TheoremStatement: Literature-sourcedProof: AI-generatedSession-authored (Fable 5 assisted)precheck passjudge pass (gpt-5.6-terra)audited 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.

The pullback of an epimorphism is an epimorphism

Statement

In a pullback square in an abelian category

if $e$ is epic, then $α$ is epic.

Facts & Assumptions

Given: The displayed pullback square in an abelian category, with $e$ epic.

[L1]

The pullback is the kernel of the difference map on $A \oplus B$ (A pullback is the kernel of the difference of the two legs, and dually for pushouts).

[L2]

Abelian categories are preadditive with zero morphisms (Abelian category, A preadditive category with a zero object has zero morphisms in the published sense).

[L3]

In an abelian category, a morphism is epic exactly when its cokernel is zero (In an abelian category, monic means zero kernel and epic means zero cokernel).

[L4]

In an abelian category every epic morphism is the cokernel of its kernel (Every monomorphism is the kernel of its cokernel, and dually every epimorphism is the cokernel of its kernel).

Proof

technique · direct

1.1

By [L1], there is a monomorphism $k : P \to A \oplus B$ with $α = p_{A} k$ , $β = p_{B} k$ , and $k$ the kernel of $d : = f p_{A} - e p_{B}$ . Since $d i_{B} = - e$ , the epicity of $e$ implies that $d$ is epic as well.

L1L2

2.1

Let $q : A \to Q$ be a cokernel of $α$ . Because $d$ is epic by step 1.1, [L4] says that $d$ is a cokernel of its kernel $k$ . Since $q α = q p_{A} k = 0$ , the map $q p_{A} : A \oplus B \to Q$ kills $k$ , so it factors through $d$ as $\overline{q} d = q p_{A}$ . Composing with $i_{B}$ gives $\overline{q} (- e) = 0$ , and since $e$ is epic, $\overline{q} = 0$ . Thus $q p_{A} = 0$ , and composing with $i_{A}$ yields $q = 0$ . So $α$ has zero cokernel and is epic by [L3].

L1L2L3L4step 1.1∎

Depends on

Used by

Dependency tree · two levels

20 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

The Stacks Project, Section 12.5, Lemma 12.5.13 (standard reference, not scraped)