False statementConstruction: AI-adaptedVerification: AI-generatedSession-authored (Fable 5 assisted)precheck passaudited 2026-08-28‡ 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.

FALSE under the Axiom of Choice: AB4 implies AB5

Statement

Assume the Axiom of Choice. Then AB4 implies AB5.

Facts & Assumptions

Given: The Axiom of Choice and the opposite category ${A b}^{o p}$ .

[A1]

The Axiom of Choice (The Axiom of Choice).

[L1]

The category ${A b}^{o p}$ does not satisfy AB5 (The opposite of abelian groups does not satisfy AB5).

[L2]

AB4 is the coproduct-monomorphism axiom (The axioms AB4 and AB4*).

[L3]

Assuming the Axiom of Choice, $A b$ satisfies AB4* (Assuming the Axiom of Choice, abelian groups satisfy AB4*).

[L4]

The opposite of an abelian category is abelian (The opposite of an abelian category is abelian).

Refutation

1.1

By [L4], the opposite category ${A b}^{o p}$ is abelian. Passing to the opposite exchanges AB4 with AB4*, so [L3] implies that ${A b}^{o p}$ satisfies AB4 in the sense of [L2].

L2L3L4algebra

2.1

But [L1] shows that ${A b}^{o p}$ does not satisfy AB5. So, even under [A1], AB4 does not imply AB5.

A1L1step 1.1∎

Depends on

Used by

Nothing in the library uses this result yet.

Dependency tree · two levels

11 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, An Introduction to Homological Algebra, Appendix A.4 (standard reference, not scraped)