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A handle decomposition gives a relative CW complex

Statement

Assume ACω. A compact triad (W;M0,M1) with a finite handle decomposition of indices k1,…,kr has a finite CW model of pairs (X,A)≃(W,M0), with one relative ki-cell per handle. Here A is a finite CW model of M0, rather than an unstated CW structure on that smooth manifold. If a finite CW structure on M0 is supplied, one can take A=M0 and the equivalence relative to M0. The relative cells may be added in the given handle order after cellular approximation of each attaching map; this order need not be a skeletal filtration. In particular a compact smooth manifold has finite CW homotopy type, with the empty incoming face giving the absolute case.

Facts & Assumptions

[F1]

Handle attachments are relative cell attachments up to homotopy replaces each handle by its core cell, as a homotopy equivalence relative to the current stage.

[F2]

Cellular approximation for maps of CW pairs is choice-free for a finite source; an attaching sphere can therefore be moved into the appropriate skeleton.

[F4]

Relative CW inclusions are cofibrations supplies HEP for disk boundaries and CW subcomplexes.

[F5]

Adapted excellent Morse functions exist on compact cobordisms and Morse functions and handle decompositions correspond give a finite handle presentation on the empty incoming face for every compact smooth manifold, also with boundary.

[F6]

The mapping-cylinder source inclusion is a closed cofibration and its target is a strong deformation retract (Mapping cylinder factorization). The relative-inverse construction proved in steps 1.1–3.1 of Cw homotopy equivalence inclusions are strong deformation retracts uses only HEP for the two inclusions and their interval products: extend an inverse homotopy to make the inverse fix the common subspace, cancel the retraced boundary track by a homotopy of homotopies, and repeat with the two maps interchanged. Thus it applies to the mapping-cylinder source inclusion when that inclusion is a homotopy equivalence, without asserting a CW structure on the original smooth base. Product HEP and the explicit disk-cylinder retraction are proved in Pushouts and products preserve the cofibrations used here, steps 1.1 and 5.1. All spaces here are finite CW models, compact smooth stages, or their closed mapping cylinders and disk attachments, so the stated compactly generated weak Hausdorff hypotheses hold.

[A1]

Hatcher, Algebraic Topology, Chapter 0, pp. 16–17 provides source context for the attachment comparison. The proof uses the local constructions in [F6], not an external prerequisite.

Proof

Given: The compact triad and its finite ordered handles.

1.1F6F3F4construct

Record the attachment comparison explicitly. If e:Y→Z is a homotopy equivalence and α:Sk−1→Y, attach a disk to its mapping cylinder Me along α in the source end. By [F6], Me retracts to Y fixing Y, so this enlarged space retracts to Y∪αDk. Inside Me, the source attaching map is homotopic along its cylinder tracks to the target map eα. For a homotopy H of attaching maps, the space formed by attaching Dk×I along H retracts to either endpoint attachment: use the disk-cylinder retraction onto (Dk×{0})∪(Sk−1×I), or its reversed version, from [F4]. Hence the enlarged space is also equivalent to the disk attached at the target end, which retracts to Z∪eαDk. This proves invariance under replacing the base by a homotopy equivalent model. For equivalences of pairs, carry the base pair through its mapping cylinder; the same retractions restrict to those of the base cylinder, giving equivalences of pairs. If the original base is retained pointwise and the initial equivalence is relative to it, the relative form of [F6] makes all these equivalences relative to it. The case k=0 is a disjoint point.

2.1F1F2F3step 1.1construct

Suppose a CW model A for M0 is available. The initial collar retracts to M0, hence has pair model (A,A). Inductively replace a handle by its core using [F1], transport its attaching map by the current equivalence, and apply step 1.1. By [F2] homotope the resulting map Ski−1→Xi−1 to a cellular one; the attaching sphere has a finite CW structure (two hemispheres, with the usual lower-dimensional cells), so the finite-source clause applies. Step 1.1 also proves invariance under this homotopy. Attaching its disk therefore gives a genuine CW complex Xi with one additional cell of dimension ki and with base subcomplex A. Finite attachments have the quotient weak topology and closure finiteness. Thus the induction gives (X,A)≃(W,M0), and retains the supplied base pointwise when A=M0.

3.1F2F5step 2.1baseihconstruct

Supply the finite model of M0 without circularity by dimension induction, simultaneously proving that every compact smooth manifold has finite CW homotopy type. In dimension zero, compactness and discreteness give finitely many points, with their zero-cell structure. In dimension d>0, present any compact smooth d-manifold relative to the empty face by [F5]. Step 2.1 uses the empty CW base and produces an absolute finite CW model, without assuming any model in dimension d. For a general d-triad, M0 is a compact boundaryless (d−1)-manifold and has a finite CW model by the already established lower-dimensional case. Step 2.1 then supplies the asserted model of pairs. This induction uses only the explicit finite attachment comparisons and finite-source cellular approximation, in addition to the ACω Morse and handle suppliers.

4.1F1F3step 2.1step 3.1discharge-induction∎

There is one relative cell for every original handle, including a disjoint point for index zero and the full-dimensional core cell for index n. An empty handle list gives the collar equivalence to the base model. The equivalence is relative to the actual M0 only when its CW structure is supplied; in general it is an equivalence of pairs to (X,A). This proves all the stated assertions and the dimension-induction conclusion.

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