ExampleConstruction: AI-generatedVerification: AI-adaptedprecheck passaudited 2026-09-04‡ 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 resolution that is not an injective resolution

Example

For the identity functor on abelian groups and a supplied projective resolution datum $P$ on a class containing the free abelian groups, the standard free resolution $0 \to Z \overset{\times 2}{\to} Z \to Z / 2 Z \to 0$ is an $F$ -acyclic resolution relative to $P$ , but it is not an injective resolution.

Facts & Assumptions

Given: The identity functor on abelian groups, a supplied projective resolution datum $P$ on a class containing the free abelian groups, and the displayed free resolution.

[L1]

Projective objects are acyclic for left derived functors (Positive left derived functors vanish on projective objects).

[L2]

An $F$ -acyclic resolution is an exact augmented resolution by $F$ -acyclic objects (An F-acyclic resolution).

[L3]

Projective and injective objects are defined by distinct lifting and extension properties (Projective object, Injective object).

Verification

technique · direct

1.1

The terms of the displayed resolution are free abelian groups, hence projective and in the domain of $P$ . By [L1], they are acyclic for the identity functor, so [L2] identifies the displayed exact sequence as an acyclic resolution relative to $P$ .

L1L2given

2.1

The term $Z$ is not injective, because the map $2 Z \to Z$ , $2 n \mapsto n$ , does not extend across $2 Z ↪ Z$ . By [L3], the resolution is therefore not an injective resolution.

L3step 1.1algebra∎

Depends on

Used by

Nothing in the library uses this result yet.

Dependency tree · two levels

12 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, Chapter 2 `Derived Functors` (standard reference, not scraped)