Math 2.0

Manifolds and cell complexes Foliations Example

Reeb foliation

Example

Let D={u∈ℝ2:∣u∣<1}D = \set{u \in \R^2 : \abs{u} < 1} be the open unit disc. On D×ℝD \times \R consider, for each c∈ℝc \in \R, the surface Lc={(u,t)∈D×ℝ:t=c+e1/(1−∣u∣2)}.L_c = \set{(u, t) \in D \times \R : t = c + e^{1/(1 - \abs{u}^2)}}. These surfaces are disjoint graphs over DD, and they fill D×ℝD \times \R. Together with the boundary cylinder ∂D×ℝ\partial D \times \R, they form a codimension one foliation of the closed solid cylinder Dˉ×ℝ\bar D \times \R. Each LcL_c is a plane that bends upwards and approaches the boundary cylinder as ∣u∣→1\abs{u} \to 1, since e1/(1−∣u∣2)→∞e^{1/(1 - \abs{u}^2)} \to \infty. The translation t↦t+1t \mapsto t + 1 maps LcL_c to Lc+1L_{c+1}, so the foliation passes to the quotient solid torus Dˉ×(ℝ/ℤ)\bar D \times (\R / \Z). This foliated solid torus is a Reeb component. Its boundary torus is one leaf, and every interior leaf is a plane that spirals towards the boundary.

The 33-sphere is the union of two solid tori glued along their boundary tori. The gluing sends the meridian of each solid torus to the longitude of the other. Putting a Reeb component on each solid torus gives a smooth codimension one foliation of S3S^3, the Reeb foliation. The leaves are one torus and infinitely many planes.

Remarks

The construction shows that S3S^3, which has Euler characteristic 00, carries a codimension one foliation. Novikov proved that every codimension one foliation of class C2C^2 on S3S^3 has a compact leaf, a torus bounding a Reeb component. The torus leaf has infinite fundamental group, so the Reeb stability theorem does not force the other leaves to be compact, and indeed they are planes.

Further reading

  1. Web https://en.wikipedia.org/wiki/Reeb_foliation
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