Alphabeta Math
TheoremStatement: Literature-sourcedProof: AI-adaptedSession-authored (Fable 5 assisted)precheck passjudge pass (deepseek-v4-pro + gpt-5.6-terra)audited 2026-08-16
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Abelian groups form a reflective full subcategory of groups

Statement

The full subcategory Ab of abelian groups is reflective in Grp, with reflector given by abelianisation.

Facts & Assumptions

Given: The full inclusion I:AbGrp.

[L1]

Abelianisation defines a functor left adjoint to I, and every map from a group to an abelian group factors uniquely through the abelianisation quotient (Abelianisation is left adjoint to the inclusion of abelian groups).

[L2]

A full subcategory is reflective when its inclusion has a left adjoint (Reflective full subcategory and reflector).

Proof

technique · direct
1.1

The adjunction of [L1] exhibits the full inclusion I as a right adjoint with abelianisation as its left adjoint.

L1
2.1

Hence [L2] makes Ab reflective in Grp with abelianisation as reflector.

step 1.1L2

Depends on

Used by

Nothing in the library uses this result yet.

Dependency tree · next 3 levels

Direct dependencies and their dependencies through the next three levels: 21 results over 5 levels. An arrow runs from a result to what uses it, and this result sits at the bottom with a heavier outline. Click the chart to enlarge it.

Sources