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Binary detection by the ordinary rank-zero K-stack comparison

Statement

Assume the Axiom of Choice and compatible site and universe bounds, with ω in the data universe. Use the source-native smooth Zariski stack category over a qcqs base S. Let E be a homotopy-commutative oriented motivic ring satisfying PBF and the coefficient hypotheses of the supplied K-stack mapping theorem. Let M be an E-module in its elementary-blowup-excision localization, and put Y=ΩP1∞M. Write B∞=colim⁡nVectn,S under addition of a trivial line, K~ for the rank-zero K-theory stack, and u:B∞→K~ for the canonical reduced object map. For every stack T, restriction induces an equivalence of unpointed mapping spaces Map⁡(K~×T,Y)≃Map⁡(B∞×T,Y). In particular u×u detects homotopy classes of maps from K~×K~ to Y. These are maps in the ordinary source-native Zariski sheaf category, before any optional hypercompletion. No additivity of the tested map is assumed.

Facts & Assumptions

[A1]

Assume The Axiom of Choice and the compatible universe closure convention of Grothendieck universe closure convention, with the separate existence and size bounds in the Statement. These are the inherited premises of the categorical and K-stack inputs in steps 1.1–2.1.

[F1]

K-stack mapping spaces and extension from smooth affine schemes supplies the pointed equivalence u∗:Map⁡∗(K~,ΩP1∞N)≃Map⁡∗(B∞,ΩP1∞N) for every excisive E-module N, natural in N. The sources and their specified basepoints are those of that supplier.

[F2]

The framework supplies presentable coefficient categories, their colimit-preserving stack action, motivic suspension and its evaluation adjunction. For an unpointed stack T, write NT+ for the cotensor, right adjoint to T+⊗−. It retains the E-module action by transposing the action on N. Evaluation gives (ΩP1∞N)T≃ΩP1∞(NT+). Here T+ adjoins a disjoint point, and the superscript on spaces is cartesian internal Hom. Source-specific non-A¹ motivic spectrum framework and hypotheses

[F3]

Source record 0 supplies the excision localization test: its generating maps are R⊗χn for all motivic spectra R, where χn is the map from the pushout of the three other corners of the elementary blowup square to An. Being local means that mapping these maps into the target gives equivalences.

[F4]

External symmetric P¹ model, orientation criterion and fundamental stabilization supplies orientation inheritance by E-modules, orientable implies fundamental, and the fundamental coefficient-stabilization equivalence. Its hypotheses are the already assumed compact generation, preservation of filtered colimits by (−)P1, and multiplicativity of the ambient coefficient category V=StS; the K-stack supplier retains these hypotheses and includes this coefficient category. The cotensor in [F2] stays in that same category. Consequently every E-module N, including NT+, has a lift N^ to motivic spectra with coefficients in Sp(StS), with N≃Ω∞N^. Here Ω∞ is ordinary coefficient evaluation, applied levelwise, distinct from ΩP1∞. Thus ΩP1∞N≃Ω∞(ΩP1∞N^) is a group-like infinite-loop object of StS, with zero, addition and inverse. Ordinary spectral conventions are those of Stable spectra and exact triangles; the passage from these motivic modules to spectral coefficients uses the fundamental-stabilization supplier. No compactness or filtered-colimit preservation of (−)T+ is required.

External input

Source record 0 imports only the excision-localization test in [F3]. The one-variable K-stack comparison is supplied by [F1], and the orientation and fundamental-stabilization inputs by [F4]. The product comparison below is a local composition using cotensor adjunctions; it is not attributed to a stated binary theorem in that paper and does not identify ordinary and hypercomplete sections.

Proof

Given: S,E,M,T,u, Choice and the compatible size bounds as in the Statement.

Goal: Apply the supplied one-variable comparison to a second source variable without changing the coefficient target or sheaf category.

1.1A1F1F2F3F4

If N is excision-local, then so is NT+. Indeed [F2] identifies the mapping test for R⊗χn into this cotensor with the test for (R⊗T+)⊗χn into N. This is an equivalence by [F3], since R⊗T+ is another permitted motivic spectrum. The action in [F2] makes the cotensor an E-module, so [F1] applies to it. For either this cotensor or the original module, [F4] supplies an orientation, hence fundamentality, and then fundamental stabilization in the same compactly generated multiplicative coefficient category with filtered-colimit-preserving P1 cotensor. For each such module Q, its evaluated target Z=ΩP1∞Q is therefore a group-like infinite-loop sheaf: its lift evaluates to a spectrum object, whose compatible circle-loop structures supply coherent addition and inverse. For a pointed stack p:1→P, let !:P→1 be the terminal map. Evaluation and subtraction define f↦(f∘p,f−(f∘p)∘!) from Map⁡(P,Z) to Map⁡(1,Z)×Map⁡∗(P,Z); the second component includes the zero homotopy supplied by inverse and unit. Its inverse sends (a,g) to a∘!+g. The coherent group identities give the two composite homotopies, so this is an equivalence of spaces, natural for pointed maps of P. Since u preserves the specified basepoints, restriction is the identity on the constant-value factor and is [F1] on the pointed factor. Hence [F1] also gives the unpointed equivalence, including paths.

2.1F1F2F4step 1.1∎

Cartesian currying and the evaluation adjunction in [F2] identify maps K~×T→Y with maps K~→ΩP1∞(MT+), and similarly with B∞ in place of K~. Apply step 1.1 to obtain the displayed equivalence. First take T=K~, and then interchange factors and take T=B∞. The resulting composite is restriction along u×u, so it is an equivalence on mapping spaces and injective on their components. In particular a map becoming nullhomotopic after that restriction is already nullhomotopic. All adjunctions and [F1] operate in the ordinary source-native category, so this conclusion uses no replacement of the coefficient sections by hypercomplete ones.

Depends on

Used by · 2

Dependency tree · 2 levels

Each result is drawn at its shortest distance from this item. Arrows run from prerequisites to the items that use them; this item has a heavier outline. External source nodes link directly to the cited text. The view shows up to two dependency steps; open a prerequisite to continue.

External dependencies

Annala–Iwasa, Motivic spectra and universality of K-theory (14 April 2025) · §3.3.2–3.3.4, printed p. 24, especially the localization-generators description in the proof of Lemma 3.3.4

Elementary-blowup excision localizes motivic spectra at all maps R tensor chi_n, with R any motivic spectrum.

Hypotheses: The source-native qcqs derived base, presentable stack-module coefficient category and compatible size bounds; chi_n is the elementary blowup-square pushout map.

Reading limits: For this repair, read the live April 14, 2025 primary PDF text of §§3.3.1–3.3.4, including the displayed proof of Lemma 3.3.4. Import only its localization test; the cotensor application is local. Also checked §§2.2.1–2.2.6, 2.2.10, Theorem 2.4.5 and its displayed proof, Corollary 2.4.8, and §§3.1.5–3.1.7 against the existing fundamental-stabilization supplier. Brown representability and the categorical foundations remain imported, not reconstructed. No independent source audit or binary theorem is claimed.

Sources