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CounterexampleConstruction: AI-generatedVerification: AI-generatedprecheck passaudited 2026-09-01 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 category with enough injectives but not enough projectives

Statement refuted

Enough injectives implies enough projectives.

Facts & Assumptions

Assume the Axiom of Choice through Baer's criterion.

Given: The category TorAb of torsion abelian groups.

[L1]

Modules over a ring form an abelian category (Modules over a ring form an abelian category).

[L2]

Over Z, injective modules are exactly divisible groups (Over a PID, injective modules are exactly divisible modules).

[L3]

Every abelian group embeds in a divisible group (Every abelian group embeds in a divisible abelian group).

Counterexample

1.1

Kernels, images, and cokernels of homomorphisms between torsion abelian groups are torsion again, so TorAb is an abelian full subcategory of the abelian category from [L1]. If A is torsion, [L3] embeds it into a divisible group D; the torsion subgroup t(D) is still divisible and still contains A. Hence [L2] makes t(D) injective, so TorAb has enough injectives.

L1L2L3construct
1.2

Suppose P were a nonzero projective torsion group. Let P×=P{0}, and for each xP× let nx be the order of x. Form G:=xP×Z/nxZ with generators ex, and define the surjection π:GP by π(ex)=x. Projectivity gives a section s:PG. Since s(P)0, some coordinate projection restricts to a nonzero map PZ/nxZ. Let H be its nonzero cyclic image, write H=d>1, and regard the resulting map u:PHZ/dZ as surjective.

givenconstruct
2.1

Choose xP with u(x)=1Z/dZ. For each n1, projectivity lifts u through the reduction Z/dnZZ/dZ to a map un:PZ/dnZ. The element un(x) is congruent to 1 modulo d, hence is relatively prime to d and has order dn. But the order of un(x) must divide the fixed finite order of x, impossible for arbitrarily large n.

step 1.2givenalgebra
3.1

Therefore TorAb has enough injectives but no nonzero projective objects, so it does not have enough projectives. This refutes the statement.

step 1.1step 2.1

Depends on

Used by

Nothing in the library uses this result yet.

Dependency tree · two levels

15 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