The monoidal category structures on graded objects #
Assuming that C is a monoidal category and that I is an additive monoid,
we introduce a partially defined tensor product on the category GradedObject I C:
given X₁ and X₂ two objects in GradedObject I C, we define
GradedObject.Monoidal.tensorObj X₁ X₂ under the assumption HasTensor X₁ X₂
that the coproduct of X₁ i ⊗ X₂ j for i + j = n exists for any n : I.
Under suitable assumptions about the existence of coproducts and the
preservation of certain coproducts by the tensor products in C, we
obtain a monoidal category structure on GradedObject I C.
In particular, if C has finite coproducts to which the tensor
product commutes, we obtain a monoidal category structure on GradedObject ℕ C.
The tensor product of two graded objects X₁ and X₂ exists if for any n,
the coproduct of the objects X₁ i ⊗ X₂ j for i + j = n exists.
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The tensor product of two graded objects.
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The inclusion of a summand in a tensor product of two graded objects.
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Constructor for morphisms from a tensor product of two graded objects.
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The morphism tensorObj X₁ Y₁ ⟶ tensorObj X₂ Y₂ induced by morphisms of graded
objects f : X₁ ⟶ X₂ and g : Y₁ ⟶ Y₂.
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The morphism tensorObj X Y₁ ⟶ tensorObj X Y₂ induced by a morphism of graded objects
φ : Y₁ ⟶ Y₂.
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The morphism tensorObj X₁ Y ⟶ tensorObj X₂ Y induced by a morphism of graded objects
φ : X₁ ⟶ X₂.
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The isomorphism tensorObj X₁ Y₁ ≅ tensorObj X₂ Y₂ induced by isomorphisms of graded
objects e : X₁ ≅ X₂ and e' : Y₁ ≅ Y₂.
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This is the addition map I × I × I → I for an additive monoid I.
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Auxiliary definition for associator.
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Auxiliary definition for associator.
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Auxiliary definition for associator.
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Given three graded objects X₁, X₂, X₃ in GradedObject I C, this is the
assumption that for all i₁₂ : I and i₃ : I, the tensor product functor - ⊗ X₃ i₃
commutes with the coproduct of the objects X₁ i₁ ⊗ X₂ i₂ such that i₁ + i₂ = i₁₂.
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Given three graded objects X₁, X₂, X₃ in GradedObject I C, this is the
assumption that for all i₁ : I and i₂₃ : I, the tensor product functor X₁ i₁ ⊗ -
commutes with the coproduct of the objects X₂ i₂ ⊗ X₃ i₃ such that i₂ + i₃ = i₂₃.
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The inclusion X₁ i₁ ⊗ X₂ i₂ ⊗ X₃ i₃ ⟶ tensorObj X₁ (tensorObj X₂ X₃) j
when i₁ + i₂ + i₃ = j.
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The inclusion X₁ i₁ ⊗ X₂ i₂ ⊗ X₃ i₃ ⟶ tensorObj (tensorObj X₁ X₂) X₃ j
when i₁ + i₂ + i₃ = j.
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The associator isomorphism for graded objects.
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Given Z : C and three graded objects X₁, X₂ and X₃ in GradedObject I C,
this typeclass expresses that functor Z ⊗ _ commutes with the coproduct of
the objects X₁ i₁ ⊗ (X₂ i₂ ⊗ X₃ i₃) such that i₁ + i₂ + i₃ = j for a certain j.
See lemma left_tensor_tensorObj₃_ext.
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The inclusion
X₁ i₁ ⊗ X₂ i₂ ⊗ X₃ i₃ ⊗ X₄ i₄ ⟶ tensorObj X₁ (tensorObj X₂ (tensorObj X₃ X₄)) j
when i₁ + i₂ + i₃ + i₄ = j.
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Given four graded objects, this is the condition
HasLeftTensor₃ObjExt (X₁ i₁) X₂ X₃ X₄ i₂₃₄ for all indices i₁ and i₂₃₄,
see the lemma tensorObj₄_ext.
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The unit of the tensor product on graded objects is (single₀ I).obj (𝟙_ C).
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The canonical isomorphism tensorUnit 0 ≅ 𝟙_ C
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tensorUnit i is an initial object when i ≠ 0.
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The left unitor isomorphism for graded objects.
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The right unitor isomorphism for graded objects.
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The monoidal category structure on GradedObject ℕ C can be inferred
from the assumptions [HasFiniteCoproducts C],
[∀ (X : C), PreservesFiniteCoproducts ((curriedTensor C).obj X)] and
[∀ (X : C), PreservesFiniteCoproducts ((curriedTensor C).flip.obj X)].
This requires importing Mathlib/CategoryTheory/Limits/Preserves/Finite.lean.