Abstract. Simple approaches to the proofs of the L 2 Castelnuovo-de Franchis theorem and the cup product lemma which give new versions are developed. For example, suppose ω 1 and ω 2 are two linearly independent closed holomorphic 1-forms on a bounded geometry connected complete Kähler manifold X with ω 2 in L 2 . According to a version of the L 2 Castelnuovo-de Franchis theorem obtained in this paper, if ω 1 ∧ ω 2 ≡ 0, then there exists a surjective proper holomorphic mapping of X onto a Riemann surface for which ω 1 and ω 2 are pull-backs. Previous versions required both forms to be in L 2 .
IntroductionAccording to the classical theorem of Castelnuovo and de Franchis (see [Be], [BarPV]), if, on a connected compact complex manifold X, there exist linearly independent closed holomorphic 1-forms ω 1 and ω 2 with ω 1 ∧ ω 2 ≡ 0, then there exist a surjective holomorphic mapping Φ of X onto a curve C of genus g ≥ 2 and holomorphic 1-forms θ 1 and θ 2 on C such that ω j = Φ * θ j for j = 1, 2. The main point is that the meromorphic function f ≡ ω 1 /ω 2 actually has no points of indeterminacy, so one may Stein factor the holomorphic map f : X → P 1 .Remark. The requirement that the forms be closed is superfluous if the compact manifold X is a surface or if X is Kähler. For, if η = √ −1g ij dz i ∧ dz j is the Kähler form for aKähler metric g and ω is a holomorphic 1-form, then, by Stokes' theorem, we havewhere n = dim X. Since the integrand is a nonnegative 2n-form, the form must vanish and it follows that dω = 0. For X a surface, the same argument with the factor η n−2 removed again yields dω = 0. In general, given a connected complex manifold X and linearly independent closed holomorphic 1-forms ω 1 and ω 2 on X with ω 1 ∧ ω 2 ≡ 0, the meromorphic function f ≡ ω 1 /ω 2 has no points of indeterminacy, f is locally constant on the analytic set Z = { x ∈ X | (ω 1 ) x = 0 or (ω 2 ) x = 0 }, and f is constant on each leaf of the holomorphic foliation determined by ω 1 and ω 2 in X \ Z (see, for example, [NR2] for an elementary proof). In particular, if the levels of the holomorphic map f : X → P 1 are compact, thenStein factorization gives a surjective proper holomorphic mapping of X onto a Riemann surface.We will say that a complete Hermitian manifold (X, g) has bounded geometry of order k if, for some constant C > 0 and for every point p ∈ X, there is a biholomorphism Ψ of the unit ball B = B(0; 1) ⊂ C n onto a neighborhood of p in X such that Ψ(0) = p and, on B, C −1 g C n ≤ Ψ * g ≤ Cg C n and |D m Ψ * g| ≤ C for m = 0, 1, 2, . . . , k.For k = 0, we will simply say that (X, g) has bounded geometry. Gromov [Gro2] observed that, for f = ω 1 /ω 2 as above, one gets compact levels if X is a bounded geometry complete Kähler manifold and the 1-forms are in L 2 and have exact real parts; thus giving an L 2 version of the Castelnuovo-de Franchis theorem. He also introduced his so-called cup product lemma, according to which, two L 2 holomorphic 1-forms ω 1 and ω 2 with exact real parts on a bounded geometry complete Kähler m...