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Mathlib.CategoryTheory.Monoidal.Rigid.Basic

Rigid (autonomous) monoidal categories #

This file defines rigid (autonomous) monoidal categories and the necessary theory about exact pairings and duals.

Main definitions #

Main statements #

Notations #

Future work #

Notes #

Although we construct the adjunction tensorLeft Y ⊣ tensorLeft X from ExactPairing X Y, this is not a bijective correspondence. I think the correct statement is that tensorLeft Y and tensorLeft X are module endofunctors of C as a right C module category, and ExactPairing X Y is in bijection with adjunctions compatible with this right C action.

References #

Tags #

rigid category, monoidal category

An exact pairing is a pair of objects X Y : C which admit a coevaluation and evaluation morphism which fulfill two triangle equalities.

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    Evaluation of an exact pairing.

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      Coevaluation of an exact pairing.

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        Evaluation of an exact pairing.

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

          A class of objects which have a right dual.

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            class CategoryTheory.HasLeftDual {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] (Y : C) :
            Type (max u₁ v₁)

            A class of objects which have a left dual.

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              The left dual of the object X.

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                The right dual of the object X.

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                  The right adjoint mate fᘁ : Xᘁ ⟶ Yᘁ of a morphism f : X ⟶ Y.

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                    The left adjoint mate ᘁf : ᘁY ⟶ ᘁX of a morphism f : X ⟶ Y.

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                      The right adjoint mate fᘁ : Xᘁ ⟶ Yᘁ of a morphism f : X ⟶ Y.

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                        The left adjoint mate ᘁf : ᘁY ⟶ ᘁX of a morphism f : X ⟶ Y.

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                          The composition of right adjoint mates is the adjoint mate of the composition.

                          The composition of left adjoint mates is the adjoint mate of the composition.

                          Given an exact pairing on Y Y', we get a bijection on hom-sets (Y' ⊗ X ⟶ Z) ≃ (X ⟶ Y ⊗ Z) by "pulling the string on the left" up or down.

                          This gives the adjunction tensorLeftAdjunction Y Y' : tensorLeft Y' ⊣ tensorLeft Y.

                          This adjunction is often referred to as "Frobenius reciprocity" in the fusion categories / planar algebras / subfactors literature.

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                            Given an exact pairing on Y Y', we get a bijection on hom-sets (X ⊗ Y ⟶ Z) ≃ (X ⟶ Z ⊗ Y') by "pulling the string on the right" up or down.

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                              If Y Y' have an exact pairing, then the functor tensorLeft Y' is left adjoint to tensorLeft Y.

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                                If Y Y' have an exact pairing, then the functor tensor_right Y is left adjoint to tensor_right Y'.

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                                  If Y has a left dual ᘁY, then it is a closed object, with the internal hom functor Y ⟶[C] - given by left tensoring by ᘁY. This has to be a definition rather than an instance to avoid diamonds, for example between category_theory.monoidal_closed.functor_closed and CategoryTheory.Monoidal.functorHasLeftDual. Moreover, in concrete applications there is often a more useful definition of the internal hom object than ᘁY ⊗ X, in which case the closed structure shouldn't come from has_left_dual (e.g. in the category FinVect k, it is more convenient to define the internal hom as Y →ₗ[k] X rather than ᘁY ⊗ X even though these are naturally isomorphic).

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                                    Transport an exact pairing across an isomorphism in the first argument.

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                                      Transport an exact pairing across an isomorphism in the second argument.

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                                        def CategoryTheory.exactPairingCongr {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] {X X' Y Y' : C} [ExactPairing X' Y'] (i : X X') (j : Y Y') :

                                        Transport an exact pairing across isomorphisms.

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                                          def CategoryTheory.rightDualIso {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] {X Y₁ Y₂ : C} (p₁ : ExactPairing X Y₁) (p₂ : ExactPairing X Y₂) :
                                          Y₁ Y₂

                                          Right duals are isomorphic.

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                                            def CategoryTheory.leftDualIso {C : Type u₁} [Category.{v₁, u₁} C] [MonoidalCategory C] {X₁ X₂ Y : C} (p₁ : ExactPairing X₁ Y) (p₂ : ExactPairing X₂ Y) :
                                            X₁ X₂

                                            Left duals are isomorphic.

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                                              A right rigid monoidal category is one in which every object has a right dual.

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                                                A left rigid monoidal category is one in which every object has a right dual.

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                                                  Any left rigid category is monoidal closed, with the internal hom X ⟶[C] Y = ᘁX ⊗ Y. This has to be a definition rather than an instance to avoid diamonds, for example between category_theory.monoidal_closed.functor_category and CategoryTheory.Monoidal.leftRigidFunctorCategory. Moreover, in concrete applications there is often a more useful definition of the internal hom object than ᘁY ⊗ X, in which case the monoidal closed structure shouldn't come the rigid structure (e.g. in the category FinVect k, it is more convenient to define the internal hom as Y →ₗ[k] X rather than ᘁY ⊗ X even though these are naturally isomorphic).

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                                                    A rigid monoidal category is a monoidal category which is left rigid and right rigid.

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