TheoremStatement: Literature-sourcedProof: 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.

A projective generator detects isomorphisms

Statement

Let $P$ be a projective generator of an abelian category. If

$A (P, f) : A (P, A) \to A (P, B)$

is an isomorphism for a morphism $f : A \to B$ , then $f$ is an isomorphism.

Facts & Assumptions

Given: A projective generator $P$ and a morphism $f : A \to B$ .

[L1]

Projectivity makes $A (P, -)$ exact on short exact sequences (Projective object characterisations).

[L2]

A generator is equivalently an object whose canonical coproduct maps are epic (The cancellation and epimorphism descriptions of a generator agree).

Proof

technique · direct

1.1

Apply [L1] to the short exact sequences $0 \to \ker (f) \to A \to im (f) \to 0$ and $0 \to im (f) \to B \to coker (f) \to 0$ . Since $A (P, f)$ is an isomorphism, the first sequence forces $A (P, \ker (f)) = 0$ and the second forces $A (P, coker (f)) = 0$ .

L1construct

2.1

Let $X$ be any object with $A (P, X) = 0$ . By [L2], the canonical map $∐_{u \in A (P, X)} P \to X$ is epic. But the indexing set is empty, so this is the zero map $0 \to X$ . An epic zero map forces $X = 0$ . Applying this to the objects in step 1.1 gives $\ker (f) = 0 = coker (f)$ .

L2step 1.1

3.1

Therefore $f$ is both monic and epic, hence an isomorphism in an abelian category.

step 2.1∎

Depends on

Used by

Nothing in the library uses this result yet.

Dependency tree · two levels

8 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

Pavel Etingof, Shlomo Gelaki, Dmitri Nikshych, and Victor Ostrik, Tensor Categories, Section 1.6 (standard reference, not scraped)