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A coherent spectral realization of derived abelian complexes

Statement

Assume the Axiom of Choice and the fixed compatible universes of the derived and spectral interfaces. Let D(Z) be the stable infinity-localization of unbounded cochain complexes of abelian groups at quasi-isomorphisms; its homotopy category is D(Z). Let HZ denote the discrete Eilenberg–Mac Lane ring spectrum of the integers. Fix the unit-preserving equivalence Φ:Mod⁡HZ≃D(Z) and an inverse with its equivalence data. Here a module spectrum is a spectrum with an HZ action including its associative and unital coherences; morphisms are coherent linear maps. Write U for the forgetful functor to spectra and define H:=UΦ−1:D(Z)⟶Sp. This is a coherent functor preserving small limits, zero objects, fibers and integer shifts, with natural identifications πiH(C)≅H−i(C),H(C[r])≃H(C)[r] for every derived complex C and all integers i,r. Different inverse data for the fixed equivalence give naturally equivalent functors. Thus composing any supplied coherent complex-valued diagram with H gives a coherent spectral diagram, including its restriction arrows, limits and fibers, independent up to equivalence of those inverse data. No lift of an arbitrary ordinary D(Z)-valued diagram is asserted.

Facts & Assumptions

Given: Choice, compatible universes, the enhanced derived category, the fixed equivalence Φ and its inverse data, and the integer indices in the Statement.

Goal: Construct the coherent realization and justify its exactness, indexing and independence from inverse data.

[F1]

External input on derived enhancements supplies D(Z) by taking the ring to be Z. Its cochain shift has C[r]q=Cq+r and differential (−1)rdC. Stable spectra and exact triangles supplies spectra, their limits and fibers, and πi(E)=π0Map⁡Sp(S[i],E).

[F2]

External stable mapping and derived linear target interfaces supplies the derived/module target and coherent mapping-space formalism. Reference 0 specifies the unit-preserving equivalence Φ, the underlying-spectrum limit criterion for all small diagrams, and the unit mapping equivalence. The proof of its Theorem 7.1.2.13 identifies Hom⁡D(Z)(Z,C[n]) with Hn(C) after reversing homological grading.

[A1]

Assume The Axiom of Choice and the same universe existence and size conventions as [F1]. Choice is inherited from the supplied unbounded enhancement and spectral foundations; no choice of a representative complex for each value of a diagram is made.

External input

Reference 0 imports the normalized derived/module equivalence, the module limit criterion and the unit mapping formula used in [F2]. Their hypotheses are checked with R=Z and spectra as ambient category. These established inputs are not proved here. Their composition into H and the indexing calculation are the local argument below.

Proof

1.1F1F2A1given

Under [A1], [F1] and [F2] supply actual enhanced categories and functors. Compose the inverse equivalence with U to obtain H on objects, arrows and all higher simplices. An equivalence preserves limits by their mapping-space universal property, and U computes them by [F2]; hence H preserves small limits. The zero module has zero underlying spectrum. It follows that H preserves fibers and loops. Suspension is inverse to loops in both stable categories, so it also preserves suspension and every integer shift. For two inverse functors E,E′ to the fixed Φ, their coherent unit and counit give E≃EΦE′≃E′. Applying U gives a natural equivalence of realizations. Applying such an equivalence to a coherent diagram and its limiting cones gives equivalent diagrams and limits, including the fiber projections.

2.1F1F2A1step 1.1∎

For N=Φ−1C, shift compatibility and the unit mapping equivalence in [F2] give πi(UN)=π0Map⁡Sp(S,UN[−i])≅π0Map⁡Mod⁡HZ(HZ,N[−i]). The unit-preserving equivalence carries this to Hom⁡D(Z)(Z,C[−i])=H−i(C) by [F2]. All identifications are natural in enhanced arrows. Precomposing H with a supplied coherent functor therefore gives the asserted coherent realization of that entire functor; step 1.1 gives its limit, fiber and choice comparisons. This proves the claim for supplied enhanced diagrams without constructing an enhancement from ordinary derived-category arrows.

Depends on

Used by · 15

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External dependencies

Jacob Lurie, Higher Algebra (September 18, 2017) · Theorem 7.1.2.13 and proof, printed pp. 1212–1213; Corollary 4.2.3.3, p. 527; Corollary 4.2.4.7, p. 531

The derived infinity-category of modules over a commutative ring is canonically equivalent, preserving tensor units, to modules over its discrete E-infinity ring spectrum. The proof identifies homology with maps from shifts of the unit. Module limits are computed on underlying spectra, and maps from the free rank-one module agree with maps from the sphere into the underlying spectrum.

Hypotheses: Use R = Z, unbounded complexes, the derived infinity-category rather than only its triangulated homotopy category, and small diagrams in the fixed universes. For the module assertions take the ambient monoidal category and its left action to be spectra.

Reading limits: In this repair, read the theorem statement and its displayed proof in the primary PDF extraction, and the statements of the two corollaries. Checked the module-limit hypotheses against the surrounding text and Proposition 4.2.3.1; both ambient categories are spectra. Their operadic, model-structure and compact-generation antecedents are imported, not reconstructed. The cochain sign convention is obtained by reversing the homological grading. The composition and choice comparison below are local arguments; no motivic construction is imported from these locators.

Sources