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In other words, a functor F : C → D is faithful if the maps
are injective for every pair of objects X and Y in C.
Note that 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, and two morphisms f : X → Y and f′ : X′ → Y′ may map to the same morphism in D.
For example, the forgetful functor U : Grp → Set is faithful but neither injective on objects or morphisms.
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