if necessary, we can assume that there is a global sections of Ln−1vanishing onY. Write div(s) =
iaiYi withai > 0 and prime divisorsYi. Use the non-archimedean induction formula developed by Chambert-Loir–
Thuillier and Gubler reviewed in “Appendix (Local intersections)”. We have
M2·L1· · ·Ln−1= t
i=1
aiM|2Yi ·L1|Yi · · ·Ln−2|Yi
−
Xan
logsc1(M)2c1(L1)· · ·c1(Ln−2).
The integral vanishes by Lemma2.5. In fact, takeMto be any vertical metrized line bundle onX and writeφ= −log1M as a real-valued function on Xan. Then the lemma gives
Xan
φc1(M)2c1(L1)· · ·c1(Ln−2)=0.
It implies that the measurec1(M)2c1(L1)· · ·c1(Ln−2)=0. Hence the integration of
−logsis also zero. By the proved inequality on everyYi, we have M|2Yi ·L1|Yi· · ·Ln−2|Yi =0.
Since one ofYi isY, the lemma is proved.
Now are ready to prove the equality part of Theorem2.1. LetMandLibe as in the theorem. We need to prove thatM is constant, or equivalently log1M is constant onXan. By induction onn, Lemma2.6immediately gives the following result.
Lemma 2.7 For any integral closed subvariety Y of codimension one in X , the restric-tion M|Y is constant on Y .
Go back to the line bundleMonX. Let|X|0be the set of closed points ofX, which is naturally a dense subset ofXan. It suffices to prove that log1Mis constant on|X|0. By the lemma, it suffices to prove that any two pointsx1,x2of|X|0 are connected by integral subvarieties of codimension one inX. By [20, Chapter III, Corollary 7.9], any hyperplane section is connected. It follows that any hyperplane section passing throughx1andx2connects them. This finishes the proof.
3 Arithmetic Hodge index theorem
In this section, we prove a Hodge index theorem for adelic metrized line bundles on projective varieties over number fields, generalizing the following theorem in the context of arithmetic intersection theory of Gillet–Soulé [15].
Theorem 3.1 ([13,21,29])Let K be a number field andπ : X → SpecOK be an arithmetic variety with a regular generic fiber of dimension n ≥ 1. Let L be an arithmetically ample hermitianQ-line bundle onX. LetMbe a hermitianQ-line bundle onXsuch that c1(MK)·c1(LK)n−1=0on the generic fiberXK. Then
c1(M)2·c1(L)n−1≤0,
and the equality holds if and only ifM=π∗M0for some hermitianQ-line bundle M0on SpecOK.
The theorem was due to Faltings [13] and Hriljac [21] forn = 1 and Moriwaki [29] for generaln.
3.1 Statements
LetX a projective variety overQ¯. In [40] and as reviewed in “Appendix (Arithmetic intersections)”, we have defined a categoryPic(X)of integrable metrized line bundles as certain limits of hermitian line bundles on models ofXoverOK, the ring of integers of number fieldsKinQ, and their intersection pairing. Then¯ Pic(X) denotes the group of isomorphism classes of objects ofPic(X), andPic(X )Q =Pic(X )⊗ZQis the group of integrable adelicQ-line bundles. We also refer to “Appendix (Arithmetic intersections)” for the notions “arithmetically positive” and “Li-bounded.” The main result of this section is the following Hodge index theorem for such metrized line bundles.
Theorem 3.2 Letπ : X → SpecQ¯ be a normal and integral projective variety of dimension n≥1. Let M be an integrable adelicQ-line bundle on X , and L1, . . . ,Ln−1
be n−1nef adelicQ-line bundles on X . Assume M·L1· · ·Ln−1=0and that each Li is big. Then
M2·L1· · ·Ln−1≤0.
Moreover, if Liis arithmetically positive and M is Li-bounded for each i , then the equality holds if and only if
M ∈π∗Pic( Q)¯ Q.
Remark 3.3 The assumption “M isLi-bounded for eachi” is necessary for the con-dition of the equality in Theorem3.2. For a counter-example, assumen =2 andL¯ is induced by an arithmetically ample line bundleLon an integral modelX of X over OK, the ring of integers in a number field. Letα: X → X be the blowing-up of a closed point inX, andMthe (vertical) line bundle onXassociated to the exceptional divisor endowed with the trivial hermitian metric1 =1. ThenMK ·LK =0 and M2·α∗L=0 by the projection formula. ButMis not coming from any line bundle on the base SpecOK. In this case,Mis notL-bounded if we convert the objects to the adelic setting.
Remark 3.4 A similar proof of Theorem3.2should give a function field analogue. In fact, letKbe an algebraic closure of the function field of a projective and smooth curve B over an algebraically closed fieldk. Fixing a point x ∈ X(K)gives a morphism i :X →alb(X)to the Albanese variety ofXoverK. See discussion in Sect. A.4. Let Abe aK/k-trace of alb(X), i.e,Ais an abelian variety defined overksuch thatAK
can be embedded into alb(X)as a maximal abelian subvariety defined overk. Then we have a surjective homomorphism alb(X)→ A⊗k K which induces a morphism j : X → AK over K. Then the theorem should hold overK with the last inclusion M ∈π∗Pic(K )Qreplaced by
M ∈π∗Pic(K )Q+ j∗Pic0(A)Q, where j∗denotes the following natural composition
Pic0(A)Q −→Pic(A)Q−→Pic(A×k B)Q−→Pic(A K)Q −→Pic(X )Q. The second arrow is the pull-back via the projection A×k B → Ato the first factor, and the last arrow is the pull-back via j : X→ AK.
Denote by Picτ(X)the group of isomorphism classes of numerically trivial line bundles onX. Assume thatMis aflatadelic line bundle onXin the sense of “Appendix (Flat metrics)”, i.e.,Mis numerically trivial and the adelic metric is flat at every place.
Then by Proposition5.19,M2·L1· · ·Ln−1does not depend on the choice of the flat metric ofM and the metrics ofL1, . . . ,Ln−1. Then it is reasonable to write
M,ML1,...,Ln−1 =M2·L1· · ·Ln−1.
The definition extends to a quadratic form on M ∈ Picτ(X)Rby bilinearity. In this case, we present anR-version of the theorem as follows.
Theorem 3.5 Let X be a normal projective variety of dimension n ≥1overQ. Let¯ M ∈Picτ(X)Rand L1, . . . ,Ln−1be n−1nefQ-line bundles on X . Then
M,ML1,...,Ln−1 ≤0.
Moreover, if each Li is ample, then the equality holds if and only if M=0.
The inequality part of Theorem3.5is implied by that of Theorem3.2. In fact, it suffices to assumeM ∈Picτ(X)Qfor the inequality.
To apply Theorem3.2, we need eachLi to be big. Thereafter, take an ample line bundle Aand a positive rational number. Define
Li, =Li+A.
LetL1,, . . . ,Ln−1,be any nef adelic line bundles extendingL1,, . . . ,Ln−1,. Then we have
M,ML1,...,Ln−1 =lim
→0M,ML1,,...,Ln−1, =lim
→0M2·L1,· · ·Ln−1, ≤0.
3.2 Curves
In this section, we prove Theorems3.2and3.5when dimX =1. Then degM =0 and the line bundlesLido not show up.
We first consider Theorem3.2. ThenM has a flat metric structure M0as defined in “Appendix (Flat metrics)”. By Proposition5.17(2)(3),M0is actually induced by a hermitian line bundle M on any regular integral model X/OK of X such that c1(M) = 0 onX(C)and thatMis perpendicular toPic(X )vert. Define a vertical adelicQ-line bundleN by
M =M0+N. NoteM0·N =0 by the flatness ofM0. It follows that
M2=M20+N2=M20+
v
N2v,
whereNvbe the restriction of NtoX⊗K Kvfor each placevof K. Now Theorems 3.2and3.5follow from the index theorem of Faltings and Hriljac forM0and the local index theorem in Theorem2.1forNvas follows:
(1) By the result Faltings and Hriljac,M20=M2≤0 where the equality is attained if and only ifM0∈π∗Pic(K )Q.
(2) By Theorem2.1,N2≤0 and the equality is attained if and only ifNvis constant for everyv.
Now we consider Theorem 3.5. Then M ∈ Pic0(X)R. It still has a flat metric structureM0, as anR-linear combinations of flat adelic line bundles. By linearity, the result of Faltings and Hriljac still gives
M20= −2hNT(M).
Here the Neron–Tate height functionhNT:Pic0(X)R→Ris extended from Pic0(X) by bilinearity. By [34, §3.8],hNT is positive definite on Pic0(X)R. This proves the theorem forn=1.
3.3 Inequality
We already known that the inequality part of Theorem3.5is implied by that of Theorem 3.2. Now we prove the inequality of Theorem3.2by induction onn =dimX. We have already treated the casen =1, so we assumen≥2 in the following discussion.
We make the following two further assumptions:
(1) X is smooth by the resolution of singularities of Hironaka and the projection formula in [40];
(2) eachLi is arithmetically positive in the sense of Definition5.3.
For the second assumption, fix an arithmetically positive adelic line bundle Aon X. Take a small rational number >0. SetLi =Li+AandM=M +δA. Here δis a number such that
M·L1· · ·Ln−1=(M+δA)·L1· · ·Ln−1=0. It determines
δ= −M·L1· · ·Ln−1 A·L1· · ·Ln−1. As→0, we haveδ→0 since
M·L1· · ·Ln−1→M ·L1· · ·Ln−1=0, A·L1· · ·Ln−1→ A·L1· · ·Ln−1>0.
Here we explain the last inequality, which uses the assumption thatLi is big and nef for eachi. We refer to [25] for bigness and nefness in the geometric setting. In particular, the bigness ofLi implies that there are ampleQ-line bundlesAi such that Li −Ai is effective. It follows that
A·L1·L2· · ·Ln−1≥ A·A1·L2· · ·Ln−1≥ · · · ≥ A·A1· · ·An−1>0.
Therefore, the inequalityM2·L1· · ·Ln−1≤0 is the limit of the inequalityM2· L1· · ·Ln−1 ≤ 0. Here every Li is arithmetically positive. This shows that we may assume (2).
By approximation, it suffices to prove
M2·L1· · ·Ln−1≤0
for a model (X,M,L1, . . . ,Ln−1)of (X,M,L1, . . . ,Ln−1)under the following assumptions:
(3) Xis smooth andXis normal;
(4) Liarithmetically ample onX in the sense of Lemma5.4.
Then the inequality was proved by Moriwaki [29] in the case that allLiare equal. The current case is similar, but we still sketch a proof as follows.
Lemma 3.6 There is a smooth metric · 0onMat archimedean places, unique up to scalars, such that the curvature form ofM=(M, · 0)on X(C)pointwise satisfies
c1(M)c1(L1)· · ·c1(Ln−1)=0,
where we omit the symbol for the wedge product of forms of degree2. Moreover, with this metric, one has pointwise
c1(M)2c1(L1)· · ·c1(Ln−2)≤0.
Proof For the first equality, writeM=(M,·)and·0=eφ·for a real-valued C∞-functionφonX(C). Then the above equation becomes
1
πi∂∂φ·c1(L1)· · ·c1(Ln−1)= −c1(M)c1(L1)· · ·c1(Ln−1).
Set = c1(L1)· · ·c1(Ln−1). Then the left-hand side is a scalar multiple of the Laplacianφwith respect to. The right-hand side is an exact form on every con-nected component Xσ(C)of X(C). In fact, its cohomology class is represented by
−M·L1· · ·Ln−1, which is zero in H2n(Xσ(C),C)=C. The solutionφexists by Gromov [14, Corollary 2.2 A2].
The inequality of the lemma is implied by the equality by Aleksandrov’s lemma. See Gromov [14, Lemma 2.1 A] by setting:=c1(L1)· · ·c1(Ln−2)andω0:=c1(Ln−1).
Write the original metric · ofMase−φ · 0. Then by the above lemma, we have
M2·L1· · ·Ln−1=M2·L1· · ·Ln−1−
X(C)φ 1
πi∂∂φ∧c1(L1)· · ·c1(Ln−1).
By integration by parts, the second term on the right
−
X(C)φ 1
πi∂∂φ∧c1(L1)· · ·c1(Ln−1)=
X(C)
1
πi∂φ∧∂φ∧c1(L1)· · ·c1(Ln−1).
It is non-positive since the volume form 1
πi∂φ∧∂φ∧c1(L1)· · ·c1(Ln−1)≤0. Hence, it suffices to prove
M2·L1· · ·Ln−1≤0.
By Moriwaki’s arithmetic Bertini theorem (cf. [28], Theorem 4.2 and Theorem 5.3), which is also stated at the beginning to [29, §1], replacingLn−1by a multiple if necessary, there is a nonzero section s ∈ H0(X,Ln−1)satisfying the following conditions:
• The supremum normssup=supx∈X(C)s(x)<1;
• The horizontal part of div(s)onX is a generically smooth arithmetic varietyY;
• The vertical part of div(s)onX is a positive linear combination
℘a℘X℘ of smooth fibersX℘ofX above (good) prime ideals℘ofOK.
Then
M2·L1· · ·Ln−1=M|2Y ·L1|Y· · ·Ln−2|Y +
℘
a℘M|2X℘·L1|X℘· · ·Ln−2|X℘
−
X(C)logsc1(M)2c1(L1)· · ·c1(Ln−2).
It suffices to prove each term on the right-hand side is non-positive. The non posi-tivity of the “main term”
M|2Y·L1|Y· · ·Ln−2|Y ≤0 follows from the induction hypothesis. By flatness ofX/OK,
M|2X℘·L1|X℘· · ·Ln−2|X℘ =(M2·L1· · ·Ln−2)log(#OK/℘)≤0.
Here we have used the Hodge index theorem in the geometric case as stated in Theorem 5.20. By the above lemma,
−
X(C)logsc1(M)2c1(L1)· · ·c1(Ln−2)≤0.
3.4 Equality
In this section, we prove the equality part of Theorem3.2and Theorem3.5. We have already treated the casen=1, so we assumen≥2 in the following discussion.
3.4.1 Reduce to flat metrics We first treat Theorem3.2.
Lemma 3.7 Under the conditions in the equality part of Theorem3.2, M is numeri-cally trivial on X .
Proof SinceLn−1is arithmetically positive,Ln−1:= Ln−1−π∗N is nef for some N ∈Pic(K)withc=deg(N) >0. Then
0=M2·L1· · ·Ln−2·Ln−1=M2·L1· · ·Ln−2·Ln−1+c M2·L1· · ·Ln−2. The two terms in the right hand are both non-positive:
(1) Apply the inequality of the theorem to(M,L1, . . . ,Ln−2,Ln−1)to obtain
M2·L1· · ·Ln−2·Ln−1≤0.
(2) Apply the geometric Hodge index theorem in Theorem5.20onXto obtain M2·L1· · ·Ln−2≤0.
It follows that
M2·L1· · ·Ln−2·Ln−1= M2·L1· · ·Ln−2=0.
By the geometric Hodge index theorem again,Mis numerically trivial.
In the following discussion, we assume thatMis numerically trivial. Then we can extend M to a flat adelicQ-line bundleM0. See “Appendix (Flat metrics)” for the definition of flat metrics again. The difference N := M −M0 is vertical. Then we have a decomposition
0=M2·L1· · ·Ln−1=M20·L1· · ·Ln−1+N2·L1· · ·Ln−1. The inequality part of Theorem3.2implies
M20·L1· · ·Ln−1=N2·L1· · ·Ln−1=0.
By the local Hodge index theorem in Theorem2.1,N ∈Pic(K )Q. Thus we only need to treat the caseM =M0.
3.4.2 Flat metric case
Here we prove the equality part of Theorem 3.2 assuming that M is flat. Let M,L1, . . . ,Ln−1be as in the equality part of the corollary. The key is the following induction result.
Lemma 3.8 For any closed integral subvariety Y of codimension one in X , we have M|2Y ·L1|Y· · ·Ln−2|Y =0.
Proof We prove this lemma by the same idea as in the proof of Lemma2.6. Replacing Ln−1by a multiple if necessary, we can assume that there is a nonzero global section sofLn−1vanishing onY. Write
div(s)= t
i=1
aiYi,
withai >0,Yi distinct, andY1 =Y. Use the sectionsto compute the intersection numberM2·L1· · ·Ln−1. Then
M2·L1· · ·Ln−1= t
i=1
ai M|2Yi ·L1|Yi· · ·Ln−2|Yi
−
v
Xanv
logsvc1(M)2c1(L1)· · ·c1(Ln−2).
By the flatness ofM, all the integrals above vanish. See Proposition5.17for more details on flat metrics. Hence,
M2·L1· · ·Ln−1= t
i=1
ai M|2Yi ·L1|Yi· · ·Ln−2|Yi.
Since every term on the right-hand side is non-positive, we obtain M|2Y ·L1|Y· · ·Ln−2|Y =0.
This proves the lemma.
Go back to the proof of Theorem3.2. Still use induction onn=dimX. We apply the lemma to a general hyperplane section Y of some very ample line bundle on X. Note thatX is normal. By a Bertini-type result of Seidenberg [32],Y is normal (and projective). By Lemma3.8and the induction hypothesis,M|Y is constant in the sense that it is a pull-back from a number field. In particular,M =0 in Pic(Y)Q. By Theorem5.21. the natural map Pic0(X)→Pic0(Y)has a finite kernel. Hence,M =0 in Pic(X)Q. ReplacingM by positive multiple, we may assume thatM =O . Now
the constancy of M on X follows from the local Hodge index theorem in Theorem 2.1.
3.4.3 Proof of Theorem3.5
The above proof also applies to the equality part of Theorem3.5. LetL1, . . . ,Ln−1
be any nef adelic line bundles extending L1, . . . ,Ln−1. By Proposition5.18, M ∈ Picτ(X)R extends to an R-linear combination M of flat adelic line bundles on X.
Lemma3.8still works in the current setting. By induction, we can assumeM|Y =0.
Then the injectivity of Pic0(X)R→Pic0(Y)RimpliesM =0.
Remark 3.9 As a consequence of the proof of Lemma3.8, we have the interpretation M,ML1,...,Ln−1 = M|C,M|C = −2hNT(M|C).
Here C is any smooth projective curve in X representing the intersection L1 · L2· · ·Ln−1 in thatC = div(s1)·div(s2)· · ·div(sn−1)for a collection of sections si ∈ H0(X,Li).
4 Algebraic dynamics
In this section, we present and prove the rigidity theorem (Theorem4.1) about the preperiodic points of algebraic dynamical systems. After the statements, we first intro-duce a theory of admissible adelic line bundles and then prove the theorem using our arithmetic Hodge index theorem.