Adjunctions and the free-group construction
Research context
postgraduate concept · established · As of 2026-10-10
An adjunction F left-adjoint to U is a natural bijection Hom-D(FX,Y)≅Hom-C(X,UY).
Key knowledge
An adjunction F left-adjoint to U is a natural bijection Hom-D(FX,Y)≅Hom-C(X,UY). The free-group functor from sets is left-adjoint to the forgetful functor from groups: every assignment of generators to a group extends uniquely to a homomorphism. Unit and counit obey triangle identities; a universal property is stronger than an informal claim that a construction is convenient.
Values and equations
Hom_D(FX,Y) ≅ Hom_C(X,UY)
- F
- Left-adjoint functor
- U
- Right-adjoint functor
- X,Y
- Objects in their respective categories
Conditions and scope
- The Hom bijection is natural in both arguments.
- The free group consists of reduced finite words with generator inverses; no additional relations are silently imposed.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Author checked this English statement, displayed equation where present and stated hypotheses against these source locations: §§1.1–1.4 categories/functors/naturality, §2.2 Yoneda, chapter 4 adjunctions. The check is bounded to the recorded claims; no complete proof reconstruction, software proof replay or universal literature completeness is claimed.
2026-10-10 · Author source-body/statement/equation/domain check, with graph associations scoped individually. Separate independent content/source/inventory review is required before branch closure.
Entry details
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