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The Whitehead torsion of an h-cobordism is well defined for a fixed presentation and its elementary moves

Statement

Assume ACω (The Axiom of Countable Choice (ACω)). Let (W;M0,M1) be a nonempty connected smooth h-cobordism with a fixed finite handle presentation H relative to M0. The class τH(W,M0) is independent of the auxiliary choices in its definition: cellular representatives, basepoint paths, the universal-cover identification and chosen lifts, core orientations, the order of handles, and the chain contraction. If H,H′ are related by the elementary handle modifications listed in Handle slides and cancelling-pair creations preserve Whitehead torsion, then τH(W,M0)=τH′(W,M0). For each fixed presentation it also agrees with AT-22's torsion of the inclusion M0↪W computed using the associated finite CW structure. No invariance under arbitrary changes of handle presentation is asserted.

Facts & Assumptions

Given: A nonempty connected smooth h-cobordism (W;M0,M1) and a fixed finite handle presentation H of (W,M0).

[F1]

The presentation-indexed torsion is the contraction torsion of the based handle complex, taken in Wh⁡(π1(M0)), and by the comparison lemma it agrees with the Whitehead torsion of the inclusion M0↪W for the CW structure associated with the presentation (Presentation-indexed Whitehead torsion of an h-cobordism, The torsion of the handle complex is the torsion of the inclusion, The handle complex of an h-cobordism is contractible over the group ring, with an explicit contraction, The based handle chain complex over the fundamental group ring).

[F2]

AT-22's independence theorem: the Whitehead torsion of a finite CW homotopy equivalence is independent of the cellular representative, the basepoint paths, the universal-cover identification, the chosen lifts, the cell orientations and order, and the chain contraction; the contraction torsion of a contractible based complex is independent of the contraction (Whitehead torsion is independent of all auxiliary choices, Whitehead torsion of a finite CW homotopy equivalence, Contraction torsion does not depend on the contraction, Finite based free complexes and contraction torsion).

[F3]

Basis changes of the displayed handle bases given by elementary matrices, permutations, signs or units ±g die in the Whitehead group (Cellular basis ambiguities vanish in the Whitehead group).

[F4]

The elementary handle modifications of the listed kinds preserve the presentation-indexed torsion class (Handle slides and cancelling-pair creations preserve Whitehead torsion, Composition and based-pair sum formulas for Whitehead torsion, h-Cobordism).

Proof

1.1F1F3

The class τH(W,M0) is defined as the contraction torsion of the based handle complex of the presentation, and by [F1] it equals the AT-22 torsion of the inclusion M0↪W for the associated finite CW structure; consequently the auxiliary choices made in the handle-complex definition (lifts of handles, core orientations, handle order) are exactly the choices controlled by the AT-22 independence theorem and the basis-change lemma.

2.1F2step 1.1

Independence of the chain contraction is the contraction-independence lemma for contraction torsion, and independence of the cellular representative, basepoint paths, cover identification, lifts, orientations and order is [F2] applied to the inclusion with the associated CW structures.

3.1F3F4step 2.1

If H and H′ differ by the listed elementary modifications, then by [F4] each modification preserves the class, so τH(W,M0)=τH′(W,M0); this argument covers exactly the listed moves, and the elementary modifications of the previous lemma include cancelling-pair creation and deletion, handle slides of equal-index handles, reordering and re-choices of oriented lifts.

4.1F1step 3.1∎

Steps 2.1 and 3.1 give fixed-presentation auxiliary-choice independence and invariance under the listed elementary moves, and step 1.1 gives agreement with AT-22's torsion of the inclusion for the associated CW structure; nothing in the argument compares presentations that are not connected by the listed moves, so no invariance under arbitrary changes of handle presentation is asserted.

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Cited to discharge well-definedness by Presentation-indexed Whitehead torsion of an h-cobordism.

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