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Suspension holonomy is the germ of the represented monodromy action
Statement
Assume Countable Choice (The Axiom of Countable Choice ()). In the suspension of a representation as in The suspension foliation of a representation of the fundamental group let be a loop at with class , let be the fixed point of used for the deck identification in that definition, let be the lift of with , so that Based loops and the fundamental group, and let . Then is a leafwise path of , and its holonomy germ between the local transversal at the start and the corresponding slice at the end is the germ at of : Moreover the leaf is diffeomorphic to with the stabiliser of , and under the holonomy representation of the leaf (as a homomorphism with traversal-order loop multiplication) is . The forward-path holonomy is the inverse germ. Both maps have the same image and kernel.
Facts & Assumptions
Given: A representation of in the diffeomorphism group of , the diagonal action on , the quotient with orbit map and suspension foliation , a loop at , a point over , its lift with , and a point .
In the suspension the quotient is a smooth manifold, is a covering map and a local diffeomorphism, and the product foliation of with leaves descends to the regular foliation whose leaves are the images of those product leaves (The suspension foliation of a representation of the fundamental group, The quotient foliation under a free and properly discontinuous foliated action).
The slice is a local transversal to the product foliation at each of its points: the product foliation has tangent distribution and the slice has tangent space , a complementary direct summand (Local transversals to a regular foliation, Plaques of a flat chart).
In a product chart of the product foliation the plaques keep the -coordinate fixed, so the transport between two slices of the form and along a leafwise path in a leaf keeps the second coordinate: it sends to (The holonomy germ is independent of the foliation chart chain, Plaques of a flat chart).
The -action on is diagonal, , so and lie in the same orbit, and identifies them; moreover holds exactly when for some (The suspension foliation of a representation of the fundamental group, The quotient foliation under a free and properly discontinuous foliated action).
Holonomy germs are well defined, invariant under leafwise homotopy relative to endpoints, and multiplicative under concatenation (The holonomy germ is independent of the foliation chart chain, Holonomy depends only on leafwise homotopy relative to endpoints, Holonomy respects path concatenation and reversal).
Intrinsic leaves are integral immersions with plaque charts (Existence and uniqueness of maximal connected integral manifolds). With traversal-order loop multiplication, the holonomy representation uses the inverse of forward-path holonomy (The holonomy representation and the holonomy group of a leaf).
For a covering-space action of a group on a path-connected space the orbit map is a covering whose deck group consists exactly of the transformations supplied by ; for a universal cover the deck group is isomorphic to the fundamental group of the base, the isomorphism moving a chosen fibre point to the lifted endpoint of the corresponding loop (The orbit map of a covering-space action is a covering, with the acting group equal to the deck group when the total space is path-connected, For a path-connected locally path-connected semilocally simply connected base, the deck group of a universal cover is isomorphic to the fundamental group, Universal covering spaces).
Proof
The lifted path is leafwise. The path lies in the product leaf ; applying the local diffeomorphism by [F1] gives a leafwise path of . Both and the end slice are local transversals to at the endpoints, being local diffeomorphic images of the transversals of [F2].
Transport upstairs. In the product foliation the transport along the path from the slice to the slice keeps the -coordinate by [F3]: a point is sent to .
The leaf through . The restriction is tangent to the descended distribution. In the quotient's local product charts it maps into plaque neighborhoods of the intrinsic leaf , so it factors smoothly as and is a local diffeomorphism between manifolds of dimension by [F7]. By [F4], its fibres are exactly the -orbits. The group acts freely and properly discontinuously on by the deck action; the quotient proposition supplies its smooth quotient and orbit local diffeomorphism. Hence descends to a bijective local diffeomorphism , and is a diffeomorphism. In particular is the corresponding orbit covering.
Descending the transport. In the point is identified by [F4] with . Since is a local diffeomorphism and the holonomy germ of is computed by transporting along the descended local product structure, which is the corresponding chart-wise transport, the holonomy germ satisfies, after identifying both the start and the end transversal with through the maps ,
The fundamental group of the leaf. The group acts on by a covering-space action (the restriction of the deck action, which consists of homeomorphisms over ) and is path-connected and simply connected, being a universal cover; the covering is then a universal cover of whose deck group consists exactly of the transformations from by [F6]. Hence .
The holonomy representation of the leaf. For , the projected path associated to a based loop representing closes because , and corresponds to under step 2.2. Its forward holonomy is by step 2.1. The representation in [F7] uses the reversed loop, and thus takes the inverse germ, namely . This is a homomorphism on ; the unreversed transport is an antihomomorphism.
Conclusion. Steps 1.1 and 2.1 give the stated holonomy germ of a base loop, and steps 1.3, 2.2 and 3.1 give the description of the leaf as together with its holonomy representation.
Depends on
- The suspension foliation of a representation of the fundamental group
- The quotient foliation under a free and properly discontinuous foliated action
- Local transversals to a regular foliation
- Leaves of a regular foliation
- Plaques of a flat chart
- The holonomy germ is independent of the foliation chart chain
- Holonomy depends only on leafwise homotopy relative to endpoints
- Holonomy respects path concatenation and reversal
- The orbit map of a covering-space action is a covering, with the acting group equal to the deck group when the total space is path-connected
- For a path-connected locally path-connected semilocally simply connected base, the deck group of a universal cover is isomorphic to the fundamental group
- Universal covering spaces
- Based loops and the fundamental group
- Diffeomorphisms and local diffeomorphisms of manifolds
- The Axiom of Countable Choice ($\mathrm{AC}_\omega$)
- The holonomy representation and the holonomy group of a leaf
- Existence and uniqueness of maximal connected integral manifolds
Used by
- Two nonhomotopic leaf loops can have the same holonomy germ Counterexample
- The flat-bundle foliation from a linear representation Example
- The Kronecker foliation of the torus has dense leaves and trivial leaf holonomy Example
- The Möbius band's central leaf has reflection holonomy Example
- The suspension of a circle diffeomorphism: leaves and return germs Example
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Sources
- Danny Calegari, Foliations and the Geometry of 3-Manifolds (Oxford Mathematical Monographs) (standard reference, not scraped)
- Eckhard Meinrenken, Lie Groupoids and Lie Algebroids, lecture notes (University of Toronto MAT1341, Fall 2017) (standard reference, not scraped)