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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 of torsion abelian groups.
Modules over a ring form an abelian category (Modules over a ring form an abelian category).
Over , injective modules are exactly divisible groups (Over a PID, injective modules are exactly divisible modules).
Every abelian group embeds in a divisible group (Every abelian group embeds in a divisible abelian group).
Counterexample
Kernels, images, and cokernels of homomorphisms between torsion abelian groups are torsion again, so is an abelian full subcategory of the abelian category from [L1]. If is torsion, [L3] embeds it into a divisible group ; the torsion subgroup is still divisible and still contains . Hence [L2] makes injective, so has enough injectives.
Suppose were a nonzero projective torsion group. Let , and for each let be the order of . Form with generators , and define the surjection by . Projectivity gives a section . Since , some coordinate projection restricts to a nonzero map . Let be its nonzero cyclic image, write , and regard the resulting map as surjective.
Choose with . For each , projectivity lifts through the reduction to a map . The element is congruent to modulo , hence is relatively prime to and has order . But the order of must divide the fixed finite order of , impossible for arbitrarily large .
Therefore has enough injectives but no nonzero projective objects, so it does not have enough projectives. This refutes the statement.
Depends on
Used by
Nothing in the library uses this result yet.
Dependency tree · two levels
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Sources
- The Stacks Project, Section 19.11: Injectives in Grothendieck categories (standard reference, not scraped)
- Romyar Sharifi, Homological Algebra (standard reference, not scraped)