CounterexampleConstruction: Literature-sourcedVerification: AI-generatedPipeline-generatedprecheck passaudited 2026-08-31‡ rests on unproved material (inherited)

How statement and proof provenance work

The first chip identifies the source of the statement or construction; the second identifies the source of its local proof or verification.

Literature-sourced: the exact statement appears in a cited source; only wording and notation differ.
AI-adapted: a semantically identical restatement of literature-sourced material, modulo indexing, notation, and boundary cases adopted by the library.
AI-generated: a genuinely novel statement formulated by AI, with no source for the claim itself.

These labels describe origin, not correctness: citations and verification chips remain separate evidence.

‡ Rests on 3 statements not proved in this library, by way of the results it cites. This item cites no such statement directly; it depends on results that do. The unproved premises it inherits are ‡ Cohen's first model: an infinite Dedekind-finite set of reals, ‡ Sierpiński 1947: the generalised continuum hypothesis implies the Axiom of Choice and ‡ The continuum hypothesis and its generalisation are independent of ZFC. Each is recorded with a citation to the literature and is not established here, because the track that would prove it has not yet been developed in this library. Everything else in this proof is proved here.

An acyclic noncontractible complex from a nonsplit extension

Statement refuted

Every acyclic complex is contractible.

Facts & Assumptions

Given: The three-term complex $0 \to Z \overset{2}{\to} Z \overset{m o d 2}{\to} Z / 2 \to 0.$

[L1]

Every contractible complex is acyclic (A contractible complex is acyclic).

[L2]

Being zero in the homotopy category is stronger than having zero homology (Zero homology does not make an object zero in the homotopy category).

[L3]

$A b$ is an abelian category (Abelian groups form an abelian category).

Counterexample

technique · direct

1.1

The complex is acyclic: multiplication by $2$ is injective, reduction mod $2$ is surjective, and its kernel is $2 Z$ , which is the image of the first map.

L3givenalgebra

2.1

If the complex were contractible, its identity map would be null-homotopic. In degree $0$ , that would force the surjection $Z \to Z / 2$ to have a section, so the short exact sequence would split. It does not split, so the complex is not contractible. Hence the displayed complex refutes the statement, exactly as [L2] warns; [L1] remains true as the forward implication.

L1L2step 1.1algebra∎

Depends on

Used by

A quasi-isomorphism with no homotopy inverse Counterexample

Dependency tree · two levels

13 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

Charles A. Weibel, Chapter 1 of An Introduction to Homological Algebra (standard reference, not scraped)