Alphabeta Math
False statementConstruction: AI-adaptedVerification: AI-generatedprecheck passaudited 2026-09-01
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FALSE: every abelian category has enough projectives and enough injectives

Statement

False. Every abelian category has enough projectives and enough > injectives.

Facts & Assumptions

Given: The abelian category FinAb of finite abelian groups.

[L1]

Enough projectives and enough injectives are extra hypotheses, not part of the definition of abelian category (A category with enough projectives and with enough injectives).

[L2]

The projective-resolution construction on this page still needs a chosen projective epimorphism at each stage (A chosen chain of projective epimorphisms gives a projective resolution).

[L3]

The Grothendieck theorem gives enough injectives only under additional Grothendieck hypotheses (Every Grothendieck category has enough injectives, and every object admits an injective resolution).

Refutation

technique · direct
1.1L1choose

The category FinAb is abelian. If it had enough projectives in the sense of [L1], then some nonzero projective object P would surject onto Z/pZ for some prime p. Choose a cyclic quotient u:P↠Z/pmZ with m maximal among all cyclic p-power quotients of P.

2.1L2L3step 1.1algebra∎

The canonical quotient q:Z/pm+1Z↠Z/pmZ is epic. If P were projective, u would lift across q, producing a surjection P↠Z/pm+1Z, contradicting maximality of m. So FinAb does not have enough projectives, and the universal statement is false. The positive statements [L2] and [L3] are therefore extra-hypothesis results, not automatic consequences of abelianity.

Depends on

Used by

Nothing in the library uses this result yet.

Dependency tree · two levels

11 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