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Mathlib.CategoryTheory.Monoidal.Braided.Multifunctor

Constructing braided categories from natural transformations between multifunctors #

This file provides an alternative constructor for braided categories, given a braiding β : -₁ ⊗ -₂ ≅ -₂ ⊗ -₁ as a natural isomorphism between bifunctors. The hexagon identities are phrased as equalities of natural transformations between trifunctors (-₁ ⊗ -₂) ⊗ -₃ ⟶ -₂ ⊗ (-₃ ⊗ -₁) and -₁ ⊗ (-₂ ⊗ -₃) ⟶ (-₃ ⊗ -₁) ⊗ -₂.

The trifunctor X₁ X₂ X₃ ↦ (X₁ ⊗ X₂) ⊗ X₃

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      The trifunctor X₁ X₂ X₃ ↦ X₁ ⊗ (X₂ ⊗ X₃)

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          The trifunctor X₁ X₂ X₃ ↦ (X₂ ⊗ X₃) ⊗ X₁

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              The trifunctor X₁ X₂ X₃ ↦ X₂ ⊗ (X₃ ⊗ X₁)

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                  The trifunctor X₁ X₂ X₃ ↦ (X₂ ⊗ X₁) ⊗ X₃

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                      The trifunctor X₁ X₂ X₃ ↦ X₂ ⊗ (X₁ ⊗ X₃)

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                          The trifunctor X₁ X₂ X₃ ↦ X₃ ⊗ (X₁ ⊗ X₂)

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                              The trifunctor X₁ X₂ X₃ ↦ (X₃ ⊗ X₁) ⊗ X₂

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                                  The trifunctor X₁ X₂ X₃ ↦ X₁ ⊗ (X₃ ⊗ X₂)

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                                      The trifunctor X₁ X₂ X₃ ↦ (X₁ ⊗ X₃) ⊗ X₂

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                                          The forward hexagon identity #

                                          Given a braiding in the form of a natural isomorphism of bifunctors β : curriedTensor C ≅ (curriedTensor C).flip (i.e. (β.app X₁).app X₂ : X₁ ⊗ X₂ ≅ X₂ ⊗ X₁), we phrase the forward hexagon identity as an equality of natural transformations between trifunctors (the hexagon on the left is the diagram we require to commute, the hexagon on the right is the same on the object level on three objects X₁ X₂ X₃).

                                                      functor₁₂₃                        (X₁ ⊗ X₂) ⊗ X₃
                                          associator /          \ secondMap₁             /           \
                                                    v            v                      v             v
                                               functor₁₂₃'    functor₂₁₃          X₁ ⊗ (X₂ ⊗ X₃)    (X₂ ⊗ X₁) ⊗ X₃
                                          firstMap₂ |            |secondMap₂            |             |
                                                    v            v                      v             v
                                               functor₂₃₁     functor₂₁₃'         (X₂ ⊗ X₃) ⊗ X₁    X₂ ⊗ (X₁ ⊗ X₃)
                                            firstMap₃\           / secondMap₃            \            /
                                                      v         v                         v          v
                                                       functor₂₃₁'                        X₂ ⊗ (X₃ ⊗ X₁)
                                          

                                          The reverse hexagon identity #

                                          Given a braiding in the form of a natural isomorphism of bifunctors β : curriedTensor C ≅ (curriedTensor C).flip (i.e. (β.app X₁).app X₂ : X₁ ⊗ X₂ ≅ X₂ ⊗ X₁), we phrase the reverse hexagon identity as an equality of natural transformations between trifunctors (the hexagon on the left is the diagram we require to commute, the hexagon on the right is the same on the object level on three objects X₁ X₂ X₃).

                                                      functor₁₂₃'                       X₁ ⊗ (X₂ ⊗ X₃)
                                          associator /          \ secondMap₁             /           \
                                                    v            v                      v             v
                                               functor₁₂₃    functor₁₃₂'          (X₁ ⊗ X₂) ⊗ X₃    X₁ ⊗ (X₃ ⊗ X₂)
                                          firstMap₂ |            |secondMap₂            |             |
                                                    v            v                      v             v
                                               functor₃₁₂'    functor₁₃₂          X₃ ⊗ (X₁ ⊗ X₂)    (X₁ ⊗ X₃) ⊗ X₂
                                            firstMap₃\           / secondMap₃            \            /
                                                      v         v                         v          v
                                                       functor₃₁₂                        (X₃ ⊗ X₁) ⊗ X₂
                                          

                                          Alternative constructor for symmetric categories, where the symmetry of the braiding is phrased as an equality of natural transformation of bifunctors.

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