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A noncompact foliation violating Novikov's compactness conclusions
Statement refuted
Assume Countable Choice . The Reeblessness conclusions of Novikov's theorem extend to noncompact three-manifolds: every Reebless cooriented codimension-one foliation of an oriented -manifold, compact or not, has all leaves with injective inclusion-induced fundamental-group homomorphisms.
Facts & Assumptions
Given: The three-torus with coordinates on the first torus factor, an irrational number , the point , and the punctured manifold .
A closed constant-rank-one form defines an integrable hyperplane field, so is the tangent field of a codimension-one foliation (Closed constant-rank one-forms define integrable hyperplane fields, Regular foliation atlases).
The product 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 ).
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 ).
Irrational torus flows have injective immersed dense orbits (The irrational torus flow is free with dense orbits). A compact oriented surface with genus and boundary circles has free fundamental group of rank (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).
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 as recorded for this pair (The countable-choice principle used in the foliation pair).
Counterexample
Let with coordinates on the first torus factor and the circle coordinate suppressed, and let with 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 of by Regular foliation atlases, the product structure being the canonical one of Products of smooth manifolds have a canonical product smooth structure.
The leaf through is parametrized intrinsically by . Equality of the first two coordinates would give an integer with an integer, so by irrationality. Plaque continuation gives the intrinsic cylinder . F5 proves density of its torus orbit, hence density of the cylinder in . No leaf is compact, so none can be the compact boundary leaf of a Reeb component. Thus is Reebless.
Remove a point and set , . Then is a noncompact three-manifold and is a codimension-one foliation of it; a Reeb component of would be a compact foliated solid torus in and hence in , forcing a compact leaf of the Reebless foliation , so is Reebless.
Let be the original cylinder containing . The restricted foliation has the connected leaf ; there is no leaf of through the removed point. Choose one intrinsic plaque disk through in a small foliation box. Dense may have other plaques in that box, so its full intersection with the box is not asserted to be this disk. Polar coordinates identify with ; if maps to , then . 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 has degree one on a sufficiently small puncture circle, proving that circle essential in by F5.
Center ambient foliation coordinates at , with the chosen plaque . Its puncture circle , , bounds the upper hemisphere , . This continuous disk avoids and lies in the coordinate box. The circle is therefore nullhomotopic in but essential in by step 4.1. Inclusion on fundamental groups is noninjective, while is smooth, cooriented by , oriented in the ambient torus and Reebless. This refutes the extension beyond the closed-manifold hypothesis.
Depends on
- Novikov's Reeb component theorem
- Reebless leaves are pi-one-injective and transverse loops are essential
- Reeb components of a codimension-one foliation
- The two-dimensional torus $T^2=(\mathbb R/\mathbb Z)^2$
- Products of smooth manifolds have a canonical product smooth structure
- Closed constant-rank one-forms define integrable hyperplane fields
- Regular foliation atlases
- Based loops and the fundamental group
- The homomorphism on fundamental groups induced by a pointed continuous map
- Euclidean spheres and closed balls as subspaces of $\mathbb{R}^n$
- The countable-choice principle used in the foliation pair
- The irrational torus flow is free with dense orbits
- Finite surface normal forms, Jordan disks, and torsion control
- A based circle loop is nullhomotopic exactly when its degree is zero
Used by
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Sources
- Danny Calegari, Foliations and the Geometry of 3-Manifolds (Oxford Mathematical Monographs; complete author-hosted PDF) (standard reference, not scraped)
- S. P. Novikov, The Topology of Foliations (English translation by J. A. Zilber; complete PDF of the translation) (standard reference, not scraped)
- Samuel Ranz, Approximately Holomorphic Techniques in Foliations: A Simple Proof of Novikov's Theorem (PhD thesis, Universidad Autonoma de Madrid, 2024; complete PDF) (standard reference, not scraped)
- Sushmita Venugopalan, Novikov's Theorem in Higher Dimensions? (arXiv:1907.05876) (standard reference, not scraped)