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A finite saddle omega-graph is strongly connected and is a finite union of polycycles

Statement

Assume Countable Choice ACω (The countable-choice principle used in the foliation pair). Let F be a C2 cooriented codimension-one foliation and let h:D2→M be a characteristic disk in the relative generic position of Relative generic position for characteristic disk maps. Let X be its planar characteristic vector field. Let U⊆R2 be an open neighborhood of the disk and let Y:U→R2 be a C1 field with Y=X and DY=DX on Γ. Assume the positive orbit of y has compact closure K0⋐U, Γ=ωY+(y) contains at least one equilibrium, all equilibria in Γ are nondegenerate characteristic saddles of X, and Γ separates two specified points of the plane. Then Γ is a finite embedded directed multigraph: its vertices are those saddles and its edges are closures of distinct nonconstant trajectories with saddle alpha- and omega-limits. The directed graph is strongly connected. Consequently every edge lies in a closed directed edge walk, and finitely many such walks cover Γ; a closed directed edge walk is allowed to repeat vertices and is called a directed saddle polycycle here.

Facts & Assumptions

Given: A C2 cooriented codimension-one foliation, a characteristic disk map h in relative generic position, its planar characteristic field X, a C1 field Y on a neighborhood U of the disk with Y=X and DY=DX on Γ, and a positive orbit with compact closure K0⋐U and ω-limit Γ whose equilibria are finitely many nondegenerate characteristic saddles of X.

[F1]

Let Y be C1 on an open U⊆R2 and let O+(y) have compact closure K0⋐U with K=ω+(y) containing only finitely many equilibria. Then either K is a singleton equilibrium, or K is one regular periodic orbit, or K is a finite set E of equilibria together with at least one regular trajectory, and every regular point of K lies on a nonconstant trajectory whose alpha- and omega-limit sets are points of E (Local generalized Poincare-Bendixson theorem for a precompact planar orbit).

[F2]

A C2 function on the plane with a nondegenerate indefinite Hessian at p has C1 coordinates centered at p in which it equals xy (A C² saddle function has C¹ Morse coordinates).

[F3]

A C1 Euclidean field has a unique maximal flow that is jointly C1, its time slices are injective, each regular point has a C1 flow box, a trajectory remaining in a compact subset of the domain has no finite maximal endpoint, and trajectories are C2 in time (C¹ Euclidean maximal flows, variational dependence and the finite C² upgrade).

[F4]

In relative generic position the characteristic singularities of the disk map are finitely many nondegenerate points in the interior of D2, each a center or a saddle (Relative generic position for characteristic disk maps).

[F5]

The standing assumption of the pair is Countable Choice ACω (The countable-choice principle used in the foliation pair).

[F6]

Closed and bounded subsets of R2 are compact; a nested decreasing family of nonempty compact subsets has nonempty intersection; a continuous real function on a nonempty compact set attains its maximum and minimum (Heine-Borel in Rn: with the Euclidean metric a subset of Rn is compact if and only if it is closed and bounded, and the proof by bisection uses no choice principle; the same holds on the real line).

Proof

technique · direct
1.1givenF2F3F4

At a saddle q∈Γ use the C1 coordinates of [F2] in which the local transverse function of the disk map is u=xy; writing the pulled-back area form as μ=m dx∧dy with m continuous and positive and the characteristic covector as β=a du with a continuous and nonzero, the field is X=(a/m)(x∂x−y∂y); replacing (x,y) by (y,x) if necessary (which changes x∂x−y∂y to its negative) makes the coefficient c(x,y)>0, continuous on a smaller compact chart, and then x˙=cx, y˙=−cy give two incoming half-branches (both y-axis rays) and two outgoing half-branches (both x-axis rays) with exponential rate bounds ∣coord(t)∣∈[r0e−c+t,r0e−c−t] for the contracting branch and the reciprocal bounds for the expanding branch on the compact chart; since Y=X on Γ, every edge of Γ through q follows one of these four half-branches, and by uniqueness of [F3] each half-branch is contained in exactly one maximal trajectory, so at most two edges leave each vertex and there are at most twice as many edges as vertices.

2.1step 1.1F1F3

Every regular point of Γ lies on a maximal trajectory whose alpha- and omega-limits are saddle points of Γ: since a singleton does not separate two points of the plane, alternative (i) of [F1] fails; alternative (ii) fails because Γ contains the given equilibrium; hence alternative (iii) holds; each such maximal trajectory is contained in Γ by invariance and closedness of Γ, has no interior equilibrium by uniqueness [F3], and its closure is one of the edge closures of step 1.1, so the edge closures together with the saddle vertices exhaust Γ, distinct edges meet only at common saddle endpoints, and the four half-branches at each saddle give the local embedded-graph structure.

3.1step 2.1F1F3

With exact endpoints, Γ is internally chain-transitive: fix p,q∈Γ, ϵ>0 and T>0; by joint continuity of the flow [F3] on the compact set K0 and the time interval [T,2T], choose δ∈(0,ϵ/3) so that δ-close points have ϵ/3-close images throughout [T,2T]; since Γ=ω+(y) and the orbit tail approaches Γ uniformly, choose s so late that Φu(y) is within δ of Γ for all u≥s, then choose s with Φs(y) within δ of p and t>s+2T with Φt(y) within δ of q; write t−s−T=Nτ with an integer N≥1 and τ∈[T,2T]; the finitely many intermediate orbit points Φs+T+kτ(y), 0≤k<N, lie within δ of Γ, so finitely many nearby points zk∈Γ exist, and x0=p, x1=z0, ..., xN=zN−1, xN+1=q together with the times T,τ,…,τ form an (ϵ,T)-chain because each flowed image is within ϵ/3 of the next orbit point and each jump is at most ϵ/3+δ<ϵ.

4.1step 2.1step 3.1F3F6

The directed graph is strongly connected: Γ is connected, being the intersection of the decreasing family of the connected closures of the orbit tails, since a separation of Γ into disjoint nonempty compact pieces has positive distance and would force a sufficiently late tail closure, which is connected, into a neighbourhood of one piece and away from the other [F6]; if the graph had more than one strongly connected component, its finite condensation would have a proper terminal component S, and if no edge entered S from outside then no edge would leave it either, so the compact carriers of S and of its complement would express the connected Γ as two disjoint nonempty closed sets, which is impossible; hence some edge enters S, and the compact set N consisting of the vertices of S, all edges internal to S and a short terminal segment of every entering edge with a trimming point in its regular part satisfies S⊆int⁡ΓN with no edge leaving S; points of N flow strictly toward S and never reach a saddle in finite time by uniqueness [F3], so Φt(N)⊆N for t≥0 and ΦT(N)⊆int⁡ΓN, whence δ0:=dist⁡(ΦT(N),Γ∖int⁡ΓN)>0 by [F6]; a chain with ϵ<δ0 starting at a point of S stays in N by induction, because Φt(N)⊆ΦT(N) for t≥T and a jump of size less than δ0 cannot leave the δ0-neighbourhood of ΦT(N); choosing the terminal point q in the omitted middle part of an entering edge gives q∉N, contradicting the exact-endpoint chain transitivity of step 3.1, so all vertices lie in one strongly connected component.

5.1step 4.1F5∎

Consequently, for each directed edge e:v→w strong connectivity supplies a directed path from w back to v, and adjoining e gives a closed directed edge walk containing e and repeating vertices only as allowed; there are finitely many edges by step 1.1, so finitely many such walks cover Γ; the construction used only finitely many points and paths of a finite graph plus the finite flow-box and compactness arguments, hence no choice principle, so the statement holds and its standing ACω hypothesis is not invoked.

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