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The signed count is invariant under framed cobordism
Statement
Assume . Let be a closed oriented smooth -manifold, , and let , be closed framed -dimensional submanifolds of with signed counts . If they are framed cobordant then . In particular a closed framed -manifold containing exactly two points of the same framing sign and no other points is not framed null-cobordant, while a pair of points of opposite framing signs lying in a common chart ball is framed null-cobordant.
Facts & Assumptions
Given: A closed oriented smooth -manifold , , framed -dimensional submanifolds with signed counts , and a framed cobordism from the first to the second in (The framing sign of a zero-dimensional regular preimage, Framed cobordism of framed submanifolds, Framings of a normal bundle).
The product orientation on orders a positive -frame before . The bottom and top face orientations are therefore and times the orientation of , respectively, by moving the outward vector past the spatial vectors (Product orientations, Induced boundary orientation, Oriented smooth manifolds and oriented charts).
For a compact oriented -manifold, the induced boundary class pushes to zero in . Since is free on path components, summing its coefficients gives zero total boundary signed count (The fundamental class of a boundary pushes forward to zero, Zero-th singular homology is free on path components, Relative fundamental class and boundary orientation).
Two closed framed -manifolds of opposite framing signs lying in a common chart ball are framed null-cobordant by a framed cobordism supported in that ball (Oppositely framed points cancel in pairs, Framed points in one component of the frame bundle are framed cobordant).
Proof
(Orientations of and of its normal bundle.) Orient the normal bundle by the framing , and orient the -manifold by the rule that a positive normal frame followed by a positive tangent frame of is a positive frame of ; this orientation exists and is unique because is connected componentwise and the rank of is . With this choice the orientation of is determined by the framing and the product orientation, and no orientation is imposed on the individual points of the .
On an end collar a normal frame given by is a frame of of sign . In the product orientation, is positive, so the rule in step 1.1 makes the positive tangent of there. At the bottom, the outward tangent is , so the boundary point sign is ; at the top it is . Consequently the signed boundary count is . This computes the signs directly on , without suppressing the dimension-dependent signs of the ambient faces.
By [F2] the signed count of the boundary of a compact oriented -manifold is zero, so and : the signed count is a framed cobordism invariant. Consequently a closed framed -manifold consisting of exactly two points of the same framing sign and no other points has signed count , while the empty framed -manifold has signed count , so it is not framed null-cobordant; a pair of opposite signs in a common chart ball is framed null-cobordant by [F3].
Depends on
- The framing sign of a zero-dimensional regular preimage
- Oppositely framed points cancel in pairs
- Framed cobordism of framed submanifolds
- Framings of a normal bundle
- Framed points in one component of the frame bundle are framed cobordant
- Oriented smooth manifolds and oriented charts
- Product orientations
- Induced boundary orientation
- Relative fundamental class and boundary orientation
- The fundamental class of a boundary pushes forward to zero
- Zero-th singular homology is free on path components
- Orientation of a finite-dimensional real vector space
- The Axiom of Countable Choice ($\mathrm{AC}_\omega$)
Used by
Dependency tree · two levels
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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)