How statement and proof provenance work
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- AI-adapted: a semantically identical restatement of literature-sourced material, modulo indexing, notation, and boundary cases adopted by the library.
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A created cancelling pair contributes a term
Example
Assume . Let be a closed smooth -manifold with a handle presentation, and let be the presentation obtained by inserting a geometrically cancelling pair of consecutive indices at an intermediate stage (), transporting the later attaching embeddings across the cancellation diffeomorphism and leaving their indices unchanged. Then presents the same manifold ; if and are adapted Morse functions inducing the two presentations, then and the correction polynomials satisfy . In the model case with the two-critical-point presentation , inserting a cancelling -pair gives and .
Facts & Assumptions
Given: A closed smooth -manifold with a finite handle presentation in stages, a geometrically cancelling pair of consecutive indices inserted at an intermediate stage, the resulting presentation , and adapted Morse functions , inducing the two presentations (empty-face convention for the closed case).
The cancellation theorem deletes any geometrically cancelling pair (Handle cancellation). The creation theorem attaches a -handle and then a -handle in the standard complementary way on a disc of the outgoing boundary and produces a diffeomorphism of with relative to the incoming boundary, so the modified presentation presents the same manifold; the two new handles are added, and later attaching embeddings are transported across this diffeomorphism without changing their indices (Creation of a cancelling handle pair, Handle decomposition relative to the incoming boundary).
Adapted Morse functions on the triad and handle presentations correspond: the Morse numbers of the function inducing a presentation equal the numbers of handles by index (Morse functions and handle decompositions correspond).
The Morse polynomial is with (Morse numbers and the Morse polynomial); the Poincare polynomial over is (Poincare polynomial of a space and of a pair over a field).
For every field there is a unique with nonnegative coefficients and (Morse polynomial identity).
The height function on has Morse polynomial . (computed below).
A homotopy equivalence induces isomorphisms on singular homology with every coefficient group (Homotopy equivalences induce isomorphisms on singular homology); in particular a diffeomorphism does so.
Verification
For on , a point away from the poles has tangent vector with , so it is not critical. In pole charts has Hessian at ; thus the south and north poles have indices , and . Sphere homology gives , so the correction polynomial is .
Apply cancellation in [F1] to the affected connected component of the intermediate stage, keeping other components fixed; for a -pair its second foot lies on that existing component by the one-intersection condition. It supplies a diffeomorphism of the new presentation's underlying manifold with the old one relative to the incoming face; transport each later attaching embedding across its boundary restriction; hence the modified presentation still presents , and its handle counts are those of the old presentation increased by one in index and one in index .
Let be the adapted Morse functions inducing the two presentations by [F2]. Their Morse numbers count the handles by index, so for every , and therefore, by [F3], .
The diffeomorphism of step 1.2 induces homology isomorphisms by [F6]. Thus the Betti numbers, and hence the Poincare polynomials defined in [F3], agree: for every field .
Apply [F4] to both functions, whose Poincare polynomials coincide by step 2.2: and . By step 2.1, the nonnegative polynomial also satisfies the second identity. Uniqueness in [F4] therefore gives .
Model case: for the two-critical-point presentation of the height function has Morse polynomial by [F5]; inserting a cancelling -pair gives and by steps 2.1 and 3.1. The Euler sum is preserved: , consistent with the Euler characteristic identity.
Remarks
- What the example shows. A birth of a cancelling pair adds to the Morse polynomial while leaving the manifold and its homology unchanged, so the correction polynomial of the Morse polynomial identity is exactly the algebraic record of such pairs.
- The perfectness defect. The pair is invisible in homology but increases the excess of Morse numbers over Betti numbers; the added term has value zero at , which is why the Euler identity cannot detect it.
Depends on
- Homology of spheres
- The Axiom of Countable Choice ($\mathrm{AC}_\omega$)
- Critical points and critical values of a smooth function
- Handle decomposition relative to the incoming boundary
- The intrinsic Hessian of a smooth function at a critical point
- Morse numbers and the Morse polynomial
- Nondegenerate critical points, nullity, index, and coindex
- Poincare polynomial of a space and of a pair over a field
- Handle cancellation
- Creation of a cancelling handle pair
- Morse functions and handle decompositions correspond
- Morse polynomial identity
- Homotopy equivalences induce isomorphisms on singular homology
Used by
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Sources
- Michele Audin and Mihai Damian, Morse Theory and Floer Homology, Part I Chapter 4 Section 4.4, printed pp. 88-91 (PDF pp. 98-100) (standard reference, not scraped)
- Liviu Nicolaescu, An Invitation to Morse Theory (2nd ed.), Chapter 2 Section 2.3, printed pp. 46-53 (PDF pp. 56-63) (standard reference, not scraped)