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Mathlib.CategoryTheory.Monoidal.Comon_

The category of comonoids in a monoidal category. #

We define comonoids in a monoidal category C, and show that they are equivalently monoid objects in the opposite category.

We construct the monoidal structure on Comon_ C, when C is braided.

An oplax monoidal functor takes comonoid objects to comonoid objects. That is, a oplax monoidal functor F : C ⥤ D induces a functor Comon_ C ⥤ Comon_ D.

TODO #

The comultiplication morphism of a comonoid object.

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      The comultiplication morphism of a comonoid object.

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          The counit morphism of a comonoid object.

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              The counit morphism of a comonoid object.

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                  A comonoid object internal to a monoidal category.

                  When the monoidal category is preadditive, this is also sometimes called a "coalgebra object".

                  • X : C

                    The underlying object of a comonoid object.

                  • comon : Comon_Class self.X
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                    The trivial comonoid object. We later show this is terminal in Comon_ C.

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                        A morphism of comonoid objects.

                        • hom : M.X N.X

                          The underlying morphism of a morphism of comonoid objects.

                        • is_comon_hom : IsComon_Hom self.hom
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                          theorem Comon_.Hom.ext {C : Type u₁} {inst✝ : CategoryTheory.Category.{v₁, u₁} C} {inst✝¹ : CategoryTheory.MonoidalCategory C} {M N : Comon_ C} {x y : M.Hom N} (hom : x.hom = y.hom) :
                          x = y
                          theorem Comon_.Hom.ext_iff {C : Type u₁} {inst✝ : CategoryTheory.Category.{v₁, u₁} C} {inst✝¹ : CategoryTheory.MonoidalCategory C} {M N : Comon_ C} {x y : M.Hom N} :
                          x = y x.hom = y.hom
                          @[reducible, inline]

                          Construct a morphism M ⟶ N of Comon_ C from a map f : M ⟶ N and a IsComon_Hom f instance.

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                              The identity morphism on a comonoid object.

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                                  def Comon_.comp {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] [CategoryTheory.MonoidalCategory C] {M N O : Comon_ C} (f : M.Hom N) (g : N.Hom O) :
                                  M.Hom O

                                  Composition of morphisms of monoid objects.

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                                      theorem Comon_.ext {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] [CategoryTheory.MonoidalCategory C] {X Y : Comon_ C} {f g : X Y} (w : f.hom = g.hom) :
                                      f = g

                                      The forgetful functor from comonoid objects to the ambient category.

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                                          @[simp]
                                          theorem Comon_.forget_map (C : Type u₁) [CategoryTheory.Category.{v₁, u₁} C] [CategoryTheory.MonoidalCategory C] {X✝ Y✝ : Comon_ C} (f : X✝ Y✝) :
                                          (forget C).map f = f.hom

                                          The forgetful functor from comonoid objects to the ambient category reflects isomorphisms.

                                          Construct an isomorphism of comonoids by giving an isomorphism between the underlying objects and checking compatibility with counit and comultiplication only in the forward direction.

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                                              Construct an isomorphism of comonoids by giving an isomorphism between the underlying objects and checking compatibility with counit and comultiplication only in the forward direction.

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                                                  Auxiliary definition for Comon_ToMon_OpOpObj.

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                                                      Turn a comonoid object into a monoid object in the opposite category.

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                                                          The contravariant functor turning comonoid objects into monoid objects in the opposite category.

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                                                              theorem Comon_.Comon_ToMon_OpOp_map (C : Type u₁) [CategoryTheory.Category.{v₁, u₁} C] [CategoryTheory.MonoidalCategory C] {X✝ Y✝ : Comon_ C} (f : X✝ Y✝) :
                                                              (Comon_ToMon_OpOp C).map f = Opposite.op { hom := f.hom.op, is_mon_hom := }
                                                              @[reducible, inline]

                                                              Auxiliary definition for Mon_OpOpToComonObj.

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                                                                  Turn a monoid object in the opposite category into a comonoid object.

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                                                                      The contravariant functor turning monoid objects in the opposite category into comonoid objects.

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                                                                          Comonoid objects are contravariantly equivalent to monoid objects in the opposite category.

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                                                                              Comonoid objects in a braided category form a monoidal category.

                                                                              This definition is via transporting back and forth to monoids in the opposite category.

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                                                                                Preliminary statement of the comultiplication for a tensor product of comonoids. This version is the definitional equality provided by transport, and not quite as good as the version provided in tensorObj_comul below.

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                                                                                The comultiplication on the tensor product of two comonoids is the tensor product of the comultiplications followed by the tensor strength (to shuffle the factors back into order).

                                                                                The forgetful functor from Comon_ C to C is monoidal when C is monoidal.

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                                                                                  The image of a comonoid object under a oplax monoidal functor is a comonoid object.

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                                                                                      A oplax monoidal functor takes comonoid objects to comonoid objects.

                                                                                      That is, a oplax monoidal functor F : C ⥤ D induces a functor Comon_ C ⥤ Comon_ D.

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                                                                                          theorem CategoryTheory.Functor.mapComon_map_hom {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] {D : Type u₂} [Category.{v₂, u₂} D] [MonoidalCategory D] (F : Functor C D) [F.OplaxMonoidal] {X✝ Y✝ : Comon_ C} (f : X✝ Y✝) :
                                                                                          (F.mapComon.map f).hom = F.map f.hom