Holomorphic vector fields tangent to foliations in dimension three

This article studies germs of holomorphic vector fields at the origin of C3 that are tangent to holomorphic foliations of codimension one. Two situations are considered. First, we assume hypotheses on the reduction of singularities of the vector field - for instance, that the final models belong to a family of vector fields whose linear parts have eigenvalues satisfying a condition of non-resonance - in order to conclude that the foliation is of complex hyperbolic type, that is, without saddle-nodes in its reduction of singularities. In the second part, we prove that a vector field that is tangent to three independent foliations is tangent to a whole pencil of foliations - hence, to infinitely many foliations - and, as a consequence, it leaves invariant a germ of analytic surface. This final part is based on a local version of a well-known characterization of pencils of foliations of codimension one in projective spaces.


Introduction
The main goal of this article is to study, at the origin of C 3 , germs of holomorphic vector fields that are tangent to holomorphic foliations of codimension one. If X as a germ of holomorphic vector field at (C 3 , 0), inducing a germ of singular holomorphic foliation of dimension one F, and ω is a germ of holomorphic 1−form which satisfies the integrability condition -ω ∧ dω = 0 -inducing a germ of singular holomorphic foliation of codimension one G, we say that X (or F) is tangent to ω (or to G) if the orbits of X are entirely contained in the two-dimensional leaves of ω, wherever both objects are defined. We also say that ω (or G) is invariant by X (or by F). In algebraic terms, this is identified by the vanishing of the contraction of ω by X, that is, i X ω = 0.
One interesting point is that, due to the the integrability condition of ω, not every germ of holomorphic vector field at (C 3 , 0) is tangent to a homomorphic foliation. Examples of this situation are presented in two recent studies where the configuration of tangency between a vector field and a foliation is considered. The first one, by F. Cano and C. Roche [6], asserts that a germ of holomorphic vector field X at (C 3 , 0) tangent to a foliation has a reduction of singularities. This means that, after a finite sequence of blow-ups with invariant centers (points or regular invariant curves in general position with the reduction divisor), the one-dimensional foliation induced by X is transformed into one for which all singularities are elementary, meaning that they are locally defined by vector fields with non-nilpotent linear part. Vector fields in a family proposed by F. Sanz and F. Sancho (see also [6]) do not admit a reduction of singularities as above and, hence, are not tangent to any germ of holomorphic foliation. In the second one, by D. Cerveau and A. Lins Neto [8], it is proved that a germ of holomorphic vector field with isolated singularity at 0 ∈ C 3 that is tangent to a holomorphic foliation always admits a separatrix, that is, an invariant analytic curve. As a consequence, vector fields in the family of X.Gómex-Mont and I. Luengo [13], which do not posses separatrices, are not tangent to holomorphic foliations.
The afore mentioned studies suggest that consequences of geometric nature arise when there is tangency between a vector field and a foliation. This perception is the main motivation for this article and shall be developed in two different and independent approaches. First, in Section 3 we present the concept of strongly diagonalizable germ of vetor field (Def. 3.1), meaning that its linear part has eigenvalues that do not satisfy any non-trivial relation of linear dependency with integer coefficients. We characterize the 1−forms that are tangent to vector fields of this type (Prop. 3.5). In the main result of the section, we consider a germ of integrable holomorphic 1−form ω at (C 3 , 0) that leaves invariant a germ of holomorphic vector field X, putting the following hypotheses on the reduction of singularities of X (which exists by Cano-Roche's result): it is composed only by punctual blow-ups (we say in this case X has an absolutely isolated singularity 0 ∈ C 3 ), the divisor associated to this sequence of blow-ups is invariant by the transformed foliation (that is, X is non-dicritical) and that all final models are strongly diagonalizable singularities. Under these assumptions on X, we prove in Theorem 3.7 that ω defines a foliation of codimension one which is complex hyperbolic -this notion is an extension for foliations of codimension one in higher dimensions of the widely studied concept of generalized curve foliations in dimension two (see definitions in Section 2).
In Section 4, we investigate the situation where a germ of holomorphic vector field at (C 3 , 0) is tangent to three independent foliations, induced by germs of integrable holomorphic 1−forms ω 1 , ω 2 and ω 3 . We prove that, in this case, up to multiplication by germs of functions in O 3 , these 1−forms define a pencil of integrable 1−forms, that is, a two-dimensional linear space -which becomes one-dimensional when projectivizedin the space of integrable holomorphic 1−forms. As a consequence, X is tangent to the infinitely many integrable 1−forms in this pencil. In Theorem 4.5, we present a geometric characterization of pencil of integrable 1−forms, stated in the more general context of foliations at (C n , 0), n ≥ 3, which is a local version of the one given by D. Cerveau in [7]. It asserts that there is a closed meromorphic 1−form θ such that dω = θ ∧ ω for all 1−forms ω in the pencil or the axis foliation-that is, the unique foliation of codimension two that is tangent to all 1−forms in the pencil -has a meromorphic first integral. As a consequence, the axis foliation always admits an invariant hypersurface. This, translated into our original three-dimensional context, gives us that a germ of vector field at (C 3 , 0) that is tangent to three independent germs of holomorphic foliations leaves invariant a germ of analytic surface. This paper contains partial results of the Ph.D thesis of the first author. She is thanks N. Corral and the University of Cantabria for hospitality during the development of part of this research.

Preliminaries
In local analytic coordinates (x 1 , x 2 , x 3 ) at (C 3 , 0), we denote a germ of holomorphic vector field at (C 3 , 0) by where A, B, C ∈ O 3 . We also consider the germ of singular one-dimensional foliation F whose leaves are the orbits of X. Thus, in order to avoid superfluous singularities, we suppose that A, B and C are without common factors. The singular sets (of X or F) are denoted by Sing(X) = Sing(F) = {A = B = C = 0}, being an analytic set of dimension at most one. We denote a germ of of holomorphic 1−form at (C 3 , 0) by where a, b, c ∈ O 3 , which are also supposed to be without common factors. If ω is integrable in the sense of Frobenius, that is, ω ∧ dω = 0, it induces a germ of singular holomorphic foliation of codimension one, denoted by G. We have Sing(ω) = Sing(G) = {a = b = c = 0}, also an analytic set of dimension at most one. A separatrix for a local holomorphic foliation is an irreducible germ of invariant analytic variety of the same dimension of the foliation. For germs of holomorphic vector fields and integrable 1−forms at (C 3 , 0), separatrices are germs of invariant curves and surfaces, respectively. We say that X (or F) is tangent to ω (or to G) if which is equivalent to saying that, outside Sing(X)∪Sing(ω), the orbits of X are contained in the two-dimensional leaves of G. If the germ of vector field X is tangent to the integrable 1−form ω, then Sing(ω) is invariant by X (see, for instance, [16,Th. 1]). Thus, the onedimensional components of Sing(ω) -if they exist -are separatrices of X.
One particular example of this configuaration of tangency is provided by vector fields with first integrals. A non-constant germ of meromorphic -or holomorphic -function Φ at (C 3 , 0) is a first integral for X if, in a small neighborhood of 0 ∈ C 3 , Φ is constant along the orbits of X. This is equivalent to saying that X is tangent to the foliation defined by Φ, which is induced by the holomorphic 1−form obtained by cancelling the components of zeros and poles of the meromorphic 1−form dΦ (we also say that Φ is a first integral for this foliation). Note that in this case, any fiber of Φ accumulating to the origin is an Xinvariant surface. We should mention that, in the article [17], the authors study germs of vector fields at (C 3 , 0) that are completely integrable -that is, that have two independent holomorphic first integrals -and prove that this property is not a topological invariant.
In dimension two, a germ of holomorphic foliation is induced in local analytic coordinates (x 1 , x 2 ) at (C 2 , 0) by a germ of holomorphic vector field X = A∂/∂x 1 +B∂/∂x 2 with isolated singularity at the origin (or by the dual 1−form ω = Bdx 1 − Adx 2 ). Recall that such a foliation has a reduction of singularities, that is, a finite sequence of blow-ups transforms it into one having a finite number of singularities which are simple or reduced [18]. Such a singularity is locally induced by a vector field whose linear part is non-nilpotent, having two eigenvalues λ 1 and λ 2 such that, if both non-zero, do not satisfy any non-trivial relation of the kind m 1 λ 1 + m 2 λ 2 = 0 with m 1 , m 2 ∈ Z ≥0 . These simple singularities are called non-degenerate, whereas the ones having one zero eigenvalue are called saddle-nodes. A foliation at (C 2 , 0) is said to be of generalized curve type [1] if it has no saddle-nodes in some (and hence in any) reduction of singularities. Several geometric properties of a foliation of generalized curve type can be read in its separatrices. For example, a sequence of blow-ups that desingularizes its set of separatrices is also a reduction of singularities for the foliation itself [1].
A foliation of codimension one at (C 3 , 0) also admits a reduction of singularities [4,3]. This means that after a finite sequence of blow-ups with invariant centers -points and regular curves satisfying a condition of general position with respect to the divisor -, the foliation is transformed into one whose singularities are all simple or reduced, which, in analogy with the two-dimensional case, are essentially of two kinds: simple complex hyperbolic singularities and simple saddle-node singularities (see, for instance, the description in [10]). Recall that the dimensional type of a codimension one foliation, τ ≥ 1, is the smallest number of variables needed to express its defining equation in some system of analytic coordinates. We say that a simple singularities is of complex hyperbolic type [5] if there are analytic coordinates (x 1 , x 2 , x 3 ) at 0 ∈ C 3 in which the foliation is defined by a holomorphic 1−form ω whose terms of lowest order are: where the residues λ i ∈ C * are non-resonant, that is there are no non-trivial relations of the kind m 1 λ 1 + m 2 λ 2 = 0 (for τ = 2) or m 1 λ 1 + m 2 λ 2 + m 3 λ 3 = 0 (for τ = 3), with m i ∈ Z ≥0 . Simple complex hyperbolic singularities, when τ = 2, correspond to simple non-degenerate singularities of foliations in dimension two. Now, a foliation G at (C 3 , 0) induced by an integrable holomorphic 1−form ω is of complex hyperbolic type if it satisfies one of the two equivalent properties [5]: • There exists a complex hyperbolic reduction of singularities for G, that is, one for which all final models are simple complex hyperbolic. In this case, every reduction of singularities of G will be complex hyperbolic. • for every holomorphic map φ : (C 2 , 0) → (C 3 , 0) generically transversal to G (that is, such that φ * ω has an isolated singularity at 0 ∈ C 2 ) the foliation π * F induced by φ * ω is of generalized curve type.
Complex hyperbolic foliations are the three-dimensional counterparts of generalized curve foliations. It is thus expected that some geometric properties enjoyed by the latter also have a formulation for the former. For instance, in the non-dicritical case (that is, if the reduction divisor is invariant by the transformed foliation), its proved in [12] that a complex hyperbolic foliation becomes reduced once its set of separatrices is desingularized.

Strongly diagonalizable vector fields
In this section we introduce the notion of strongly diagonalizable germs of vector fields. A vector field in this family has a linear part with eigenvalues satisfying a hypothesis of non-resonance, being, as a consequence, linearizable in formal coordinates. We show that by assuming hypothesis on the reduction of singularities of a germ of vector field at (C 3 , 0) that is tangent to a holomorphic foliation of codimension one -among them, that the final models belong to this family of strongly diagonalizable vector fields -we are able to conclude that the foliation is of complex hyperbolic type.
In the next definition, we work with the following notion: a vector α = (α 1 , · · · , α n ) ∈ C n is said to be strongly non-resonant if there are no non-trivial relations of the form ℓ 1 α 1 + · · · + ℓ n α n = 0, with ℓ 1 , . . . , ℓ n ∈ Z. Such a non-trivial relation will be called strong resonance. We have: Definition 3.1. A germ of holomorphic vector field at (C n , 0), n ≥ 2, is said to be strongly diagonalizable if its associated eigenvalues are strongly non-resonant.
We denote the family of strongly diagonalizable vector fields by X sd . Clearly, a vector field in X sd satisfies the usual condition of non-resonance, being linearizable in formal coordinates. Further, the associated eigenvalues are nonzero and pairwise distinct, implying that its linear part is diagonalizable. We also say that a germ of one-dimensional foliation F at (C n , 0) is strongly diagonalizable if it is induced by a vector field in X sd . Evidently, this definition does not depend on the choice of the vector field in X sd inducing F.
In the sequel, we restrain ourselves to ambient dimension n = 3. Thus, if X is a germ of holomorphic vector field at (C 3 , 0) in X sd , we can take local formal coordinates ( We start by proving a simple lemma: A vector field in X ∈ X sd has exactly three formal smooth separatrices, which correspond to the coordinate axes in its diagonalized form. Proof. We take X in its diagonal form (1). Suppose, without loss of generality, that an where a ∈ C * and f, g, h ∈Ô 1 are non-units, with f of order at least two. We have to prove that g = h = 0. Suppose, for instance, g = 0. The condition of invariance is expressed as Comparing residues in this formula, we find α 2 /α 1 = m, where m = ν 0 (g). This, however, gives a strong resonance for the vector (α 1 , α 2 , α 3 ), in contradiction with our hypothesis. Therefore g = 0 and, in a similar way, h = 0, giving that γ is contained in the x 1 -axis.
Blow-ups preserve the family of strongly diagonalizable vector fields. More precisely, we have the following: Then the strict transform of the one-dimensional foliation induced by X by a blow-up with smooth invariant center is locally given by vector fields in X sd .
Proof. Take formal diagonalizing coordinates for X. For a punctual blow-up at 0 ∈ C 3 , consider coordinates In these coordinates, the strict transform of X is We only have to check that the eigenvalues of X are strongly non-resonant. However, a relation of the kind for c 1 , c 2 , c 3 ∈ Z, is possible if and only if c 1 = c 2 = c 3 = 0, since the eigenvalues of X are strongly non-resonant.
In the case of a monoidal blow-up, its smooth invariant center must be one of the coordinate axes, by Lemma 3.2. For instance, fixing the x 3 -axis as the blow-up center and taking blow-up charts Again, the absence of strong resonances for the eigenvalues ofX follows from that of X.
As a consequence, we have: The only formal separatrices of a vector field in X sd are those corresponding to the coordinate axes in its diagonal form.
Proof. Let γ be a formal separatrix for X ∈ X sd . If γ is smooth, this has been proved in Lemma 3.2. If γ is singular, we desingularize it through a sequence of punctual blowups. By the previous lemma, the strict transform of the foliation induced by X has local models in X sd . Its smooth invariant curves are either in the desingularization divisor or are contained in the strict transforms of the coordinate axis in diagonalizing coordinates for X. The transform of γ is evidently not in the desingularization divisor. This means that γ is smooth contained in one of the coordinate axes, which is not our case.
Our objective now is to prove the following result: Let ω be a germ of integrable holomorphic 1−form at (C 3 , 0) with codim Sing(ω) ≥ 2. Suppose that ω is invariant by a vector field in X ∈ X sd . Then, in formal diagonalizing coordinates for X and up to multiplication by a unit inÔ 3 , we have either formal diagonalizing coordinates for X as in (1) and write where a, b, c ∈Ô 3 are without common factors. Since X is tangent to ω, the contraction of ω by X gives The integrability condition in its turn reads The differentiation of (3) with respect to each of the variables x 1 , x 2 and x 3 produces the following set of equations: We have the following (this was shown to us by M. Fernández-Duque): Assertion 1. In the above conditions, X leaves invariant each ratio of coefficients of ω.
We have just found that These equations can be rewritten as . Therefore, we can find functions R 1 , R 2 , R 3 ∈Ô 3 such that From equations (8) we have where λ i ∈ C and f i ∈Ô 3 is a non-unity. From the above equations, we have Suppose that a = 0 and denote by a ν be its initial part, that is, the homogeneous part of order ν = ν 0 (a) of its Taylor series. Taking initial parts in both sides of X(a) = (λ 1 + f )a and considering the fact that the derivation by X preserves the degree -actually, the multidegree -of each monomial, we have X(a ν ) = λ 1 a ν .
Proof of the Assertion. Write the power series a = ℓ≥ν a ℓ , where a ℓ assembles the homogeneous terms of degree ℓ. We will show by induction that a ν = b 1 x i 1 x j 2 x k 3 divides each a ℓ . There is nothing to prove for ℓ = ν. Let m > ν and suppose that a ℓ is divisible by a ν for all ℓ = ν, . . . , m − 1. Let ̺ be a monomial of a m . We have X(̺) = λ̺ for some λ ∈ C. Considering the calculation in the above paragraph, since λ 1 has already been determined by the multidegree of a ν , we must have λ = λ 1 . On the other hand, separating all monomials of the same multidegree of ̺ in the expression X(a) = λ 1 a + f a, we have (9) X(̺) = λ 1 ̺ +̺, where̺ assembles all monomials coming from f a. Notice that̺ can be seen a combination of monomials of a of order smaller than m having monomials of f as coefficients. Hence, by the induction hypothesis,̺ is divisible by a ν . Rewriting (9) as λ̺ = λ 1 ̺ +̺, we find (λ − λ 1 )̺ =̺, from where we deduce that ̺ is also divisible by a ν . We then conclude that a ν divides a m , proving the general step of the induction.
Suppose that the coefficients a, b, and c of ω are non-zero. By Assertion 2, we can write where b 1 , b 2 , b 3 ∈ C * and g 1 , g 2 , g 3 ∈Ô 3 are non-units. Since X is tangent to ω, we have which implies that Since a, b, c do not have common factors, we find straight that i 1 = j 2 = k 3 = 0, giving Dividing equation (10) by 1 + g 1 /b 1 , we can rewrite, abusing, notation, Let us apply the relation X(b/a) = −µ 1 b/a of (8) to this writing of ω. Write as a sum of meromorphic monomials. Since the derivation by X preserves monomials, we have that α 1 i + α 2 j + α 3 k = −µ 1 whenever α ijk = 0. Again, the fact that (α 1 , α 2 , α 3 ) is free from strong resonances implies that b/a is a monomial. Thus g 2 = 0 and In an analogous way, we can also prove that c = b 3 x 1 x 2 . This leads to the form (II) in the statement of the proposition. The case where one of the coefficients of ω is zero, for example, c = 0, is treated following the same steps above, giving form (I) in the statement.
Proposition 3.6. Let ω be a germ of integrable holomorphic 1−form at (C 3 , 0) invariant by a vector field in X ∈ X sd . Then ω is complex hyperbolic.
Proof. We apply Proposition 3.5. If the dimensional type is two, it is straight to see that ω is simple complex hyperbolic. If the dimensional type is three, then, except for a possible resonance of its residues, ω has the form of a simple complex hyperbolic singularity. However, these resonances can be eliminated by punctual or monoidal blow-ups [3,11], obtaining simple singularities of complex hyperbolic type. We then conclude that ω is complex hyperbolic.
Next result exemplifies how vector fields and codimension one foliations satisfying a relation of tangency can be geometrically entwined. Before stating it, we set a definition: a germ holomorphic vector field -or its associated one-dimensional holomorphic foliation -at (C 3 , 0) has an absolutely isolated singularity at 0 ∈ C 3 if it admits a reduction of singularities having only punctual blow-ups. We call the corresponding composition of blow-up maps an absolutely isolated reduction of singularities.
Theorem 3.7. Let F be a germ of one-dimensional holomorphic foliation at (C 3 , 0) admitting a non-dicritical absolutely isolated reduction of singularities whose associated final models are all strongly diagonalizable. If G is a germ of foliation of codimension one invariant by F, then G is of complex hyperbolic type.
Proof. The proof goes by induction on n, the minimal length of all absolutely isolated reductions of singularities for F as in the theorem's assertion. If n = 0 the result follows from Proposition 3.6. Suppose then that n > 0 and that the result is true for onedimensional foliations having non-dicritical absolutely isolated reductions of singularities with strongly diangonal final models of length less than n. Denote by π : (M, E) → (C 3 , 0) the first punctual blow-up of the corresponding reduction of singularities of F. If G 1 = π * G were non-singular over the divisor E = π −1 (0) ≃ P 2 , then Sing(G) would be an isolated singularity at 0 ∈ C 3 . As a consequence of Malgrange's Theorem [15], in this case G would have a holomorphic first integral, being of complex hyperbolic type. We can then suppose that Sing(G 1 ) ∩ E = ∅ and pick p ∈ Sing(G 1 ) ∩ E. If p ∈ Sing(F 1 ), then, by the induction hypothesis, we must have that G 1 is of complex hyperbolic type at p. Suppose then that p is regular for F 1 . In this case, since F 1 is tangent to G 1 , the foliation G 1 has dimensional type two at p and the leaf of F 1 at p is a curve contained in the one-dimensional a analytic set Sing(G 1 ). Since E is invariant by F 1 , the component of Sing(G 1 ) containing this leaf is contained in E and, hence, it is an algebraic curve in E ≃ P 2 , that we denote by γ. Now, the sum of Camacho-Sad indices of F 1|E along γ is the self-intersection number γ · γ > 0 (see [2,19]). This assures the existence of a singularity q ∈ Sing(F 1|E ), which is obviously a singularity of F 1 . By the induction hypothesis, q is of complex hyperbolic type for G 1 . In view of this, the transversal model of G 1 along (the generic point of) γ is of complex hyperbolic type, leading to the conclusion that G 1 is of complex hyperbolic type at p. We have found that each singularity of G 1 over E is of complex hyperbolic type, admitting a complex hyperbolic reduction of singularities. This means that G itself has a complex hyperbolic reduction of singularities, being a foliation of complex hyperbolic type.

Integrable pencils of 1−forms
The goal of this section is characterize the situation in which a germ of holomorphic vector field at (C 3 , 0) is tangent to three independent holomorphic foliations. We show that, when this happens, the vector field is tangent to infinitely many foliations and it leaves invariant a germ of analytic surface. To this end, we work with the notion of pencil of integrable 1-forms or pencil of foliations. We will formulate our results in the broader context of holomorphic foliations of codimension one at (C n , 0), n ≥ 3, that leave invariant foliations of codimension two.
We start with a definition. Let ω 1 and ω 2 be independent germs of holomorphic 1−forms at (C n , 0), that is, such that ω 1 ∧ω 2 = 0. The pencil of 1−forms with generators ω 1 and ω 2 is the linear subspace P = P(ω 1 , ω 2 ) of the complex vector space of germs of holomorphic 1−forms (C n , 0) formed by all 1−forms ω (a,b) = aω 1 + bω 2 , where a, b ∈ C. We have the following lemma, which is a local version of [16,Lem. 2]: If the independent germs of holomorphic 1−forms ω 1 and ω 2 do not have common components of codimension one in their singular sets, then the 1−forms aω 1 + bω 2 ∈ P = P(ω 1 , ω 2 ) have singular sets of codimension at least two, except possibly for a finite number of values (a : b) ∈ P 1 .
Proof. Suppose, by contradiction, that the result is false. Then, for infinitely many values of t ∈ C, the 1−form ω 1 + tω 2 ∈ P has some component of codimension one in its singular set, defined by and irreducible g t ∈ O n . Let us consider these values of t. Writing we have that, for each pair i, j, with 1 ≤ i < j ≤ n, both A i + tB i = 0 and A j + tB j = 0 are zero over g t = 0, implying that A i B j − A j B i = 0 over this same set. However, the fact that Sing(ω 1 ) and Sing(ω 2 ) do not have a common component of codimension one imply that independent functions g t are associated to different values of t. Hence A i B j − A j B i ≡ 0 and, consequently, A i /B i ≡ A j /B j for each pair i, j. By setting Φ = A i /B i -which is independent of the chosen i -we have a germ of meromorphic function at (C n , 0) such that ω 1 = Φω 2 . This contradicts the fact that ω 1 and ω 2 are independent 1−forms.
Consider a pair of germs of holomorphic 1−forms ω 1 and ω 2 as in the lemma. We say that P = P(ω 1 , ω 2 ) is a pencil of integrable 1−forms or an integrable pencil if all its elements are integrable 1−forms, that is, ω ∧ dω = 0 for all ω ∈ P. This is equivalent to the following fact, which will be referred to as pencil condition: Observe that, after possibly cancelling codimension one components in the singular set, we associate to each ω (a,b) ∈ P a germ of singular holomorphic foliation F t , where t = (a : b) ∈ P 1 . For this reason, we also treat this object as pencil of holomorphic foliations. The 2−form ω 1 ∧ ω 2 is also integrable, defining, after cancelling singular components of codimension one, a singular holomorphic foliation of codimension two which is tangent to all foliations (associated to 1−forms) in P = P(ω 1 , ω 2 ). This codimension two foliation is called axis of P. . . , f k ∈ O n , where k ≥ 2. Consider also (λ 1 , . . . , λ k ) and (µ 1 , . . . , µ k ) two C-linearly independent k-uples of numbers in C * . Then, the holomorphic 1−forms are generators of a pencil of integrable 1−forms. The axis foliation is defined by the 2−form 1 where h ij = f 1 · · · f i · · · f j · · · f k is the product of all gems f ℓ with the exception of f i and f j . Remark that the germs of analytic hypersurfaces {f i = 0} are invariant by all foliations in the integrable pencil, as well as by the axis foliation.
Next, in the framework of the previous result, we add integrability as an ingredient. We obtain that if a distribution of codimension two is tangent to three independent foliations, then it is tangent to infinitely many foliations that are in a pencil. More precisely, we have: Proposition 4.4. Let ω 1 , ω 2 and ω 3 be independent germs of integrable 1−forms at (C n , 0), n ≥ 3, with singular sets of codimension at least two. Suppose that there exists a non-zero holomorphic 2−form η, locally decomposable outside its singular set, that is tangent to each ω i , for i = 1, 2, 3. Then ω 1 , ω 2 and ω 3 define foliations that are in a pencil. Furthermore, η is integrable, defining the axis foliation of this pencil. Proof. We start by applying Proposition 4.3, finding that ω 3 = λ 1 ω 1 + λ 2 ω 2 , where λ 1 and λ 2 are germs of meromorphic functions in (C n , 0). Write λ i = ψ i /ϕ i , i = 1, 2, with ψ i , ϕ i ∈ O n without common factors. Let ϕ = lcm(ϕ 1 , ϕ 2 ), where lcm denotes the least common multiple. We then have Writing we have three integrable 1−forms, defining the same foliations as ω 1 , ω 2 and ω 3 , satisfying so that the pencil condition (12) holds for η 1 and η 2 . Thus, P(η 1 , η 2 ) will be an integrable pencil of 1−forms if its generic element has a singular set of codimension at least two. This will follow straight from Lemma 4.1 if we prove that Sing(η 1 ) and Sing(η 2 ) do not have common components of codimension one. Indeed, looking at (14), the possible common irreducible components of codimension one of Sing(η 1 ) and Sing(η 2 ) are also components of Sing(η 3 ). Fix an equation for such a component. It must be a factor of ϕ, since codim Sing(ω 3 ) ≥ 2. By definition of least common multiple, it cannot be a factor of both ϕ/ϕ 1 and ϕ/ϕ 2 . Suppose, for instance, that it is not a factor of ϕ/ϕ 1 . Then it is evidently a factor of ϕ 1 and, since it defines a component of zeroes of η 1 , it must be also a factor of ψ 1 . This gives a contradiction, since ψ 1 and ϕ 1 where chosen without common factors. Finally, the distributions of codimension two subspaces induced by η and by the integrable 2−form η 1 ∧ η 2 coincide outside Sing(η) ∪ Tang(η 1 , η 2 ), giving the last part of the proposition's statement.
In the sequel we present a characterization of pencils of integrable 1−forms at (C n , 0). It is a local version of a result by D. Cerveau on pencils of foliations in P 3 [7]. Our proof essentially follows the same arguments, adapting them to the local setting.
Theorem 4.5. Let P be pencil of integrable 1−forms at (C n , 0), n ≥ 3. Then, at least one of the following conditions is satisfied: (a) There exists a closed meromorphic 1−form θ such that dω = θ ∧ ω for every 1−form ω ∈ P. When θ is holomorphic, all foliations in P admit holomorphic first integrals. (b) The axis foliation of P is tangent to the levels of a non-constant meromorphic function. In particular, there exists a germ of hypersurface at (C n , 0) that is tangent to the axis foliation of P.
Proof. The two cases in the assertion correspond to the pencil curvature k(P) being zero or non-zero. Case 1 : k(P) = 0, that is, the 1−form θ in (13) is closed. In the purely meromorphic case, we can write, from [9], where f i ∈ O n are irreducible equations of the components of the polar set of θ, h ∈ O n , λ i ∈ C, n i ∈ N for i = 1, · · · , k, with λ i = 0 only if n i > 0. Condition (13) then says that each hypersurface f i = 0 in invariant by every ω ∈ P and, hence, also by the axis of P. Suppose, on the other hand, that θ holomorphic. Since dθ = 0, there exists h ∈ O n such that θ = dh and hence dω = dh ∧ ω for every ω ∈ P. Setting h ′ = exp(h) ∈ O * n , we have dh ′ /h ′ = dh and thus dω = (dh ′ /h ′ ) ∧ ω. Then d (ω/h ′ ) = 0, implying that there exists f ∈ O n such that ω/h = df . Hence, each foliation in P has a holomorphic first integral, and this implies that the axis of P is completely integrable, that is, it has two independent holomorphic first integrals. In particular, the axis foliation leaves invariant germs of analytic hypersurfaces.
Case 2: k(P) = 0. Let ω 1 and ω 2 be generators of the pencil. We have the following: Assertion. There exists a germ of meromorphic function α at (C n , 0) such that Proof of the Assertion. First note that, taking differentials in both sides of dω i = θ ∧ ω i , we have dθ ∧ ω i = 0 for i = 1, 2. Now, let q be a point near 0 ∈ C n such that (ω 1 ∧ ω 2 )(q) = 0. This means that ω 1 and ω 2 are non-singular and linearly independent at q, so that we can find analytic coordinates (x 1 , x 2 , · · · , x n ) at q such that ω 1 = A 1 dx 1 and ω 2 = A 2 dx 2 , where A 1 , A 2 are invertible germs of holomorphic functions at q.
Subtracting (16), we obtain, respectively, This allows us to conclude that he meromorphic functions k 2 1 /α and k 2 2 /α are constant on the leaves of the axis foliation of P. If one of these two functions is non-constant, we have a meromorphic first integral for the axis foliation and the proof of the theorem is accomplished. We claim that this actually happens. Indeed, suppose, by contradiction, that both k 2 1 /α and k 2 2 /α are constant. This would imply that k 1 /k 2 (if k 2 = 0) is also constant. Writing k 1 /k 2 = c 1 ∈ C, we get, from (17), and, since k 2 2 /α is constant, we find that (21) θ = − dk 2 k 2 + k 2 (c 1 ω 1 + ω 2 ).
Let us put the previous discussion our initial three-dimensional context: Theorem 4.6. Let X be a germ of holomorphic vector field at (C 3 , 0) tangent to three independent foliations of codimension one. Then there exists an integrable pencil P such that X is tangent to all foliations in P. Furthermore, at least one of the following two conditions holds: i) there exists a closed meromorphic 1−form θ such that dω = θ ∧ ω for every ω ∈ P; ii) X has a non-constant meromorphic (possibly holomorphic) first integral. In particular, there exists a germ of analytic surface at 0 ∈ C 3 which is invariant by X.
We close this article with an example.
Example 4.7. (Jouanolou's Example) Consider, for m ≥ 2, the vector field X = x m 3 ∂/∂x 1 + x m 1 ∂/∂x 2 + x m 2 ∂/∂x 3 . Then ω = i R i X Ω, where Ω = dx 1 ∧ dx 2 ∧ dx 3 is the volume form and R = x 1 ∂/∂x 1 + x 2 ∂/∂x 2 + x 3 ∂/∂x 3 is the radial vector field, is an integrable 1−form, with homogeneous coefficients, that is invariant by X. Since i R ω = 0, ω defines a foliation on the complex projective plane P 2 C which leaves invariant no algebraic curve [14]. This is equivalent to saying that, at 0 ∈ C 3 , the vector vector field X leaves invariant no homogeneous surface -that is, a surface defined by the vanishing of a homogeneous polynomial in the coordinates (x 1 , x 2 , x 3 ). Since X itself is a homogeneous vector field, this implies that X is not tangent to any germ of analytic surface at 0 ∈ C 3 . By Theorem 4.6, X is cannot be tangent to three independent foliations.