Reflective functors #

Basic properties of reflective functors, especially those relating to their essential image.

Note properties of reflective functors relating to limits and colimits are included in CategoryTheory.Monad.Limits.

    A functor is reflective, or a reflective inclusion, if it is fully faithful and right adjoint.


      For a reflective functor i (with left adjoint L), with unit η, we have η_iL = iL η.

      When restricted to objects in D given by i : D ⥤ C, the unit is an isomorphism. In other words, η_iX is an isomorphism for any X in D. More generally this applies to objects essentially in the reflective subcategory, see Functor.essImage.unit_isIso.

      If A is essentially in the image of a reflective functor i, then η_A is an isomorphism. This gives that the "witness" for A being in the essential image can instead be given as the reflection of A, with the isomorphism as η_A.

      (For any B in the reflective subcategory, we automatically have that ε_B is an iso.)

      (Implementation) Auxiliary definition for unitCompPartialBijective.

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        If i has a reflector L, then the function (i.obj (L.obj A) ⟶ B) → (A ⟶ B) given by precomposing with η.app A is a bijection provided B is in the essential image of i. That is, the function λ (f : i.obj (L.obj A) ⟶ B), η.app A ≫ f is bijective, as long as B is in the essential image of i. This definition gives an equivalence: the key property that the inverse can be described nicely is shown in unitCompPartialBijective_symm_apply.

        This establishes there is a natural bijection (A ⟶ B) ≃ (i.obj (L.obj A) ⟶ B). In other words, from the point of view of objects in D, A and i.obj (L.obj A) look the same: specifically that η.app A is an isomorphism.

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          theorem CategoryTheory.equivEssImageOfReflective_counitIso {C : Type u₁} {D : Type u₂} [CategoryTheory.Category.{v₁, u₁} C] [CategoryTheory.Category.{v₂, u₂} D] {i : CategoryTheory.Functor D C} [CategoryTheory.Reflective i] :
          CategoryTheory.equivEssImageOfReflective.counitIso = CategoryTheory.NatIso.ofComponents CategoryTheory.equivEssImageOfReflective_counitIso_app

          If i : D ⥤ C is reflective, the inverse functor of i ≌ F.essImage can be explicitly defined by the reflector.

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