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Trivial holonomy gives a product foliated neighbourhood

Statement

Assume ACω (The countable-choice principle used in the foliation pair). Let F be a regular foliation and L a compact leaf whose holonomy representation is trivial (equivalently, whose holonomy group is the trivial group) (The holonomy representation and the holonomy group of a leaf). Then the finite-holonomy normal model with H={1} is L^×D with L^=L, and L has arbitrarily small saturated neighbourhoods U foliated-diffeomorphic to products L×D with the product foliation by the slices L×{t}. In particular every leaf of F∣U is compact and diffeomorphic to L, and L has a fundamental system of product foliated neighbourhoods.

Facts & Assumptions

Given: A regular foliation F and a compact leaf L whose holonomy representation ρx is trivial.

[F1]

The holonomy cover p:L^→L is the connected covering with p∗π1(L^,x^)=ker⁡ρx; when ρx is trivial, ker⁡ρx=π1(L,x), so p has degree one and L^=L (The holonomy cover of a leaf, The holonomy representation and the holonomy group of a leaf).

[F2]

The finite-holonomy normal model of (L,T,D,H) is (L^×D)/H with the diagonal action; for H={1} it is the product L×D with the product foliation by the slices, and the product carries its canonical product smooth structure (The finite-holonomy normal model of a compact leaf, Products of smooth manifolds have a canonical product smooth structure).

[F3]

A compact leaf with finite holonomy is stable: every neighbourhood contains a saturated neighbourhood foliated-diffeomorphically onto a neighbourhood of the central leaf of the normal model (Local Reeb stability for compact leaves with finite holonomy, Saturated neighbourhoods of a leaf).

[F4]

A diffeomorphism of a neighbourhood of the central leaf onto a product L×D′ restricts to the slices, which are diffeomorphic to L (Diffeomorphisms and local diffeomorphisms of manifolds).

Proof

technique · direct
1.1F1

(The holonomy cover is trivial.) With ρx trivial, the kernel is all of π1(L,x), so the covering p:L^→L associated with the kernel has p∗π1(L^,x^)=π1(L,x); a covering of degree one is a diffeomorphism, and we identify L^=L [F1].

2.1F1F2F3step 1.1

(The model and the neighbourhood.) With H={1} the finite-holonomy normal model is the product L×D with the product foliation by slices [F2]. The local Reeb stability theorem applies because the holonomy group is finite (indeed trivial), and gives, for every neighbourhood W of L, a saturated neighbourhood U⊆W foliated-diffeomorphically onto a neighbourhood of the central leaf, which after shrinking D is a product L×D′ with the product foliation [F2, F3].

3.1F2F3F4step 2.1∎

(Fundamental system and leaves.) The product neighbourhoods L×D′ for shrinking transverse disks D′ form a fundamental system of neighbourhoods of the central leaf, and each leaf of the product foliation is a slice L×{t}, compact and diffeomorphic to L [F2, F4]. Hence L has a fundamental system of product foliated neighbourhoods whose leaves are compact and diffeomorphic to L.

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