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A handle decomposition gives a relative CW complex
Statement
Assume . A compact triad with a finite handle decomposition of indices has a finite CW model of pairs , with one relative -cell per handle. Here is a finite CW model of , rather than an unstated CW structure on that smooth manifold. If a finite CW structure on is supplied, one can take and the equivalence relative to . 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
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.
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.
Cell attachment by a characteristic map and Skeleta, CW subcomplexes, and relative CW complexes give cell attachments and relative CW pairs.
Relative CW inclusions are cofibrations supplies HEP for disk boundaries and CW subcomplexes.
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.
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.
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.
Record the attachment comparison explicitly. If is a homotopy equivalence and , attach a disk to its mapping cylinder along in the source end. By [F6], retracts to fixing , so this enlarged space retracts to . Inside , the source attaching map is homotopic along its cylinder tracks to the target map . For a homotopy of attaching maps, the space formed by attaching along retracts to either endpoint attachment: use the disk-cylinder retraction onto , or its reversed version, from [F4]. Hence the enlarged space is also equivalent to the disk attached at the target end, which retracts to . 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 is a disjoint point.
Suppose a CW model for is available. The initial collar retracts to , hence has pair model . 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 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 with one additional cell of dimension and with base subcomplex . Finite attachments have the quotient weak topology and closure finiteness. Thus the induction gives , and retains the supplied base pointwise when .
Supply the finite model of 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 , present any compact smooth -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 . For a general -triad, is a compact boundaryless -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 Morse and handle suppliers.
There is one relative cell for every original handle, including a disjoint point for index zero and the full-dimensional core cell for index . An empty handle list gives the collar equivalence to the base model. The equivalence is relative to the actual only when its CW structure is supplied; in general it is an equivalence of pairs to . This proves all the stated assertions and the dimension-induction conclusion.
Depends on
- Handle decomposition relative to the incoming boundary
- Handle attachments are relative cell attachments up to homotopy
- Cell attachment by a characteristic map
- Skeleta, CW subcomplexes, and relative CW complexes
- Relative CW inclusions are cofibrations
- Cofibration and homotopy extension property
- Cellular approximation for maps of CW pairs
- Adapted excellent Morse functions exist on compact cobordisms
- Morse functions and handle decompositions correspond
- Mapping cylinder factorization
- Cw homotopy equivalence inclusions are strong deformation retracts
- Pushouts and products preserve the cofibrations used here
Used by
- Homotopy spheres of dimension at least five bounding a contractible manifold are standard spheres Corollary
- Morse Euler characteristic identity Corollary
- A homology cobordism need not be an h-cobordism Counterexample
- Algebraic Lefschetz number via rational homology traces Definition
- The based handle chain complex over the fundamental group ring Definition
- A contractible relative group-ring complex with a pi-one isomorphism detects a homotopy equivalence Lemma
- Compact smooth manifolds have finite CW models under countable choice Lemma
- Compactified unstable manifolds give the Morse--Smale CW decomposition Lemma
- Finite C2 surface carriers have smooth normal forms and relative cap approximations Lemma
- Finite tangent index count and inward boundary sum Lemma
- Handle slides act by elementary basis change on handle chains Lemma
- Immersion extension on a disk: absolute and relative parametric forms Lemma
- The orientation-twisted diagonal realizes the Lefschetz trace Lemma
- Zero- and one-handles are eliminated in a simply connected h-cobordism Lemma
- Surgery below the middle dimension improves connectivity Proposition
- The relative handle chain complex computes H_*(W,M₀) and has the intersection matrix as its differential Proposition
- The relative Morse complex of an adapted cobordism Proposition
- Handle slides are not handle cancellations Remark
- The Smale–Hirsch immersion theorem Theorem
Dependency tree · two levels
68 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- Allen Hatcher, Algebraic Topology, Chapter 0, Propositions 0.18–0.19 and Corollaries 0.20–0.21, pp. 16–17 (standard reference, not scraped)
- C. T. C. Wall, Differential Topology (Cambridge Studies in Advanced Mathematics 156), Sections 5.1-5.4, printed pp. 129-148 (standard reference, not scraped)
- Andrei Pajitnov, Circle-Valued Morse Theory (de Gruyter Studies in Mathematics 32), Chapter 5 Sections 1-3 (pp. 163-189) and Chapter 4 Section 3 (pp. 132-162) (standard reference, not scraped)