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Mathlib.CategoryTheory.Monoidal.Center

Half braidings and the Drinfeld center of a monoidal category #

We define Center C to be pairs ⟨X, b⟩, where X : C and b is a half-braiding on X.

We show that Center C is braided monoidal, and provide the monoidal functor Center.forget from Center C back to C.

Implementation notes #

Verifying the various axioms directly requires tedious rewriting. Using the slice tactic may make the proofs marginally more readable.

More exciting, however, would be to make possible one of the following options:

  1. Integration with homotopy.io / globular to give "picture proofs".
  2. The monoidal coherence theorem, so we can ignore associators (after which most of these proofs are trivial).
  3. Automating these proofs using rewrite_search or some relative.

In this file, we take the second approach using the monoidal composition ⊗≫ and the coherence tactic.

structure CategoryTheory.HalfBraiding {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] (X : C) :
Type (max u₁ v₁)

A half-braiding on X : C is a family of isomorphisms X ⊗ U ≅ U ⊗ X, monoidally natural in U : C.

Thinking of C as a 2-category with a single 0-morphism, these are the same as natural transformations (in the pseudo- sense) of the identity 2-functor on C, which send the unique 0-morphism to X.

Instances For
    def CategoryTheory.Center (C : Type u₁) [Category.{v₁, u₁} C] [MonoidalCategory C] :
    Type (max u₁ u₁ v₁)

    The Drinfeld center of a monoidal category C has as objects pairs ⟨X, b⟩, where X : C and b is a half-braiding on X.

    Instances For

      A morphism in the Drinfeld center of C.

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        theorem CategoryTheory.Center.Hom.ext_iff {C : Type u₁} {inst✝ : Category.{v₁, u₁} C} {inst✝¹ : MonoidalCategory C} {X Y : Center C} {x y : X.Hom Y} :
        x = y x.f = y.f
        theorem CategoryTheory.Center.Hom.ext {C : Type u₁} {inst✝ : Category.{v₁, u₁} C} {inst✝¹ : MonoidalCategory C} {X Y : Center C} {x y : X.Hom Y} (f : x.f = y.f) :
        x = y
        theorem CategoryTheory.Center.ext {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] {X Y : Center C} (f g : X Y) (w : f.f = g.f) :
        f = g
        theorem CategoryTheory.Center.ext_iff {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] {X Y : Center C} {f g : X Y} :
        f = g f.f = g.f
        noncomputable def CategoryTheory.Center.isoMk {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] {X Y : Center C} (f : X Y) [IsIso f.f] :
        X Y

        Construct an isomorphism in the Drinfeld center from a morphism whose underlying morphism is an isomorphism.

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          @[simp]
          theorem CategoryTheory.Center.isoMk_hom {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] {X Y : Center C} (f : X Y) [IsIso f.f] :
          (isoMk f).hom = f
          @[simp]
          theorem CategoryTheory.Center.isoMk_inv_f {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] {X Y : Center C} (f : X Y) [IsIso f.f] :
          (isoMk f).inv.f = inv f.f

          Auxiliary definition for the MonoidalCategory instance on Center C.

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            def CategoryTheory.Center.whiskerLeft {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] (X : Center C) {Y₁ Y₂ : Center C} (f : Y₁ Y₂) :
            X.tensorObj Y₁ X.tensorObj Y₂

            Auxiliary definition for the MonoidalCategory instance on Center C.

            Instances For
              def CategoryTheory.Center.whiskerRight {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] {X₁ X₂ : Center C} (f : X₁ X₂) (Y : Center C) :
              X₁.tensorObj Y X₂.tensorObj Y

              Auxiliary definition for the MonoidalCategory instance on Center C.

              Instances For
                def CategoryTheory.Center.tensorHom {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] {X₁ Y₁ X₂ Y₂ : Center C} (f : X₁ Y₁) (g : X₂ Y₂) :
                X₁.tensorObj X₂ Y₁.tensorObj Y₂

                Auxiliary definition for the MonoidalCategory instance on Center C.

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                  @[simp]
                  theorem CategoryTheory.Center.tensorHom_f {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] {X₁ Y₁ X₂ Y₂ : Center C} (f : X₁ Y₁) (g : X₂ Y₂) :

                  Auxiliary definition for the MonoidalCategory instance on Center C.

                  Instances For

                    Auxiliary definition for the MonoidalCategory instance on Center C.

                    Instances For

                      Auxiliary definition for the MonoidalCategory instance on Center C.

                      Instances For

                        Auxiliary definition for the MonoidalCategory instance on Center C.

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                          @[simp]
                          theorem CategoryTheory.Center.tensor_f {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] {X₁ Y₁ X₂ Y₂ : Center C} (f : X₁ Y₁) (g : X₂ Y₂) :

                          The forgetful monoidal functor from the Drinfeld center to the original category.

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

                            Auxiliary definition for the BraidedCategory instance on Center C.

                            Instances For

                              Auxiliary construction for ofBraided.

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                                The functor lifting a braided category to its center, using the braiding as the half-braiding.

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                                  @[simp]
                                  theorem CategoryTheory.Center.ofBraided_map_f (C : Type u₁) [Category.{v₁, u₁} C] [MonoidalCategory C] [BraidedCategory C] {X✝ Y✝ : C} (f : X✝ Y✝) :
                                  ((ofBraided C).map f).f = f