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Mathlib.CategoryTheory.Monoidal.Cartesian.Mon_

Yoneda embedding of Mon C #

We show that monoid objects in Cartesian monoidal categories are exactly those whose yoneda presheaf is a presheaf of monoids, by constructing the yoneda embedding Mon C ⥤ Cᵒᵖ ⥤ MonCat.{v} and showing that it is fully faithful and its (essential) image is the representable functors.

If X represents a presheaf of monoids, then X is a monoid object.

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    @[deprecated MonObj.ofRepresentableBy (since := "2025-09-09")]

    Alias of MonObj.ofRepresentableBy.


    If X represents a presheaf of monoids, then X is a monoid object.

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      @[deprecated MonObj.ofRepresentableBy (since := "2025-03-07")]

      Alias of MonObj.ofRepresentableBy.


      If X represents a presheaf of monoids, then X is a monoid object.

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        @[deprecated MonObj.ofRepresentableBy (since := "2025-09-09")]

        Alias of MonObj.ofRepresentableBy.


        If X represents a presheaf of monoids, then X is a monoid object.

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          @[reducible, inline]

          If M is a monoid object, then Hom(X, M) has a monoid structure.

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            theorem CategoryTheory.Functor.map_mul {C : Type u_1} {D : Type u_2} [Category.{v, u_1} C] [CartesianMonoidalCategory C] [Category.{w, u_2} D] [CartesianMonoidalCategory D] {M X : C} [MonObj M] (F : Functor C D) [F.Monoidal] (f g : X M) :
            F.map (f * g) = F.map f * F.map g

            Functor.map of a monoidal functor as a MonoidHom.

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              @[simp]

              Functor.map of a fully faithful monoidal functor as a MulEquiv.

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                @[simp]
                @[reducible, inline]

                If M is a commutative monoid object, then Hom(X, M) has a commutative monoid structure.

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                  If M is a monoid object, then Hom(-, M) is a presheaf of monoids.

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                    @[simp]
                    theorem yonedaMonObj_map {C : Type u_1} [CategoryTheory.Category.{v, u_1} C] [CategoryTheory.CartesianMonoidalCategory C] (M : C) [MonObj M] {X Y₂ : Cᵒᵖ} (φ : X Y₂) :
                    (yonedaMonObj M).map φ = MonCat.ofHom { toFun := fun (x : Opposite.unop X M) => CategoryTheory.CategoryStruct.comp φ.unop x, map_one' := , map_mul' := }

                    If X represents a presheaf of monoids F, then Hom(-, X) is isomorphic to F as a presheaf of monoids.

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                      The yoneda embedding of Mon_C into presheaves of monoids.

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                        @[simp]
                        theorem yonedaMon_map_app {C : Type u_1} [CategoryTheory.Category.{v, u_1} C] [CategoryTheory.CartesianMonoidalCategory C] {M N : Mon C} (ψ : M N) (Y : Cᵒᵖ) :
                        (yonedaMon.map ψ).app Y = MonCat.ofHom { toFun := fun (x : Opposite.unop Y M.X) => CategoryTheory.CategoryStruct.comp x ψ.hom, map_one' := , map_mul' := }

                        The yoneda embedding for Mon_C is fully faithful.

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                          @[deprecated MonObj.one_comp (since := "2025-09-09")]

                          Alias of MonObj.one_comp.

                          @[deprecated MonObj.mul_comp (since := "2025-09-09")]

                          Alias of MonObj.mul_comp.

                          @[deprecated MonObj.pow_comp (since := "2025-09-09")]

                          Alias of MonObj.pow_comp.

                          @[deprecated MonObj.comp_one (since := "2025-09-09")]

                          Alias of MonObj.comp_one.

                          @[deprecated MonObj.comp_mul (since := "2025-09-09")]

                          Alias of MonObj.comp_mul.

                          @[deprecated MonObj.comp_pow (since := "2025-09-09")]

                          Alias of MonObj.comp_pow.

                          @[deprecated MonObj.one_eq_one (since := "2025-09-09")]

                          Alias of MonObj.one_eq_one.

                          If M and N are isomorphic as monoid objects, then X ⟶ M and X ⟶ N are isomorphic monoids.

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