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The signed count is invariant under framed cobordism

Statement

Assume ACω. Let M be a closed oriented smooth m-manifold, m≥1, and let (N0,φ0), (N1,φ1) be closed framed 0-dimensional submanifolds of M with signed counts Σ0,Σ1. If they are framed cobordant then Σ0=Σ1. In particular a closed framed 0-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 m-manifold M, m≥1, framed 0-dimensional submanifolds (Ni,φi) with signed counts Σi=∑x∈Niε(x), and a framed cobordism (W,ε,Ψ) from the first to the second in M×I (The framing sign of a zero-dimensional regular preimage, Framed cobordism of framed submanifolds, Framings of a normal bundle).

[F1]

The product orientation on M×I orders a positive M-frame before ∂t. The bottom and top face orientations are therefore (−1)m+1 and (−1)m times the orientation of M, respectively, by moving the outward vector ∓∂t past the m spatial vectors (Product orientations, Induced boundary orientation, Oriented smooth manifolds and oriented charts).

[F2]

For a compact oriented 1-manifold, the induced boundary class pushes to zero in H0(W;Z). Since H0 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).

[F3]

Two closed framed 0-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

technique · direct
1.1F1given

(Orientations of W and of its normal bundle.) Orient the normal bundle ν(W⊆M×I) by the framing Ψ, and orient the 1-manifold W by the rule that a positive normal frame followed by a positive tangent frame of W is a positive frame of T(M×I); this orientation exists and is unique because W is connected componentwise and the rank of ν(W) is m. With this choice the orientation of W is determined by the framing and the product orientation, and no orientation is imposed on the individual points of the Ni.

2.1F1step 1.1

On an end collar a normal frame given by Ψ−1 is a frame b of TxM of sign ε(x). In the product orientation, (b,ε(x)∂t) is positive, so the rule in step 1.1 makes ε(x)∂t the positive tangent of W there. At the bottom, the outward tangent is −∂t, so the boundary point sign is −ε(x); at the top it is +ε(x). Consequently the signed boundary count is −Σ0+Σ1. This computes the signs directly on W, without suppressing the dimension-dependent signs of the ambient faces.

3.1F2F3step 1.1step 2.1∎

By [F2] the signed count of the boundary of a compact oriented 1-manifold is zero, so −Σ0+Σ1=0 and Σ0=Σ1: the signed count is a framed cobordism invariant. Consequently a closed framed 0-manifold consisting of exactly two points of the same framing sign and no other points has signed count ±2≠0, while the empty framed 0-manifold has signed count 0, so it is not framed null-cobordant; a pair of opposite signs in a common chart ball is framed null-cobordant by [F3].

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Dependency tree · two levels

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