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Framed points in one component of the frame bundle are framed cobordant
Statement
Assume . Let be a closed smooth -manifold, , and let be a smooth path in the frame bundle from to . Then the framed points and , regarded as closed framed -dimensional submanifolds of of codimension with framings , are framed cobordant in .
Facts & Assumptions
Given: A closed smooth -manifold , , points , linear isomorphisms , and a smooth path with , (The frame bundle of a smooth manifold).
The frame bundle is a smooth manifold with smooth projection and smooth right action; writing , both and are smooth, and composing with a smooth nondecreasing reparametrization with near and near gives a smooth path with the same endpoints that is constant near the ends (The frame bundle of a smooth manifold, Smooth embeddings, The chain rule for differentials of smooth maps). The interval is compact and its graph image is compact (Heine-Borel in : with the Euclidean metric a subset of is compact if and only if it is closed and bounded, and the proof by bisection uses no choice principle; the same holds on the real line, A continuous image of a compact space is compact; a continuous real-valued map on a nonempty compact space attains a maximum and a minimum; and a continuous bijection from a compact space to a Hausdorff space is a homeomorphism).
A framed cobordism from a closed framed codimension- submanifold to in a closed is data : a compact neat embedded with , product ends and , and a framing of the pullback of over each end collar, along which the -direction is tangent to (Framed cobordism of framed submanifolds, Framings of a normal bundle, Normal and conormal bundles of an embedded submanifold, Neat submanifolds of a manifold with boundary, The Axiom of Countable Choice ()).
Proof
Choose a smooth nondecreasing given explicitly by , with the standard smooth step function (The standard smooth step function) and , and replace by the reparametrized smooth path with the same endpoints, so that and for and , for .
Define . The map is smooth and injective (the second coordinate separates points) with derivative having second component , so is a compact embedded -submanifold with boundary the two endpoints and ; it is neat in , and by step 1.1 its ends are exactly and .
Write . At define by . Its kernel is precisely , and it is surjective since ; hence it induces a smooth isomorphism of the normal quotient with . The framing is on that quotient. On each end collar and , so is exactly the product pullback of . This supplies the required quotient map and the framing in the trivialization direction of [F2].
Therefore satisfies all the data of a framed cobordism from the framed point to in the sense of [F2], and reading the framings through the frame-bundle dictionary the two framed points and are framed cobordant. No choice beyond the inherited countable choice and the finite choice of and is used.
Depends on
- The frame bundle of a smooth manifold
- Framed cobordism of framed submanifolds
- Framings of a normal bundle
- Normal and conormal bundles of an embedded submanifold
- Neat submanifolds of a manifold with boundary
- Smooth embeddings
- Diffeomorphisms and local diffeomorphisms of manifolds
- The chain rule for differentials of smooth maps
- The Axiom of Countable Choice ($\mathrm{AC}_\omega$)
- The standard smooth step function
- Heine-Borel in $\mathbb{R}^n$: with the Euclidean metric a subset of $\mathbb{R}^n$ is compact if and only if it is closed and bounded, and the proof by bisection uses no choice principle; the same holds on the real line
- A continuous image of a compact space is compact; a continuous real-valued map on a nonempty compact space attains a maximum and a minimum; and a continuous bijection from a compact space to a Hausdorff space is a homeomorphism
Used by
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Sources
- Daniel S. Freed, Bordism: Old and New (lecture notes, UT Austin, Fall 2012) (standard reference, not scraped)
- John Milnor, Topology from the Differentiable Viewpoint (standard reference, not scraped)