Cartesian products of categories #
We define the category instance on C × D when C and D are categories.
We define:
sectl C Z: the functorC ⥤ C × Dgiven byX ↦ ⟨X, Z⟩sectr Z D: the functorD ⥤ C × Dgiven byY ↦ ⟨Z, Y⟩fst: the functor⟨X, Y⟩ ↦ Xsnd: the functor⟨X, Y⟩ ↦ Yswap: the functorC × D ⥤ D × Cgiven by⟨X, Y⟩ ↦ ⟨Y, X⟩(and the fact this is an equivalence)
We further define evaluation : C ⥤ (C ⥤ D) ⥤ D and evaluationUncurried : C × (C ⥤ D) ⥤ D,
and products of functors and natural transformations, written F.prod G and α.prod β.
prod C D gives the cartesian product of two categories.
See
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- CategoryTheory.prod C D = CategoryTheory.Category.mk ⋯ ⋯ ⋯
Two rfl lemmas that cannot be generated by @[simps].
The isomorphism between (X.1, X.2) and X.
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Construct an isomorphism in C × D out of two isomorphisms in C and D.
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- CategoryTheory.Iso.prod f g = { hom := (f.hom, g.hom), inv := (f.inv, g.inv), hom_inv_id := ⋯, inv_hom_id := ⋯ }
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Category.uniformProd C D is an additional instance specialised so both factors have the same
universe levels. This helps typeclass resolution.
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sectl C Z is the functor C ⥤ C × D given by X ↦ (X, Z).
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sectr Z D is the functor D ⥤ C × D given by Y ↦ (Z, Y) .
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fst is the functor (X, Y) ↦ X.
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- CategoryTheory.Prod.fst C D = { toPrefunctor := { obj := fun (X : C × D) => X.1, map := fun {X Y : C × D} (f : X ⟶ Y) => f.1 }, map_id := ⋯, map_comp := ⋯ }
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snd is the functor (X, Y) ↦ Y.
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- CategoryTheory.Prod.snd C D = { toPrefunctor := { obj := fun (X : C × D) => X.2, map := fun {X Y : C × D} (f : X ⟶ Y) => f.2 }, map_id := ⋯, map_comp := ⋯ }
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The functor swapping the factors of a cartesian product of categories, C × D ⥤ D × C.
Equations
- CategoryTheory.Prod.swap C D = { toPrefunctor := { obj := fun (X : C × D) => (X.2, X.1), map := fun {X Y : C × D} (f : X ⟶ Y) => (f.2, f.1) }, map_id := ⋯, map_comp := ⋯ }
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Swapping the factors of a cartesian product of categories twice is naturally isomorphic to the identity functor.
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The equivalence, given by swapping factors, between C × D and D × C.
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- CategoryTheory.Prod.swapIsEquivalence C D = inferInstance
The "evaluation at X" functor, such that
(evaluation.obj X).obj F = F.obj X,
which is functorial in both X and F.
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The "evaluation of F at X" functor,
as a functor C × (C ⥤ D) ⥤ D.
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The constant functor followed by the evaluation functor is just the identity.
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The cartesian product of two functors.
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Similar to prod, but both functors start from the same category A
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The product F.prod' G followed by projection on the first component is isomorphic to F
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The product F.prod' G followed by projection on the second component is isomorphic to G
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The diagonal functor.
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The cartesian product of two natural transformations.
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- CategoryTheory.NatTrans.prod α β = { app := fun (X : A × C) => (α.app X.1, β.app X.2), naturality := ⋯ }
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The cartesian product of two natural isomorphisms.
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- CategoryTheory.NatIso.prod e₁ e₂ = { hom := CategoryTheory.NatTrans.prod e₁.hom e₂.hom, inv := CategoryTheory.NatTrans.prod e₁.inv e₂.inv, hom_inv_id := ⋯, inv_hom_id := ⋯ }
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The cartesian product of two equivalences of categories.
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F.flip composed with evaluation is the same as evaluating F.
Equations
- CategoryTheory.flipCompEvaluation F a = CategoryTheory.NatIso.ofComponents (fun (b : B) => CategoryTheory.eqToIso ⋯) ⋯
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The forward direction for functorProdFunctorEquiv
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The backward direction for functorProdFunctorEquiv
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The unit isomorphism for functorProdFunctorEquiv
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The counit isomorphism for functorProdFunctorEquiv
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The equivalence of categories between (A ⥤ B) × (A ⥤ C) and A ⥤ (B × C)
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The equivalence between the opposite of a product and the product of the opposites.
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