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A created cancelling pair contributes a (1+t)tk term

Example

Assume ACω. Let M be a closed smooth n-manifold with a handle presentation, and let M′ be the presentation obtained by inserting a geometrically cancelling pair of consecutive indices k,k+1 at an intermediate stage (0≤k≤n−1), transporting the later attaching embeddings across the cancellation diffeomorphism and leaving their indices unchanged. Then M′ presents the same manifold M; if f and f′ are adapted Morse functions inducing the two presentations, then Mf′(t)=Mf(t)+tk+tk+1=Mf(t)+(1+t)tk,PM′,F=PM,F, and the correction polynomials satisfy Q′=Q+tk. In the model case M=S2 with the two-critical-point presentation Mf(t)=1+t2, inserting a cancelling (0,1)-pair gives Mf′(t)=2+t+t2 and Q′=1.

Facts & Assumptions

Given: A closed smooth n-manifold M with a finite handle presentation in stages, a geometrically cancelling pair of consecutive indices k,k+1 inserted at an intermediate stage, the resulting presentation M′, and adapted Morse functions f, f′ inducing the two presentations (empty-face convention for the closed case).

[F1]

The cancellation theorem deletes any geometrically cancelling pair (Handle cancellation). The creation theorem attaches a k-handle and then a (k+1)-handle in the standard complementary way on a disc of the outgoing boundary and produces a diffeomorphism of W∪hk∪hk+1 with W 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).

[F2]

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).

[F3]

The Morse polynomial is Mf(t)=∑kmk(f)tk with mk(f)=#{p:ind⁡(p)=k} (Morse numbers and the Morse polynomial); the Poincare polynomial over F is PX,F(t)=∑kdim⁡FHk(X;F)tk (Poincare polynomial of a space and of a pair over a field).

[F4]

For every field F there is a unique Q∈Z[t] with nonnegative coefficients and Mf=PM,F+(1+t)Q (Morse polynomial identity).

[F5]

The height function on S2 has Morse polynomial 1+t2. (computed below).

[F6]

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

technique · presentation-comparison
1.1givenalgebra

For h(x)=x3 on S2, a point away from the poles has tangent vector v=e3−x3x with dh(v)=1−x32>0, so it is not critical. In pole charts h(u)=±1−∣u∣2 has Hessian ∓I2 at u=0; thus the south and north poles have indices 0,2, and Mh=1+t2. Sphere homology gives PS2,F=1+t2, so the correction polynomial is Q=0.

1.2F1given

Apply cancellation in [F1] to the affected connected component of the intermediate stage, keeping other components fixed; for a (0,1)-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 M, and its handle counts are those of the old presentation increased by one in index k and one in index k+1.

2.1F2F3step 1.2

Let f,f′ be the adapted Morse functions inducing the two presentations by [F2]. Their Morse numbers count the handles by index, so mj′=mj+δjk+δj,k+1 for every j, and therefore, by [F3], Mf′(t)=Mf(t)+tk+tk+1=Mf(t)+(1+t)tk.

2.2F3F6step 1.2

The diffeomorphism of step 1.2 induces homology isomorphisms by [F6]. Thus the Betti numbers, and hence the Poincare polynomials defined in [F3], agree: PM′,F(t)=PM,F(t) for every field F.

3.1F4step 2.1step 2.2

Apply [F4] to both functions, whose Poincare polynomials coincide by step 2.2: Mf=PM,F+(1+t)Q and Mf′=PM,F+(1+t)Q′. By step 2.1, the nonnegative polynomial Q+tk also satisfies the second identity. Uniqueness in [F4] therefore gives Q′=Q+tk.

4.1F5step 2.1step 3.1∎

Model case: for M=S2 the two-critical-point presentation of the height function has Morse polynomial 1+t2 by [F5]; inserting a cancelling (0,1)-pair gives Mf′(t)=1+t2+t0+t1=2+t+t2 and Q′=0+1=1 by steps 2.1 and 3.1. The Euler sum is preserved: 2−1+1=2=1−0+1, consistent with the Euler characteristic identity.

Remarks

  • What the example shows. A birth of a cancelling pair adds (1+t)tk 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 (1+t)tk has value zero at t=−1, which is why the Euler identity cannot detect it.

Depends on

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