On Parameter Space of Complex Polynomial Vector Fields in C

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On Parameter Space of Complex Polynomial Vector Fields in C

Kealey Dias, Lei Tan

To cite this version:

Kealey Dias, Lei Tan. On Parameter Space of Complex Polynomial Vector Fields in C. 2014. �hal-

01027414�

On Parameter Space of Complex Polynomial Vector Fields in C

Kealey Dias^a,∗, Tan Lei^b,1

aBronx Community College of the City University of New York, 2155 University Ave., Bronx, New York, USA 10453

bUniversit´e d’Angers, Facult´e des sciences, LAREMA, 2 Boulevard Lavoisier, 49045 Angers cedex 01, France

Abstract

The space Ξ_d of degree dsingle-variable monic and centered complex polyno- mial vector fields can be decomposed into loci in which the vector fields have the same topological structure. This paper analyzes the geometric structure of these loci and describes some bifurcations. In particular, it is proved that new homoclinic separatrices can form under small perturbation. By an example, we show that this decomposition of parameter space by combinatorial data is not a cell decomposition.

The appendix to this article, joint work with Tan Lei, shows that landing separa- trices are stable under small perturbation of the vector field if the multiplicities of the equilibrium points are preserved.

Keywords: holomorphic foliation, holomorphic vector field, bifurcations, qualitative dynamics, Abelian differential, quadratic differential

2010 MSC: 37F75, 34Cxx, 34C23, 34C37

1. Introduction

The objects we consider are the vector fields inCthat in a global chart take the form P(z)_dz^d, withP(z) = z^d+ad−2z^d−2+· · ·+a0, with z and ai ∈ C. We are interested in the global qualitative dynamics of the integral curves of these vector fields, or equivalently, solutions to the real-time, first order ordinary differential equations ˙z=P(z) (withPas above), where the dot is the derivative with respect to time, t ∈ R. The space Ξ_d ≃ C^d−1 of these vector fields of degree d can be decomposed into loci C in which the vector fields have the same combinatorial data set (to be defined). We will prove that each of these

IThis article contains a co-authored appendix.

∗Corresponding author

Email addresses: [email protected](Kealey Dias),[email protected] (Tan Lei)

1Co-author on appendix only.

0000 00 1111

11∞ γ₀ γ₄

γ7

γ1

γ₂ γ6

γ₅

γ3

e₁ e₀ e₆ e₅ e4

e3 e2

e₇

Figure 1.1: The point at∞is a pole of orderd−2 for vector fieldsξP ∈Ξd. There are 2(d−1) trajectoriesγℓwhich meet at infinity with asymptotic angles _2(d−1)^2πℓ ,ℓ∈ {0,1, . . . ,2d−3}. There are 2d−2 accesses to∞defined by the trajectories at infinity. An endeℓis infinity with access betweenγℓ−1 andγℓ. Anodd endis an endek labelled by an odd indexk, and aneven end is an endejlabelled by an even indexj.

combinatorial classes is a connected manifold with well-defined (real) dimension q, which is the dimension of the combinatorial class as a subspace in Ξ_d.

We present now a summary of some necessary concepts and definitions. It can be shown that∞is a pole of orderd−2 for vector fieldsξP ∈Ξd. There are 2(d−1) trajectories γℓ which meet at infinity with asymptotic angles _2(d^2πℓ₋₁₎, ℓ∈ {0,1, . . . ,2d−3}. When the labelling indexℓ is even, the trajectories are calledincomingto∞, and when the indexℓodd, they are calledoutgoing from

∞(see Figure 1.1).

There are 2d−2 accesses to ∞ defined by the trajectories at infinity. An end e_ℓis infinity with access betweenγ_ℓ−1andγ_ℓ(see Figure 1.1). Anodd end is an ende_k labelled by an odd index k, and aneven end is an end e_j labelled by an even indexj.

Separatrices sℓare the maximal trajectories ofξP incoming to and outgoing from∞(in finite time). They are labelled also by the 2(d−1) asymptotic angles.

A separatrixsℓ is calledlanding if ¯sℓ\sℓ=ζ, whereζ is an equilibrium point forξP (equivalently, a zero ofP). A separatrixsℓ=sk,j is calledhomoclinic if

¯

sk,j\sk,j =∅. See Figures 1.2, 1.3, and 1.4 for some examples of landing and homoclinic separatrices. A separatrix for a polynomial vector fieldξP ∈Ξd can only be either homoclinic or landing. A homoclinic separatrix sk,j is labelled by the one odd index k and the one even index j corresponding to its two asymptotic directions at infinity. TheSeparatrix graph: ΓP =

2d∪−3 ℓ=0

ˆ

sℓ, that is, the union of all separatrices and any equilibrium points at which they land, as well as the point at infinity. completely determines the topological structure of the trajectories of a vector field (see, for instance, [1], [2]).

1.1. Zones

The connected components Z of C\Γ_P are called zones. There are three types of zones for vector fields in Ξd, and the types of zones are determined by the types of their boundaries:

• A center zone Z contains an equilibrium point, which is a center, in its interior. Its boundary consists of one or several homoclinic separatrices and the point at infinity. If a center zone is on the left of n homoclinic separatrices sk₁,j₁, . . . , sk_n,j_n on the boundary ∂Z, then the center zone hasnodd endsek₁, . . . , ek_n at infinity on∂Zand the zone is called either a counter-clockwise center zoneor an odd center zone. If a center zone is on the right ofnhomoclinic separatricessk1,j1, . . . , skn,jnon the boundary

∂Z, then the center zone hasneven endse_j1, . . . , e_jnat infinity on∂Zand the zone is called either a clockwise center zone or an even center zone (see Figure 1.2).

• Asepal zone Z has exactly one equilibrium point on the boundary, which is both the α-limit point and ω-limit point for all trajectories in Z (i.e.

ζα = ζω). This equilibrium point is necessarily a multiple equilibrium point. The boundary∂Z contains exactly one incoming and one outgo- ing landing separatrix, the point at infinity, and possibly one or several homoclinic separatrices. If a sepal zone is to the left ofnhomoclinic sep- aratrices sk₁,j₁, . . . , sk_n,j_n on its boundary, then it has n+ 1 odd ends on the boundary: ek₁, . . . , ek_n and ej_i+1 for some corresponding ji, de- pending on how one orders the separatrices. In this case, it is called an odd sepal zone. Similarly, if a sepal zone is on the right of nhomoclinic separatricessk1,j1, . . . , skn,jn on its boundary, then it hasn+ 1 even ends on the boundary, e_j1, . . . , e_jn ande_ki+1 for some correspondingk_i, again depending on the ordering of the separatrices. In this case, it is called an even sepal zone (see Figure 1.3).

• Anαω-zone Z has two equilibrium points on the boundary,ζ_α̸=ζ_ω, the α-limit point andω-limit point for all trajectories inZ. The boundary∂Z contains one or two incoming landing separatrices and one or two outgoing landing separatrices, possibly one or several homoclinic separatrices, and the point at infinity. If an αω-zone is both on the left of n1 homoclinic separatricessk₁,j₁, . . . , sk_n1,j_n1 and on the right ofn2 homoclinic separa- tricessk₁,j₁, . . . , sk_n2,j_n2 on the boundary, then theαω-zone hasn1+1 odd ends (ek₁, . . . , ek_n1 andej_i+1 for some correspondingji) andn2+ 1 even ends (ej₁, . . . , ej_n2 andek_i+1 for some correspondingki) on the boundary (see Figure 1.4).

Remark 1. It will be important to note for anαω-zone, there are exactly one odd end and one even end, neither of whose indices coincide with any index of a homoclinic separatrix (in the notation above the odd and even ends aree_ji+1 ande_ki+1 respectively).

1.2. Transversals

There are several ways to encode the combinatorial structure of a vector field. The author’s preferred descriptions rely on objects calledtransversals. We define in this section the important structures needed to understand definitions of acombinatorial data set.

In any simply connected domain avoiding zeros of P, the differential _P(z)^dz has an antiderivative, unique up to addition by a constant

Φ(z) =

∫ z z0

dw P(w). Note that

Φ_∗(ξP) = Φ^′(z)P(z) d dz = d

dz. (1.1)

The coordinatesw= Φ(z) are, for this reason, calledrectifying coordinates. We will call the images of zones under rectifying coordinatesrectified zones. The rectified zones and corresponding boundaries are of the following types:

• The image of a center zone (minus a curve contained in the zone which joins the center ζ and∞) under Φ is a vertical half strip. It is an upper vertical half strip for a counterclockwise center zone, and the odd ends and homoclinic separatrices are mapped to the lower boundary. It is a lower vertical half strip for a clockwise center zone, and the even ends and homoclinic separatrices are mapped to the upper boundary of this half strip (see Figure 1.2).

• The image of an odd sepal zone under Φ is an upper half plane, where odd ends and homoclinic separatrices are mapped to the lower boundary of this half plane. The image of an even sepal zone under Φ is a lower half plane, where even ends and homoclinic separatrices are mapped to the upper boundary of this half plane (see Figure 1.3).

• The image of an αω-zone under Φ is a horizontal strip (see Figure 1.4).

The lower boundary of the strip consists of two landing separatrices, odd ends, and counterclockwise homoclinic separatrices on the boundary of the zone. The upper boundary of the strip consists of two landing separatrices, even ends, and clockwise homoclinic separatrices on the boundary of the zone.

Via the rectifying coordinates, it is evident that there are a number of closed geodesics through ∞ in ˆC\ {equilibrium pts} in the metric with length ele- ment _|P(z)^|dz|_|. Among these are the h homoclinic separatrices, and there are s distinguished transversals (defined below).

Definition 1. Thedistinguished transversal T_k,j is the geodesic in the metric

|dz|

|P(z)| joining the ends e_k and e_j, avoiding the separatrices and equilibrium points, whereej is the left-most end on the upper boundary andekis the right- most end on the lower boundary of the strip that is the image of theαω-zone in which the transversal is contained (see Figure 1.5).

e1 e3 e5 e1

s1,2 s3,4 s5,0 d

dz

s3,4 e5

Φ_e1 s5,0

P(z)_dz^d

e₁ s1,2

e3

Figure 1.2: Pictured are the trajectories of a vector field with four center zones: one odd center zone (shaded) with homoclinic separatricess5,0,s1,2, ands3,4and endse1,e3, ande5

on the boundary; and three even center zones, each with one homoclinic separatrix and one end on the boundary. The image of a center zone (minus a curve contained in the zone which joins the centerζand∞) under Φ is a vertical half strip. It is an upper vertical half strip for a counterclockwise center zone, and a lower vertical half strip for a clockwise center zone. In this figure, there is an odd center zone mapped to an upper vertical half strip.

s4 s5

e4 e2 e0 e6

s_3,2 s_1,0 s_7,6

s5 e6 s7,6

e₀ s1,0

Φ_e4 P(z)_dz^d

e₄

s₄

e2

s3,2

Figure 1.3: Pictured are the trajectories of a vector field with an even sepal zone (shaded).

On the boundary of the sepal zone is the double equilibrium point which is both theαandω limit point of the trajectories; one incoming landing separatrixs4 and one outgoing landing separatrixs5; three homoclinic separatricess1,0,s3,2, ands7,6; and four ends at infinitye0, e2,e4, ande6. There is an odd sepal zone (not shaded) which shares the equilibrium point and the landing separatrices with the shaded sepal zone, but it has no homoclinic separatrices and only one odd ende5 on the boundary. The image of an odd sepal zone under Φ is an upper half plane, and the image of an even sepal zone is a lower half plane. In this figure, there is an even sepal zone mapped to a lower half plane.

d dz

s₀

s0

s₃

s3 e0

e1 e3

e6 e4

s7,6

s1,2

s5,4

e1

s₀

s5,4 e6

s7,6

e0

Φe1

P(z)_dz^d e4

s3

e₃

s1,2

Figure 1.4: Pictured are the trajectories of a vector field with an αω-zone (shaded). On the boundary of the zone are two equilibrium points: one which is theα-limit point of the trajectories and the other is theω-limit point of the trajectories. Also on the boundary are one incoming landing separatrixs0 and one outgoing landing separatrixs3. The zone is to the left of the homoclinic separatrixs1,2 and on the right of the two homoclinic separatrices s5,4, ands7,6. Finally, the boundary contains two odd endse1 and e3 and three even ends e0,e4, ande6at infinity. The image of anαω-zone under Φ is a horizontal strip.

Note that the way in which the distinguished transversal is chosen, the indices of the ends it joins are exactly those ends whose indices will never coincide with the indices of any homoclinic separatrices.

1.3. Combinatorial and Analytic Data

One way to describe the topological structure of a vector field is by the union of homoclinic separatricess_k,jand distinguished transversalsT_k,j. It was proved in [3] that this description is equivalent to the one presented in the classification (from [4]). Essentially, we want to use the numbers Z/(2d−2) to stand for indices of separatrices for homoclinics, and indices of ends for distinguished transversals otherwise. The indices of transversals were chosen in a way to never conflict with the indices of homoclinic separatrices. Acombinatorial data set can be described as a bracketing on the string 0 1 2. . .2d−3, where the elements paired by parentheses correspond to the labels of the separatrices or distinguished transversals we want to pair. Round parentheses (· · ·) are used to mark pairings corresponding to homoclinic separatrices, square parentheses [· · ·] are used to mark pairings corresponding to a distinguished transversal in each αω-zone, and elements that are not paired correspond to the ends in sepal-zones (see Figure 1.6 for some examples).

The analytic invariants are an (s+h)-tuple in H^s ×R^h+ where to each homoclinic separatrix is assigned a number τ = ∫

sk,j

dz

P(z) > 0, and to each distinguished transversal is assigned a numberα=∫

T dz P(z) ∈H.

s3

s0

s1,2

s7,6

s5,4

∫

dζ

P(ζ) T3,0

e₃

s₃

e6 e4

s3

d dz

e₁ e0

s0

s₀

s_1,2 s_7,6 s_5,4

Figure 1.5: Eachαω-zone is isomorphic to a strip. There may be several transversals which avoid the equilibrium points and separatrices (the dashed curves), but there is exactly one distinguished transversal for eachαω-zone (in this case,T3,0). We define the distinguished transversal to be the geodesic in the metric|dz|/|P(z)|joining the endsek and ej (in this figure,e3 ande0) whereejis the left-most end on the upper boundary of the strip andekis the right-most end on the lower boundary of the strip. Since these indices are the same as the indices for the two landing separatrices on the upper left and lower right boundary of the strip, they can never coincide with the indices for a homoclinic separatrix.

s0

e2

s3

s4

e5

s5

e0

e4

e1

e3

s2 s1

s0

e2

s3

s4

e5

s5

e0

e4

e1

e3

s2 s1

s0

e2

s3

s4

e5

s5

e0

e4

e1

e3

s2 s1

[0(1 2)3][4 5] (0[1(2 3)4]5) (0 1 2 3)[4 5]

Figure 1.6: Disk models for three examples of vector fields of degreed= 4 having sepal zones or/and homoclinic separatrices. The pairing of the ends is marked by the dashed curves. The representation of the combinatorics in brackets is displayed below each figure.

0 1 2

3 4 5

τ = 3 α₁= 1 + i

α2= 3i

Figure 1.7: Example of possible metric graph defining a complex polynomial vector field. The combinatorics can be described by the bracketing (0[1[2 3]4]5) and the analytic invariants are the (2 + 1)-tuple (1 + i,3i,3)∈H²+×R¹+.

Putting the combinatorial and analytic data together, one can uniquely de- scribe a vector field in Ξ_dby a metric graph with a single vertex (corresponding to the pole at infinity),hsolid loops (corresponding to homoclinic separatrices), ands dashed loops (corresponding to distinguished transversals). Each of the solid loops is assigned a positive real number and each dashed loop is assigned a complex number, corresponding to the analytic invariants (see Figure 1.7). Such a metric graph is a complete set of realizeable invariants for the classification of these vector fields ([4]). Another interesting fact about this presentation is that each connected component of the plane minus this transversal flower contains exactly one equilibrium point.

We decompose parameter space into classesC of vector fields that have the same separatrix graph with the labeling.

Our goal is to understand bifurcations of the global trajectory structure, which means we need to understand changes in separatrix structure under small perturbation. In this paper, we will partially answer this question. That is, we will pick and arbitrary ξ_P0 ∈ Ξ_d, and try to answer which classes C intersect every arbitrarily small neighborhood ofξ_P0.

2. Topological and Analytic Structure of the Loci

The classification in [4] gives a bijection between a combinatorial classCand H^s×R^h+. The following theorem proves the type of bijection.

Theorem 2.1. There exists a real analytic isomorphism G_C : H^s×R^h+ → C, which is C-analytic in the first s coordinates and R-analytic in the last h coordinates. It is the restriction of a holomorphic mapping in(s+h) complex variables: G˜_C:H^s×V_R^h₊(ϵ)→C˜, whereC ⊃ C˜ .

In particular, each C is naturally foliated byC-analytic leaves of complex dimensions.

Proof. We prove, that G_C is a restriction of a holomorphic function in s+h variables. Let

V_R+(ϵ) ={z| ℜ(z)>0,|ℑ(z)|< ϵ}, (2.1)

forϵsufficiently small. We prove the existence of a holomorphic function G:H^s×V_R^h

+(ϵ)→Ξ_d

α7→ξ_α, (2.2)

α= (α1, . . . , αs, τ1, . . . , τh)∈H^s×V_R^h₊(ϵ).

By Hartog’s Theorem, it is enough to show thatGis holomorphic in eachα andτin the single-variable sense (we drop the indices on theαandτto simplify notation) in order to concludeG is holomorphic in the (s+h)-variable sense.

We will construct families of surfaces ¯Mα, ¯Mτ and mapsG_α, G_τ such that Gα: ¯Mα→C¯, (Gα)_∗

(d dz

)

=ξα, ξα∈Ξd, (2.3) Gτ : ¯Mτ →C¯, (Gτ)_∗

( d dz

)

=ξτ, ξτ ∈Ξd, (2.4) and we will prove that each family is holomorphic in the one complex variable αor τ by utilizing holomorphic dependence of parameters in the Measurable Riemann Mapping Theorem ([5]).

We first define the rectified surface M0(C) associated to the vector field ξ₀∈ Cand with analytic invariant the (s+h)-tupleα₀= (α⁰₁, . . . , α⁰_s, τ₁⁰, . . . , τ_h⁰).

Without loss of generality, we can takeα₀ = (i, . . . ,i,1, . . . ,1) to simplify pre- sentation. This combinatorial class has a number of rectified zonesZ with ana- lytic invariantsα₀ (see the left side of Figure 2.1). Each separatrix has exactly two representations on the boundary of the rectified zones: one on the upper boundary of a rectified zone and one representation on the lower boundary of a (possibly the same) rectified zone. There are also several representations of∞ on the boundaries of the rectified zones, called theends. Let

M^∗0(C) :=(⊔

Z )

/∼, (2.5)

where ∼ is the appropriate identification of the two representations of each separatrix and the identification of all ends. We let M0(C)≃ C¯ be the com- pactification (for details, see [4]).

We now define distorted rectified surfaces Mα(C) and Mτ(C) respectively by the following. We consider first the case where we allow oneα₀= i to vary.

Choose the strip Z₀ associated to α₀. Choose a complex number α∈ H. We define a piecewise affine mappingA_α,α∈H, on the rectified zonesZas follows.

Let Aα be the piecewise affine mapping which is the identity on all rectified zonesZ̸=Z0, and onZ0, it is defined by i7→αand 17→1. Then onZ0

Aα(z) = 1

2(1−iα)z+1

2(1 + iα)¯z, (2.6)

The mappingAα maps Z0 onto the distorted rectified zone Z₀^′ :=Aα(Z0). As before, we defineMα(C) as the compactification of

M^∗α(C) :=



Z₀^′ ⊔ ⊔

Z̸=Z₀

Z



/∼. (2.7)

−1

−2

2 +i 1 +i i

−3

−2 −1

0

0

i 1 +i 2 +i

−1−τ−1

−2−τ

0

0

−1

−2

1 +i 2 +i 3 +i

i α α+ 1

0

α+ 2

n0+i

Aα

Aτ

Aτ

Aτ

Z0

Z^′₀

α+ 3

Zk,j

Zk,j

Zk,j

Figure 2.1: Some examples of canonical rectified zones (left) and their images underAα or Aτ, the distorted rectified zones (right).

The argument is similar but slightly more complicated for Mτ(C), where we allow exactly one τ₀ ∈ R+ to vary. A homoclinic separatrix s_k,j is on the boundary of exactly two zones, so we have two rectified zonesZ₁ and Z₂ with the rectified homoclinic separatrixs_k,j with lengthτ₀= 1 on their boundaries that will be distorted when we allowτ₀to vary holomorphically (see the right- hand side of Figure 2.1). These two zones can be a combination of strips, half-planes, and vertical half-strips (cylinders). For an sk,j on the boundary of either an upper (respectively lower) half-plane or vertical half-strip, letZk,j

be the vertical half-strip of width τ0 = 1, such that sk,j is on the boundary.

We letAτ, τ ∈V_R+(ϵ), be the piecewise affine map that is the identity on all rectified zonesZ ̸=Z1 or Z2 and the identity (perhaps with some translation) onZ1\Zk,jorZ2\Zk,j, and onZk,j, it is defined by 17→τ∈V_R+and±i7→ ±i.

This affine map takes the form Aτ(z) = 1

2(τ+ 1)z+1

2(τ−1)¯z. (2.8)

Ifs_k,j is on the lower boundary of a strip, then we distort the strip by a (three- piece) piecewise mapping by the construction below. Details are included for completeness, but the idea is much easier to understand by consulting Figures 2.2 and 2.3. Let ∆j be the triangle in the strip with vertices i,−j, and−j+ 1 on the boundary of the strip. One edge of ∆j is on the lower boundary of the strip. Let∇j be the triangle in the strip with vertices 0, i + (j−1), and i +j.

One edge of∇j is on the upper boundary of the strip. In either case, letUℓ be

i i+ 1 i+ 2

i+ 3 i+ 4

-1 0 -3 -2

Uℓ ∆2 Ur

i i+ 1 i+ 2

-1 0

−1−τ

i+ 3

Figure 2.2: The triangle ∆2 has vertices i,−2, and−1 on the boundary of the strip. LetUℓ

be the part of the strip to the left of ∆2andUrto the right. If we distort someτ0= 1 on the lower edge of ∆2, then onUℓ, the affine mapAτ is defined by−17→ −1 and 2 + i7→τ+ 1 + i.

On ∆2, the affine map is defined by−17→ −τ and 1 + i7→1 + i. OnUr,Aτ is the identity.

i i+ 1 i+ 2 i+ 4

-1 0 -3 -2

Uℓ Ur

∇3

i i+ 1 i+ 2

0

i+ 2 +τ i+ 3

-3

-3 -2 -1

Figure 2.3: The triangle∇³ has vertices 0, i + 2, and i + 3 on the boundary of the strip. Let Uℓbe the part of the strip to the left of∇3andUr to the right. If we distort someτ0= 1 on the upper edge of∇3, then onUℓ, the affine mapAτ is the identity. On∇3, the affine map is defined by 17→τand−2−i7→ −2−i. OnUr,Aτ is defined by 17→1 and−3−i7→ −τ−2−i.

the part of the strip to the left of either ∆j or ∇j, and Ur to the right. If we distort someτ0= 1 on the lower edge of some ∆j, then on Uℓ, the affine map Aτ is defined by−17→ −1 and j+ i7→τ+j−1 + i. This corresponds to the affine map

Aτ(z) =1

2(2 + i−iτ)z+1

2(−i + iτ)¯z. (2.9) On ∆j, the affine map is defined by−17→ −τ and j−1 + i7→j−1 + i. This corresponds to the affine map

Aτ(z) = 1

2(τ+ 1 + i(j−1)[τ−1])z+1

2(τ−1−i(j−1)[τ−1])¯z. (2.10) OnUr, Aτ is the identity. The construction is similar for∇j.

The mapping Aτ sends Z1 and Z2 to the distorted rectified zones Z₁^′ :=

Aτ(Z1) andZ₂^′ :=Aτ(Z2). As before, we defineMτ(C) as the compactification of

M^∗τ(C) :=



(Z₁^′ ⊔Z₂^′)⊔ ⊔

Z̸=Z1,Z2

Z



/∼. (2.11)

The exact expressions defining Aα andAτ are not so important. What is im- portant is that the associated Beltrami coefficientsµα andµτ are holomorphic inαandτ respectively and satisfy∥µα∥∞<1,∥µτ∥∞<1.

We endowMα with the standard complex structureσ0 and the vector field

d

dz. We pullback by Aα, giving us a new complex structure σα in M0 that depends holomorphically onα. The rectifying coordinates extend to a mapping ϕ:C→

(⊔

Z

Z¯ )

/∼. Under pullback, we induce a new almost complex structure

˜

σα inC, analytic inα, and a vector field (ϕ◦Aα)^∗(_d

dz

)in C. Let ζ_i⁰ be the equilibrium points of (ϕ◦Aα)^∗(_d

dz

). By the Measurable Riemann Mapping Theorem (MRMT), there exists a family of quasiconformal mapsfα:C→C, normalized such that

fα(∞) =∞, (2.12)

∑

i

fα(ζ_i⁰) = 0, and (2.13)

f_α(s₀) is asymptotic toR+, (2.14) such that (f_α)^∗σ₀= ˜σ_α. The mapping G_α= (f_α◦ϕ⁻¹◦A⁻_α¹) is holomorphic in z, and by MRMT, f_α is holomorphic inα. Then (G_α)_∗(_d

dz

)=P_α(z)_dz^d, where Pα is holomorphic inC. The above is summarized in the diagram

(Mα, σ0,_dz^d) A_α

←−−−− (

M0, σα, A^∗_α(_dz^d))



y^Gα x^ϕ (C, σ0, Pαd

dz) ←−−−−^fα (C,σ˜α,(ϕ◦Aα)^∗(_dz^d)).

The index of the vector field at infinity is −(d−2) (look about the ends in rectifying coordinates), so infinity must be the only pole of orderd−2 for the vector field. We can conclude that P is a degree dpolynomial, which by the above normalizations is monic and centered. So for fixedα,Pαtakes the form Pα(z) =

∏d i=1

(z−ζi), ζi =fα(ζ_i⁰). We need to show that Pα is holomorphic in α, and it is enough to show that the ζ_i are analytic functions of α. We can conclude that the rootsζ_iare analytic functions ofαsincef_αis holomorphic in αfor fixedz.

Therefore,Gis holomorphic in eachα, τ and is hence holomorphic in (s+h) complex variables. Therefore,Gis an open mapping. The restrictionG_C :H^s× R^h+ → Cis an open mapping and is furthermore bijective by the classification in [4]. HenceG_C is an isomorphism which isC-analytic in the first scoordinates, andR-analytic in the lasthcoordinates.

Corollary 2.2 (Corollary of Theorem 2.1). Each C is connected. The (real) dimension of eachC isdim_R(C) = 2s+h, and the codimension (with respect to Ξd) iscodim_R(C) = 2(d−1)−(2s+h) = 2m^∗+h.

Remark 2. By Corollary 2.2 and the enumeration of combinatorial classes in [3], we know exactly how many loci there are altogether and how many loci there are of a particular dimension.

2.1. Cone Structure of Loci

Each combinatorial classC ∼=H^s×R^h+is anR+ cone withz^{d d}_dz ∈Ξdas base point. We need the following proposition stated in Pilgrim [6].

Proposition 2.3. Let P(z) =

∏d j=1

(z−ζj)andC ∋ξP. For everyc >0,ξP˜∈ C forP(z) =˜

∏d j=1

(z−cζj).

Proof. If γ(t) is a real trajectory of the vector field given byP(z), i.e. γ^′(t) = P(γ(t)), then for everyc >0,η(t) =cγ(c^d−1t) is a real trajectory of the vector field given by ˜P(z). Indeed,η^′(t) =c^dγ^′(c^d−1t) =c^dP(c^d−1t) = ˜P(cγ(c^d−1t)) = P(η(t)). Since˜ c > 0, the trajectories η(t) are reparameterizations by time of theγ(t), preserving orientation.

Corollary 2.4. The minimal stratum 0 ∈ C^d−1 corresponding to the vector fieldz^{d d}_dz is adjacent to all other loci.

Proof. We use Lemma 2.3, note that ˜P is continuous inc, and letc→0.

This cone structure is also reflected in the analytic invariants for a class. We note what happens to the analytic invariants when roots of the polynomial are multiplied by the constantc.

Proposition 2.5. The analytic invariants for P˜ are equal to1/c^d−1 times the analytic invariants forP.

Proof. The Res(1/P, ζ) are conformal invariants (Brickman and Thomas [7]), so the analytic invariants are too. Therefore, ˜P_dz^d has the same analytic invariants asc^d−1P_dz^d, and

˜ α=

∫

γ

dz

c^d−1P(z) = 1

c^d−1α, (2.15)

where ˜α and α are the corresponding analytic invariants for ˜P_dz^d and P_dz^d respectively.

3. Structural Stability and Bifurcations

It is natural to consider the possible bifurcations for these vector fields.

More specifically, we want to understand: given an arbitrary ξ0 ∈ Ξd, which combinatorial classes intersect every arbitrarily small neighborhood of ξ0. So we need to consider changes in the separatrix structure for small perturbations ofξ₀.

3.1. Structurally Stable Vector Fields

Proposition 3.1. The structurally stable vector fields inΞdare the vector fields with neither multiple equilibrium points nor homoclinic separatrices.

Proof. The vector fields without homoclinic separatrices or multiple equilib- rium points are structurally stable, which follows immediately from Theorem A.1. By Theorem 2.1, the vector fields with either a homoclinic separatrix or multiple equilibrium point form loci of dimension strictly less than the maximal dimension and must therefore belong to the bifurcation locus.

Corollary 3.2. The structurally stable vector fields are dense inΞd.

In general, bifurcations can be complicated when we allow multiple equilib- rium points to split (see Section 4 for an example). We therefore consider first the possible bifurcations when the multiplicities of the equilibrium points are preserved under small perturbation. Theorem A.1 in the appendix of this paper proves that a landing separatrix cannot be lost under small perturbation when preserving multiplicity, so the non-splitting bifurcations must be those involving only breakings of one or more homoclinic separatrices.

3.2. Some Non-splitting Bifurcations

The construction in the proof of Theorem 2.1 in fact tells us more than is stated in the theorem. Since the only non-splitting bifurcations can involve breakings of homoclinic separatrices, all non-splitting bifurcations can be un- derstood by analyzing the combinatorics of the deformed zones. We describe in the following certain non-splitting bifurcations, and an exhaustive analysis of these is to be considered in a future paper.

We start by explaining what can happen if exactly one analytic invariant associated to a homoclinic separatrix is allowed to take values in±H, instead of being restricted to R+, while the rest of the analytic invariants are preserved.

That is, we consider the possible bifurcations when exactly one homoclinic sep- aratrixsk,j breaks. A homoclinic separatrixsk,j is on the boundary of exactly two zones. We consider thedistorted zones as the proof of Theorem 2.1, where we allowτ₀(s_k,j)7→τ∈V_R+(ϵ)\R+. We know that the distorted zones endowed with the vector field_dz^d correspond to some monic and centered polynomial vec- tor fields in a neighborhood of the given combinatorial class. When we allow a singleτ0(sk,j) to vary holomorphically, then this causes the separatricessk and sj to land. Ifτ∈+H, then instead of coming back into infinity (resp. outgoing from) infinity, the separatrixsk(resp. sj) now lands at the equilibrium point on the boundary of the zone havingsk,jas part of its upper (resp. lower) boundary in rectifying coordinates. If τ ∈ −H, then instead of coming back into (resp.

outgoing from) infinity, the separatrixsk (resp. sj) now lands at the equilib- rium point on the boundary of the zone having s_k,j as part of its lower (resp.

upper) boundary in rectifying coordinates. The equilibrium point at whichs_k (resp. s_j) lands is either a sink (resp. source) or multiple equilibrium point, depending on whether the lower or upper (resp. upper or lower) rectified zone havings_k,j on the boundary was a vertical half-strip or strip in the first case, or in the latter case, a half plane. Notice that if sk,j is on the boundary of a vertical half-strip, then the associated center becomes either a sink or source.

See Figure 3.1 for some examples.

If we allow more than one analytic invariant associated to a homoclinic separatrix to vary at the same time, more complicated things can happen. In particular, new homoclinic separatrices can form. In order to understand this situation, we need to define H-chains. These H-chains turn out to be the structures we need to understand exactly which homoclinic separatrices can form under small perturbation, so we define them here

Definition 2. AnH-chain of lengthnis a sequence ofnconsecutive homoclinic separatrices{sk_i,j_i}, i= 1, . . . , n, i.e. homoclinic separatricessk_i,j_i such that for eachi, eitherki+1=ji+ 1 (upper) orki+1=ji−1 (lower). In particular, a sequenceski,ji such thatki+1=ji+ 1 for alliis called aclockwiseH-chain, and a sequence s_ki,ji such that k_i+1 =j_i−1 for alli is called a counter-clockwise H-chain.

Remark 3. Note that any counter-clockwiseH-chain is necessarily contained in the lower boundary of a single zone, and a clockwiseH-chain is contained in the upper boundary of a single zone.

Definition 3. AclosedH-chainof lengthnis anH-chain in whichsk_i+n,j_i+n = sk_i,j_i, for alli= 1, . . . , n. Anopen H-chain is one that is not closed.

Remark 4. The separatrices in an openH-chain have a natural ordering, ac- cording to the ordering from left to right in the rectifying coordinates (direction of the flow). The separatrices in a closedH-chain do not have a well-defined ordering.

−1−τ

−2−τ

sk

sk

s_j

0

i 1 +i 2 +i

−1

Figure 3.1: Some examples of distorted zones endowed with the vector field _dz^d. The separa- trices are the trajectories going out of and coming into the ends, so these may not necessarily be on the boundary of the distorted zones. Each of the two separatrices which were a ho- moclinic separatrix for the non-distorted zones enter opposite zones on which the homoclinic separatrix was part of the boundary. In the top picture,sjlands at either the source or mul- tiple equilibrium point to which the strip is associated. In the middle picture,s_k,j belonged to the lower boundary of an upper half-strip, and after perturbation, sk now lands at the equilibrium point which was on the boundary of the half-strip, making the center a sink (one should see the points marked by the crosses and circles in the figure as being identified). In the bottom picture,sk now lands at the multiple equilibrium point which hadsk,j on the uppper boundary of one of its associated half-planes.