European Journal of Mathematics

, Volume 2, Issue 1, pp 87–95

# Rational connectedness and order of non-degenerate meromorphic maps from $${{\mathbb C}}^n$$

• Frédéric Campana
• Jörg Winkelmann
Research Article

## Abstract

We show that an n-dimensional compact Kähler manifold X admitting a non-degenerate meromorphic map $$f:{\mathbb C}^n\rightarrow X$$ of order is rationally connected.

## Keywords

Rational connectedness Order of a non-degenerate meromorphic map

## Mathematics Subject Classification

32A22 14E08 32H04 14J40 32Q45

## 1 Introduction

The purpose of this paper is the following result.

### Theorem 1.1

Let X be a compact Kähler manifold of fixed dimension n. Let $$f:{\mathbb C}^n\dasharrow X$$ be a non-degenerate meromorphic map of order (see the next section for the definition of ). Then X is rationally connected, hence projective.

(Here a meromorphic map $$f:{\mathbb C}^n\dasharrow X$$ is called non-degenerate if there is a point v such that f is defined in v and is surjective.)

This result belongs to a series of similar statements relating the existence and growth of maps from $${\mathbb C}^n$$ to algebro-geometric properties of the target space X. These statements are better expressed by introducing the following invariant $$\rho (X)$$, which suggests many questions, some of which are raised in the last section.

### Definition 1.2

Let X be an n-dimension connected compact complex manifold. Define . It is understood that , and that if there exists no non-degenerate meromorphic map $$f:{\mathbb {C}}^n\rightarrow X$$.

The invariant $$\rho$$ is easily seen to be bimeromorphic, preserved by finite étale covers, and increasing (i.e., $$\rho (X)\geqslant \rho (Y)$$ if there exists a dominant meromorphic map $$g:X\rightarrow Y$$). It is obvious that $$\rho (X)=0$$ if X is unirational, because for every rational map f.

Kobayashi and Ochiai proved that the existence of a non-degenerate meromorphic map from $${\mathbb C}^n$$ to a projective manifold X implies that X is not of general type [9]. Thus if X is of general type. It is proved in [4], more generally, that X is ‘special’ if there exists a non-degenerate meromorphic map . In particular, X must be special if .

If X is Kähler and $$K_X$$ is pseudo-effective of numerical dimension $$\nu \in \{0,1,\dots , n\}$$, then  [6]. This implies the previous result of Kobayashi–Ochiai (since X is of general type if and only if $$\nu =n$$). Using [2], it also implies that if X is projective, then X is uniruled if $$\rho (X)<2$$.

If $$h^0\bigl (X,\mathrm{Sym}^k(\mathrm{\Omega }^p_X)\bigr )\ne 0$$, for some $$k,p>0$$, then $$\rho (X)\geqslant 2$$ [11]. The Kähler condition is not required here. If, in addition, X is assumed to be a Kähler surface, then the condition $$h^0\bigl (X,\mathrm{Sym}^k(\mathrm{\Omega }^p_X)\bigr )=0$$ for all pk implies that X is rational. Thus, if X is a Kähler surface and if $$\rho (X)<2$$, then X must be rational [11]. On the other hand, some Hopf (thus non-rational, non-Kähler) surfaces have $$\rho (X)= 1$$ [11].

Mumford conjectured that for a projective manifold (of arbitrary dimension) the condition “$$h^0\bigl (X,\mathrm{Sym}^k(\mathrm{\Omega }^p_X)\bigr )\ne 0$$ for some $$k,p>0$$” is equivalent to X being not rationally connected. However, it is still unknown whether this conjecture holds.

The present result generalizes these two results from [6, 11], avoiding the deep methods of [6]. The estimate is optimal, since for an Abelian variety A we have $$\rho _\tau =2$$ for the universal covering map $$\tau :{\mathbb C}^g\rightarrow A$$.

All these results provide lower bounds for $$\rho (X)$$ deduced from the geometry of X. Producing upper bounds for $$\rho (X)$$ (i.e., the existence of non-degenerate f) turns out to be a completely open topic, except in the trivial case of X unirational or a torus.

The case of rationally connected vs unirational manifolds when $$n\geqslant 3$$ is of great interest. For example, what is $$\rho (X)$$ if X is a ‘general’ smooth quartic in $${\mathbb {P}}^4$$? If $$\rho (X)>0$$, then X is not unirational. More generally, are there projective manifolds X such that $$\rho (X)\in (0,2)$$? Such a manifold X would necessarily be rationally connected, but not unirational. See the last section for some more questions.

## 2 Characteristic function and order of a non-degenerate meromorphic map

We start with some preparations. Let X be a compact complex manifold and let be a meromorphic map. The map f is said to be (differentiably) non-degenerate if there is a point $$p\in {\mathbb C}^n$$ such that f is holomorphic at p and is surjective.

Let on $${\mathbb C}^n$$, and let $$\omega$$ be a positive (1, 1)-form on X. The characteristic function of f (with respect to $$\omega$$) is defined as
Here $$B_t=\{z\in {\mathbb C}^n:\Vert z\Vert <t\}$$. Observe that the integral over $$B_t$$ is well defined even if f is only meromorphic, not necessarily holomorphic.
If $$\omega$$ and $$\widetilde{\omega }$$ are any two positive (1, 1)-forms on X, then (by the compactness of X) there are constants $$C_1,C_2>0$$ such thatand, consequently,
The characteristic function is a very fine tool for measuring the growth of a map. A coarser invariant is the order which is defined as
where $$\omega$$ is a positive (1, 1)-form on X. Due to inequalities (1) this number is independent of the choice of the positive (1, 1)-form $$\omega$$ used in the definition of .

The well-known Crofton’s formula (see e.g. [8]) implies that equals the average of taken over all complex lines $$L\subset {\mathbb C}^n$$. This permits to extend many properties of characteristic functions from the case of entire curves to the higher-dimensional domains. For instance, let $$\tau :X'\rightarrow X$$ be a bimeromorphic holomorphic map between compact complex manifolds. Then is again a non-degenerate meromorphic map, and .

Now let $$\alpha :X\rightarrow Y$$ be a dominant holomorphic map. Then it follows directly from the definitions that (if $$\omega _X\geqslant \alpha ^*(\omega _Y)$$) and, consequently, .

The order is easily seen to behave nicely with respect to products. Let , $$\omega _i$$ be positive (1, 1)-forms in $$X_i$$ and let $$f_i:{\mathbb C}^{n_1}\rightarrow X_i$$ be non-degenerate meromorphic maps. Define , $$n=n_1+n_2$$ and $$f:{\mathbb C}^n\rightarrow X$$ as $$f(v_1,v_2)=(f_1(v_1),f_2(v_2))$$. Then . This implies , because for any $$t_1,t_2>0$$ we have

## 3 Rational connectedness

As usual, a compact complex manifold X (usually Kähler) is called rationally connected (RC) if every two points can be linked by a (possibly singular) irreducible rational curve. (Equivalently in the Kähler case: can be linked by a chain of rational curves). Unirational manifolds are RC. There are unirational threefolds which are not rational, e.g. smooth cubic hypersurfaces in $${\mathbb P}_4$$. However, it is not known whether there exist RC manifolds which are not unirational, although it is expected that these should exist (the case of ‘general’ quartics in $${\mathbb P}_4$$ being one of the first open cases) .

Rationally connected compact Kähler manifolds are projective (since $$h^{2,0}=0$$, using Kodaira’s criterion).

Let X be a compact connected Kähler manifold. Then there exists an “almost holomorphic” rational dominant map (called the RC-reduction, or rational quotient [3], or the MRC-fibration [10] if X is projective), such that the fibers are RC, and maximal with this property.

When X is projective, it is known (by [7]) that the base Y is not uniruled. In fact, [7] shows that if is a surjective meromorphic map with fibres and base Y which are both RC, then X is RC if it is compact Kähler (remark first that $$h^{2,0}(X)=0$$, and that X is thus projective). The base Y of the RC-reduction is not uniruled also when X is compact Kähler. Let indeed $$r:Y\dasharrow Z$$ be the RC-reduction of Y. The fibres of are thus RC, and so $$Y=Z$$, which means that Y is not uniruled.

Due to [2] a projective manifold is uniruled if and only if $$K_X$$ is not pseudo-effective. Based on this, another recent criterion is the following (see [5]): Let X be a compact Kähler manifold. Then X is rationally connected if and only if there is no pseudo-effective invertible subsheaf $$F\subset \mathrm{\Omega }^p_X ($$for some $$p\in {\mathbb N})$$.

The proof consists in observing that X is not RC precisely if its RC-reduction has $$\dim Y=p>0$$. Define then $$F=\rho ^*(K_Y)$$, which is pseudo-effective since Y is not uniruled.

It is conjectured that a compact Kähler (or equivalently, projective) manifold X is RC if (and only if) there is no $${\mathbb Q}$$-effective (instead of pseudo-effective) invertible subsheaf $$F\subset \mathrm{\Omega }^p_X$$ (for some $$p\in {\mathbb N}$$). By means of the RC-reduction as above, this conjecture is equivalent to the ‘non-vanishing conjecture’, claiming that if $$K_X$$ is pseudo-effective, it is $${\mathbb Q}$$-effective.

## 4 Pseudo-effective line bundles

A singular hermitian metric h on a complex line bundle is given in the form , where s is the standard metric for some local holomorphic trivialization and $$\phi$$ is a $$L^1_\mathrm{loc}$$-function. The $$L^1_\mathrm{loc}$$-condition ensures that the curvature makes sense (in the sense of currents) and represents the Chern class of the line bundle.

A line bundle L on a compact complex manifold is called pseudo-effective if there is a singular hermitian metric h with semipositive curvature $$\mathrm{\Theta }_h\geqslant 0$$. This condition means that the metric is locally given via a weight function $$e^{-\phi }$$ with $$\phi$$ being plurisubharmonic. Now let $$F^*$$ be the dual line bundle equipped with the dual metric $$h^*$$, i.e., the metric with weight factor $$e^\phi$$. If F is pseudo-effective and s is a holomorphic section in $$F^*$$, plurisubharmonicity of $$\phi$$ implies that $$\log \Vert s\Vert _{h^*}$$ is plurisubharmonic.

## 5 Measuring the derivative

Let VW be complex vector spaces equipped with hermitian inner products. The norm $$\Vert F\Vert$$ of a complex linear map $$F:V\rightarrow W$$ is defined by where $$F^\dagger$$ denotes the adjoint of F. If A is the matrix describing F with respect to orthonormal bases on V and W, then
\begin{aligned} \Vert F\Vert ^2 = \sum _{i,j} |A_{ij}|^2\!. \end{aligned}
The inner products on V and W naturally induce inner products on the dual vector spaces $$V^*$$, respectively, $$W^*$$. Observe that, given a linear map $$F:V\rightarrow W$$, the dual map $$F^*:W^*\rightarrow V^*$$ fulfills $$\Vert F^*\Vert =\Vert F\Vert$$.
Let $$k\in {\mathbb N}$$. Then F induces linear maps $$\otimes ^kF:\otimes ^kV\rightarrow \otimes ^kW$$ and $$\mathrm{\Lambda }^k:\mathrm{\Lambda }^k V \rightarrow \mathrm{\Lambda }^k W$$. An orthonormal base $$e_1,\ldots ,e_n$$ of V induces an orthonormal base of $$\otimes ^kV$$ given by all tensors of the form . Using such bases, it is clear that . As a consequence,because $$\mathrm{\Lambda }^kV\subset \otimes ^kV$$.
We now compare this norm with the operator norm which we denote as $$\Vert F\Vert _\mathrm{op}$$. It is defined as
Clearly, $$\Vert F(e_j)\Vert \!\leqslant \!\Vert F\Vert _\mathrm{op}$$ if $$e_j$$ form an orthonormal base. Since $$\Vert F\Vert ^2\!=\!\sum _i\Vert F(e_i)\Vert ^2$$, we have . On the other hand, evidently $$\Vert F(e_j)\Vert ^2\!\leqslant \!\Vert F\Vert ^2$$ for all j and therefore for all $$v\in V$$. Thus $$\Vert F\Vert _\mathrm{op}\leqslant \Vert F\Vert$$.
We continue with a local observation. Let UV be open subsets in $${\mathbb C}^n$$, let be the Kähler form for the euclidean metric and let $$f:U\rightarrow V$$ be a holomorphic map. Then
where

### Proposition 5.1

Let $$U\subset {\mathbb C}^n$$ be an open subset, X an n-dimensional compact complex manifold equipped with a hermitian metric h and a positive (1, 1)-form $$\omega$$ and let $$f:U\rightarrow X$$ be a holomorphic map. Let . Then there is a constant $$C>0$$ such that
Here the norm $$\Vert Df\Vert$$ is calculated in each point $$x\in U$$ with respect to h on and the euclidean norm on .

### Proof

We cover X with finitely many open subsets $$V_k$$ such that each $$V_k$$ admits an embedding $$j_k:V_k\rightarrow {\mathbb C}^n$$ and each $$V_k$$ contains a relatively compact open subset $$W_k\subset V_k$$ such that $$X=\bigcup _kW_k$$. Let $$h_k$$ denote the hermitian metric on $$V_k$$ induced by the euclidean metric via its embedding in $${\mathbb C}^n$$. We choose $$C_1>0$$ such that $$h_k\leqslant C_1h$$ everywhere on each $$W_k$$ and $$C_2>0$$ such that $$\omega \geqslant C_2 j_k^*\alpha$$ on each $$W_k$$. Then the claim (with $$C=C_1C_2$$) follows from the preceding local observation.$$\square$$

## 6 Proof of Theorem 1.1

Assume by contradiction that X is not rationally connected. Then there is a pseudo-effective invertible subsheaf $${\mathscr {F}}\subset \mathrm{\Omega }^p_X$$ (for some $$p\in {\mathbb N}$$). We fix a hermitian metric h on X. The hermitian metric on $$T_X$$ induces a hermitian metric $$h_p$$ on $$\mathrm{\Omega }_X^p$$.

The injection of sheaves $${\mathscr {F}}\hookrightarrow \mathrm{\Omega }^p_X$$ corresponds to a non-zero vector bundle homomorphism $$\xi _0:F\rightarrow \mathrm{\Omega }^p_X$$, where F is a pseudo-effective line bundle on X. Since F is pseudo-effective, there is a singular hermitian metric g on F such that $$\mathrm{\Theta }_g\geqslant 0$$, i.e, with positive curvature current.

We claim: There is a constant $$K\in {\mathbb R}^+$$ such that
\begin{aligned} \Vert \xi _0(v)\Vert _{h_p}\le K \Vert v\Vert _g \end{aligned}
(3)
for all $$v\in F_x$$, $$x\in X$$.
Indeed, locally may be written as for some plurisubharmonic function u. Since u is upper semicontinuous with values in , the map $$v\mapsto \Vert v\Vert _g$$ is lower semicontinuous with values in for $$v\ne 0$$. Hence
\begin{aligned} v\mapsto \frac{\Vert \xi _0(v)\Vert _{h_p}}{\Vert v\Vert _g} \end{aligned}
is upper semicontinuous with values in for $$v\ne 0$$. Since upper semicontinuous functions with values in $${\mathbb R}$$ assume a maximum on compact sets, compactness of X now implies the claim.

Next we consider the pull-back bundles and $$f^*\mathrm{\Omega }^p_X$$. They are defined on , where I(f) denoting the indeterminacy set of the meromorphic map f. We consider these bundles equipped with the pull-backed metrics, which we denote by $$\widetilde{g}=f^*g$$ and $$\widetilde{h}_p=f^*h_p$$.

The meromorphic map and the vector bundle homomorphism $$\xi _0:F\rightarrow \mathrm{\Omega }^p_X$$ induce vector bundle homomorphisms
on . Let . Now $$\beta$$ is a holomorphic section in
and is the dual of a pseudo-effective line bundle. Therefore $$\log \Vert \beta \Vert _{\widetilde{g}}$$ defines a plurisubharmonic function on . Plurisubharmonic functions extend through closed analytic subsets of codimension at least two. Hence $$\log \Vert \beta \Vert _{\widetilde{g}}$$ extends to a plurisubharmonic function $$\zeta _0$$ defined on the whole $${\mathbb C}^n$$.
Let and . Let $$w=\xi (v)$$. Then combined with inequality (3) implies
Observe also (see inequality (2)) that
Define . The plurisubharmonicity of $$\zeta _0$$ implies the plurisubharmonicity of $$\zeta$$. Thus we obtain the existence of a plurisubharmonic function $$\zeta$$ on $${\mathbb C}^n$$ such that $$\Vert Df\Vert _{h}\geqslant \zeta$$.
Using Proposition 5.1, we may deduce that . It follows that
\begin{aligned} T_f(r;\omega )\geqslant \int ^r_1\! \frac{dt}{t^{2n-1}}\int _{B_t}\!\zeta \alpha ^n\!. \end{aligned}
Since moving the origin, i.e., replacing f by , where $$\tau$$ denotes a translation, does not affect the order , we may assume that $$c=\zeta (0,\ldots ,0)>0$$. Using the sub-mean value property of plurisubharmonic functions, it follows that
where $$\nu$$ denotes the volume of the unit ball. Therefore,

## 7 Non-Kähler manifolds

Our result is not valid for non-Kähler manifolds. In fact, there are Hopf surfaces X admitting a non-degenerate holomorphic map $$f:{\mathbb C}^n\rightarrow X$$ of order (see [11]). Of course, these Hopf surfaces are non-Kähler and not rationally connected; they do not contain any rational curve at all. More precisely, in [11] the following is proved

### Theorem 7.1

Let $$\lambda \in {\mathbb C}$$ with $$|\lambda |>1$$ and let $$\sim$$ denote the equivalence relation on given by
\begin{aligned} v\sim w \qquad \iff \qquad \text {there exists }k\in {\mathbb Z}v=\lambda ^k w. \end{aligned}
Let and let $$f:{\mathbb C}^2\rightarrow X$$ be the map induced by $$(z,w)\mapsto (z,1+zw)$$. Then .

This result has been generalized by Amemiya [1] to the class of Hopf surfaces defined by an equivalence $$(z,w)\sim (\lambda ^kz,\mu ^kw)$$, $$k\in {\mathbb Z}$$, where $$\lambda$$ may be different from $$\mu$$ (but $$|\lambda |,|\mu |>1$$).

## 8 Questions

The following questions are not expected to have necessarily positive answers. Let X be an n-dimensional compact Kähler manifold, and let be a meromorphic non-degenerate map.
1. 1.

If there exists such , can it be chosen so that ? In other words, if there exists an f as above, is ?

2. 2.

If , does there exist some with ?

3. 3.

If X is rationally connected, does there exist a non-degenerate meromorphic map ?

4. 4.

If X is RC, and if there exists a non-degenerate , is X unirational (i.e., can f be chosen algebraic)? A positive answer would imply that there exists no X with $$\rho (X)\in (0,2)$$.

5. 5.

Is X unirational if ? Is X unirational if $$\rho (X)=0$$? (It should be remarked that for every algebraic map, but the condition is substantially weaker than algebraicity, as seen by appropriate power series in one variable.)

6. 6.

Is the estimate in [6] optimal if $$K_X$$ is pseudo-effective with $$\nu (X)=\nu$$? In other words, does there exist $$X_n$$ with $$\nu (X)=\nu$$ (or better, with $$\kappa (X)=\nu$$) and with for any $$n>0$$ and $$\nu \in \{0,1,\dots ,n\}$$?

The questions 3 and 4 were raised for $$n=3$$ in [4, Question 9.5].

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