Reeb sphere theorem

In mathematics, Reeb sphere theorem, named after Georges Reeb, states that

A closed oriented connected manifold M ⁿ that admits a singular foliation having only centers is homeomorphic to the sphere Sⁿ and the foliation has exactly two singularities.

Morse foliation

A singularity of a foliation F is of Morse type if in its small neighborhood all leaves of the foliation are levels of a Morse function, being the singularity a critical point of the function. The singularity is a center if it is a local extremum of the function; otherwise, the singularity is a saddle.

The number of centers c and the number of saddles $s$, specifically c − s, is tightly connected with the manifold topology.

We denote ind p = min(k, n − k), the index of a singularity $p$, where k is the index of the corresponding critical point of a Morse function. In particular, a center has index 0, index of a saddle is at least 1.

A Morse foliation F on a manifold M is a singular transversely oriented codimension one foliation of class C² with isolated singularities such that:

• each singularity of F is of Morse type,
• each singular leaf L contains a unique singularity p; in addition, if ind p = 1 then $L\setminus p$ is not connected.

Reeb sphere theorem

This is the case c > s = 0, the case without saddles.

Theorem: Let $M^n$ be a closed oriented connected manifold of dimension $n\ge 2$. Assume that $M^n$ admits a $C^1$-transversely oriented codimension one foliation $F$ with a non empty set of singularities all of them centers. Then the singular set of $F$ consists of two points and $M^n$ is homeomorphic to the sphere $S^n$.

It is a consequence of the Reeb stability theorem.

Generalization

More general case is $c>s\ge 0.$

In 1978, E. Wagneur generalized the Reeb sphere theorem to Morse foliations with saddles. He showed that the number of centers cannot be too much as compared with the number of saddles, notably, $c\le s+2$. So there are exactly two cases when $c>s$:

(1) $c=s+2, \,$

(2) $c=s+1. \,$

He obtained a description of the manifold admitting a foliation with singularities that satisfy (1).

Theorem: Let $M^n$ be a compact connected manifold admitting a Morse foliation $F$ with $c$ centers and $s$ saddles. Then $c\le s+2$. In case $c=s+2$,

• $M$ is homeomorphic to $S^n$,

• all saddles have index 1,

• each regular leaf is diffeomorphic to $S^{n-1}$.

Finally, in 2008, C. Camacho and B. Scardua considered the case (2), $c=s+1$. Interestingly, this is possible in a small number of low dimensions.

Theorem: Let $M^n$ be a compact connected manifold and $F$ a Morse foliation on $M$. If $s = c + 1$, then

• $n=2,4,8$ or $16$,

• $M^n$ is an Eells–Kuiper manifold.