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Framed zero-dimensional bordism in an oriented manifold is the integers
Statement
Assume . Let be a nonempty closed connected oriented smooth -manifold, . Then the signed count is a bijection from the set of framed cobordism classes of closed framed -dimensional submanifolds of to , it is additive under disjoint union, and is framed null-cobordant if and only if . Framed cobordism classes of closed framed -manifolds in therefore form a commutative monoid isomorphic to .
Facts & Assumptions
Given: and a nonempty closed connected oriented smooth -manifold , .
The signed count is invariant under framed cobordism (The signed count is invariant under framed cobordism, The framing sign of a zero-dimensional regular preimage).
Same-sign frames lie in one component of and are joined by smooth paths; a frame path gives a graph cobordism with product ends (The components of the frame bundle of a connected manifold, Framed points in one component of the frame bundle are framed cobordant).
An opposite-sign pair in a chart ball cancels by a framed cobordism supported there (Oppositely framed points cancel in pairs).
Framed cobordisms have literal product ends and compose along them; finitely many cobordisms with disjoint images may be united, since their normal bundles and framings restrict to the pieces (Framed cobordism of framed submanifolds, Framed cobordism is an equivalence relation, Framings of a normal bundle).
A nonempty closed connected smooth -manifold is a circle: the finite circle-and-interval classification has no interval component when its boundary is empty, and exactly one circle component by connectedness (Boundary of a compact 1-manifold has even cardinality, Separation of a topological space, connected and disconnected spaces, clopen sets, and connected subsets, Open cover, subcover, and compact topological space; a compact subset is a subspace that is compact in its own right).
Same-orientation frames are smoothly path-connected, and the standard smooth step function makes the paths constant near endpoints (Positively oriented bases of an oriented vector space are path-connected, The standard smooth step function).
Proof
The count descends to classes by [F1]. For any , a chart ball contains distinct points carrying frames of sign ; the empty set represents . Their signed count is . This proves surjectivity without choosing a preferred framing at every point of .
Suppose . A Euclidean ball minus finitely many points is path-connected: for two allowed endpoints choose an intermediate point off the finitely many lines through an endpoint and a removed point; the two straight segments lie in the convex ball and avoid the removed points. Such an intermediate point exists because a finite union of lines has empty interior in dimension at least two (in a small ball choose a line direction distinct from the finitely many directions, then exclude its finitely many intersections). Consequently a frame path can be replaced by one avoiding any prescribed finite set disjoint from its endpoint base points: subdivide the original path into finitely many tangent trivializations, move its subdivision frames slightly off inside the chart overlaps, and join the new endpoints inside each punctured chart, keeping the frame coordinate in its original determinant component by [F6]. Small moves in the overlaps preserve that component. The finitely many local paths glue; smoothing with fixed endpoints as in [F2] on the open manifold gives a smooth frame path avoiding . Its graph cobordism is disjoint from every stationary cylinder , , and their union is therefore embedded by [F4].
For , identify with an oriented circle using [F5]. If both signs occur in a finite configuration, some cyclically adjacent pair has opposite signs. The arc between them with a small extension at either end is a chart interval containing no other occupied point. Apply [F3] inside that interval and adjoin the stationary cylinders of the other points, whose images are disjoint from its support. Repeat until only points of one sign remain. To compare two remaining configurations of points, choose cyclically increasing real lifts and in a period-one circle coordinate, matching the cyclic orders. The paths remain distinct modulo one: successive gaps, including the final cyclic gap, are convex combinations of positive gaps. The oriented circle coordinate supplies a smooth nonzero tangent frame; choose the constant model frame of the required sign along each trajectory. At the fixed endpoints, adjust the prescribed frames to these models by [F6] on disjoint stationary cylinders. Flatten the parameter at the ends and use the quotient graph framing from [F2]. The resulting disjoint graphs and endpoint cylinders give a cobordism of the two configurations. For both reduced configurations are empty.
For , fix a chart ball and distinct target points there avoiding . Apply step 1.2 successively to move each framed point of to a target point of the same sign, taking to be the other currently occupied points. Thus every move extends to a cobordism of the entire configuration. Arrange each positive-negative pair at two points in its own small ball in , disjoint from all the other points. By [F3] cancel these pairs one at a time, adjoining only stationary cylinders outside the supporting ball. The remaining configuration has points all of sign , where . Two such configurations with the same can both be moved to the same distinct target points (chosen to avoid both initial finite sets), with the same chosen frames there, again using step 1.2. Thus their classes agree.
Steps 2.1 and 1.3 show that configurations with the same signed count are cobordant; [F1] gives the converse. Together with step 1.1 this proves the bijection and the null-cobordism criterion. Every two classes admit disjoint representatives by placing the required finite sets in separate small balls. Define their sum by the class of that union: its count is the sum of the two counts, so the bijection proves independence of the disjoint representatives. Associativity, commutativity and the empty unit follow from integer addition, giving the asserted monoid isomorphic to . This uses no disjointness inference for arbitrary cobordisms.
Depends on
- The framing sign of a zero-dimensional regular preimage
- Oppositely framed points cancel in pairs
- The signed count is invariant under framed cobordism
- The components of the frame bundle of a connected manifold
- Framed points in one component of the frame bundle are framed cobordant
- Framed cobordism of framed submanifolds
- Framings of a normal bundle
- Framed cobordism is an equivalence relation
- Separation of a topological space, connected and disconnected spaces, clopen sets, and connected subsets
- Paths, path-connected spaces and path components
- A connected, locally path-connected space is path-connected, because its path components are open
- Oriented smooth manifolds and oriented charts
- Open cover, subcover, and compact topological space; a compact subset is a subspace that is compact in its own right
- Orientation of a finite-dimensional real vector space
- Euclidean spaces and Euclidean open subsets as smooth manifolds
- Smooth manifolds and their smooth charts
- The Axiom of Countable Choice ($\mathrm{AC}_\omega$)
- Boundary of a compact 1-manifold has even cardinality
- Positively oriented bases of an oriented vector space are path-connected
- The standard smooth step function
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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)
- Victor Guillemin and Alan Pollack, Differential Topology (standard reference, not scraped)