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CounterexampleConstruction: AI-generatedVerification: AI-generatedprecheck passaudited 2026-09-01
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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

1.1L1L2L3construct

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.

1.2givenconstruct

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

2.1step 1.2givenalgebra

Choose x∈P with u(x)=1∈Z/dZ. For each n≥1, projectivity lifts u through the reduction Z/dnZ↠Z/dZ to a map un:P→Z/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.

3.1step 1.1step 2.1∎

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

Depends on

Used by

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