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
).
funtor₁₂₃ (X₁ ⊗ X₂) ⊗ X₃
associtator/ \ secondMap₁ / \
v v v v
functor₁₂₃' functor₂₁₃ X₁ ⊗ (X₂ ⊗ X₃) (X₂ ⊗ X₁) ⊗ X₃
firsMap₂ | |secondMap₂ | |
v v v v
functor₂₃₁ functor₂₁₃' (X₂ ⊗ X₃) ⊗ X₁ X₂ ⊗ (X₁ ⊗ X₃)
firstMap₃\ / secondMap₃ \ /
v v v v
functor₂₃₁' X₂ ⊗ (X₃ ⊗ X₁)
The middle left map in the forward hexagon identity.
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The bottom left map in the forward hexagon identity.
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The top right map in the forward hexagon identity.
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The middle right map in the forward hexagon identity.
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The bottom right map in the forward hexagon identity.
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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
).
funtor₁₂₃' X₁ ⊗ (X₂ ⊗ X₃)
associtator/ \ secondMap₁ / \
v v v v
functor₁₂₃ functor₁₃₂' (X₁ ⊗ X₂) ⊗ X₃ X₁ ⊗ (X₃ ⊗ X₂)
firsMap₂ | |secondMap₂ | |
v v v v
functor₃₁₂' functor₁₃₂ X₃ ⊗ (X₁ ⊗ X₂) (X₁ ⊗ X₃) ⊗ X₂
firstMap₃\ / secondMap₃ \ /
v v v v
functor₃₁₂ (X₃ ⊗ X₁) ⊗ X₂
The middle left map in the reverse hexagon identity.
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The bottom left map in the reverse hexagon identity.
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The top right map in the reverse hexagon identity.
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The middle right map in the reverse hexagon identity.
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The bottom right map in the reverse hexagon identity.
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Given a braiding β : curriedTensor C ≅ (curriedTensor C).flip
as a natural isomorphism between
bifunctors, and the two equalities hexagon_forward
and hexagon_reverse
of natural
transformations between trifunctors, we obtain a braided category structure.
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