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A noncompact foliation violating Novikov's compactness conclusions

Statement refuted

Assume Countable Choice ACω. The Reeblessness conclusions of Novikov's theorem extend to noncompact three-manifolds: every Reebless C2 cooriented codimension-one foliation of an oriented 3-manifold, compact or not, has all leaves with injective inclusion-induced fundamental-group homomorphisms.

Facts & Assumptions

Given: The three-torus N0=T2×S1 with coordinates (x,y) on the first torus factor, an irrational number λ, the point p∈N0, and the punctured manifold M=N0∖{p}.

[F1]

A closed constant-rank-one form defines an integrable hyperplane field, so ker⁡(dy−λ dx) is the tangent field of a codimension-one foliation (Closed constant-rank one-forms define integrable hyperplane fields, Regular foliation atlases).

[F2]

The product T2×S1 carries its canonical product smooth structure and the product coordinates decompose its tangent space (Products of smooth manifolds have a canonical product smooth structure, The two-dimensional torus T2=(R/Z)2).

[F3]

A Reeb component is a compact saturated solid torus whose boundary is a single compact leaf (Reeb components of a codimension-one foliation); the inclusion-induced homomorphism on fundamental groups and its injectivity are as in The homomorphism on fundamental groups induced by a pointed continuous map and Based loops and the fundamental group; a Euclidean ball and its punctured version are the standard model (Euclidean spheres and closed balls as subspaces of Rn).

[F5]

Irrational torus flows have injective immersed dense orbits (The irrational torus flow is free with dense orbits). A compact oriented surface with genus g and b>0 boundary circles has free fundamental group of rank 2g+b−1 (Finite surface normal forms, Jordan disks, and torsion control). A circle loop of degree one is essential (A based circle loop is nullhomotopic exactly when its degree is zero).

[F4]

Novikov's theorem and its corollary are stated for closed manifolds (Novikov's Reeb component theorem, Reebless leaves are pi-one-injective and transverse loops are essential), and the standing assumption is Countable Choice ACω as recorded for this pair (The countable-choice principle used in the foliation pair).

Counterexample

technique · direct verification
1.1F1F2givenconstruct

Let N0=T2×S1 with coordinates (x,y) on the first torus factor and the circle coordinate suppressed, and let ω=dy−λ dx with λ irrational. The form is closed and nowhere vanishing, so by the closed-constant-rank-one-forms criterion its kernel is integrable and defines a codimension-one foliation F0 of N0 by Regular foliation atlases, the product structure being the canonical one of Products of smooth manifolds have a canonical product smooth structure.

2.1F3F5step 1.1

The leaf through ([x0],[y0],[z0]) is parametrized intrinsically by (u,[v])↦([x0+u],[y0+λu],[v]). Equality of the first two coordinates would give an integer k with λk an integer, so k=0 by irrationality. Plaque continuation gives the intrinsic cylinder R×S1. F5 proves density of its torus orbit, hence density of the cylinder in N0. No leaf is compact, so none can be the compact boundary leaf of a Reeb component. Thus F0 is Reebless.

3.1F3givenstep 2.1

Remove a point p∈N0 and set M=N0∖{p}, F=F0∣M. Then M is a noncompact three-manifold and F is a codimension-one foliation of it; a Reeb component of F would be a compact foliated solid torus in M and hence in N0, forcing a compact leaf of the Reebless foliation F0, so F is Reebless.

4.1F3F5step 2.1step 3.1construct

Let L be the original cylinder containing p. The restricted foliation has the connected leaf L′=L∖{p}; there is no leaf of F through the removed point. Choose one intrinsic plaque disk through p in a small foliation box. Dense L may have other plaques in that box, so its full intersection with the box is not asserted to be this disk. Polar coordinates identify L with R2∖{0}; if p maps to a≠0, then L′≅R2∖{0,a}. Remove small disjoint disks about these two points and cut off the outer end. The resulting compact pair of pants is a deformation retract along the three end collars, so F5 gives free rank two. The map z↦(z−a)/∣z−a∣ has degree one on a sufficiently small puncture circle, proving that circle essential in L′ by F5.

5.1F3F4step 3.1step 4.1construct∎

Center ambient foliation coordinates (u,v,w) at p, with the chosen plaque w=0. Its puncture circle u2+v2=ε2, w=0, bounds the upper hemisphere u2+v2+w2=ε2, w≥0. This continuous disk avoids p and lies in the coordinate box. The circle is therefore nullhomotopic in M but essential in L′ by step 4.1. Inclusion on fundamental groups is noninjective, while F is smooth, cooriented by ω, oriented in the ambient torus and Reebless. This refutes the extension beyond the closed-manifold hypothesis.

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