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In category theory, a faithful functor (resp. a full functor) is a functor that is injective (resp. surjective) when restricted to each set of morphisms that have a given source and target.

Explicitly, let C and D be (locally small) categories and let F : CD be a functor from C to D. The functor F induces a function

FX,Y:Hom𝒞(X,Y)Hom𝒟(F(X),F(Y))

for every pair of objects X and Y in C. The functor F is said to be

for each X and Y in C.

A faithful functor need not be injective on objects or morphisms. That is, two objects X and X′ may map to the same object in D (which is why the range of a full and faithful functor is not necessarily isomorphic to C), and two morphisms f : XY and f′ : X′ → Y′ (with different domains/codomains) may map to the same morphism in D. Likewise, a full functor need not be surjective on objects or morphisms. There may be objects in D not of the form FX for some X in C. Morphisms between such objects clearly cannot come from morphisms in C.

A full and faithful functor is necessarily injective on objects up to isomorphism. That is, if F : CD is a full and faithful functor and F(x)F(y) then xy.

Examples

  • The forgetful functor U : GrpSet is faithful as each group maps to a unique set and the group homomorphism are a subset of the functions. This functor is not full as there are functions between groups which are not group homomorphisms. A category with a faithful functor to Set is (by definition) a concrete category; in general, that forgetful functor is not full.
  • Let F : SetSet be the functor which maps every set to the empty set and every function to the empty function. Then F is full, but is neither injective on objects nor on morphisms.
  • The inclusion functor AbGrp is fully faithful.

See also

Notes

  1. Mac Lane (1971), p. 15
  2. 2.0 2.1 Jacobson (2009), p. 22
  3. Mac Lane (1971), p. 14

References

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  • 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

    My blog: http://www.primaboinca.com/view_profile.php?userid=5889534

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