PropositionStatement: Literature-sourcedProof: AI-adaptedPipeline-generatedprecheck passjudge pass (gpt-5.6-terra)audited 2026-09-06‡ 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.

Extension classes are contravariant in the quotient and covariant in the subobject

Statement

For fixed extension classes, pullback in the quotient object and pushout in the subobject define additive maps; thus ${YExt}^{1} (-, -)$ is contravariant in its first and covariant in its second variable.

Facts & Assumptions

Given: Morphisms $a : M^{'} \to M$ and $b : N \to N^{'}$ .

Proof

technique · direct

1.1

Pullback along $a$ and pushout along $b$ are well-defined on equivalence classes by Pullback and pushout descend to extension classes. Their universal properties make their composites agree with pullback and pushout along composite maps.

givenconstruct

2.1

Both constructions commute with the direct-sum, diagonal, and codiagonal steps defining Baer addition. Hence they are homomorphisms for the group structure of Baer sum makes extension classes an abelian group, with the stated variance.

step 1.1algebra∎

Depends on

Used by

Yoneda Ext one is naturally isomorphic to derived Ext one Theorem

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

Charles A. Weibel, An Introduction to Homological Algebra, Chapters 3–4 (standard reference, not scraped)