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Mathlib.CategoryTheory.Monoidal.Grp_

The category of groups in a cartesian monoidal category #

We define group objects in cartesian monoidal categories.

We show that the associativity diagram of a group object is always cartesian and deduce that morphisms of group objects commute with taking inverses.

We show that a finite-product-preserving functor takes group objects to group objects.

A group object internal to a cartesian monoidal category. Also see the bundled Grp_.

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    The inverse in a group object

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      The inverse in a group object

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        A group object in a cartesian monoidal category.

        • X : C

          The underlying object in the ambient monoidal category

        • grp : Grp_Class self.X
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          A group object is a monoid object.

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            Make a group object from Grp_Class.

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              @[simp]

              The map (· * f).

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                The associativity diagram of a group object is cartesian.

                In fact, any monoid object whose associativity diagram is cartesian can be made into a group object (we do not prove this in this file), so we should expect that many properties of group objects follow from this result.

                The forgetful functor from group objects to the ambient category.

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

                  A finite-product-preserving functor takes group objects to group objects.

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                    @[simp]
                    theorem CategoryTheory.Functor.mapGrp_map_hom {C : Type u₁} [Category.{v₁, u₁} C] [CartesianMonoidalCategory C] {D : Type u₂} [Category.{v₂, u₂} D] [CartesianMonoidalCategory D] (F : Functor C D) [F.Monoidal] {X✝ Y✝ : Grp_ C} (f : X✝ Y✝) :
                    (F.mapGrp.map f).hom = F.map f.hom

                    The identity functor is also the identity on group objects.

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                      The composition functor is also the composition on group objects.

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                        Natural transformations between functors lift to group objects.

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                          Natural isomorphisms between functors lift to group objects.

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                            mapGrp is functorial in the left-exact functor.

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                              An adjunction of monoidal functors lifts to an adjunction of their lifts to group objects.

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                                An equivalence of categories lifts to an equivalence of their group objects.

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