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Mathlib.AlgebraicTopology.DoldKan.Compatibility

Tools for compatibilities between Dold-Kan equivalences

The purpose of this file is to introduce tools which will enable the construction of the Dold-Kan equivalence SimplicialObject C ≌ ChainComplex C ℕ for a pseudoabelian category C from the equivalence Karoubi (SimplicialObject C) ≌ Karoubi (ChainComplex C ℕ) and the two equivalences SimplicialObject C ≌ Karoubi (SimplicialObject C) and ChainComplex C ℕ ≌ Karoubi (ChainComplex C ℕ).

It is certainly possible to get an equivalence SimplicialObject C ≌ ChainComplex C ℕ using a composition of the three equivalences above, but then neither the functor nor the inverse would have good definitional properties. For example, it would be better if the inverse functor of the equivalence was exactly the functor Γ₀ : ChainComplex C ℕ ⥤ SimplicialObject C which was constructed in FunctorGamma.lean.

In this file, given four categories A, A', B, B', equivalences eA : A ≌ A', eB : B ≌ B', e' : A' ≌ B', functors F : A ⥤ B', G : B ⥤ A equipped with certain compatibilities, we construct successive equivalences:

When extra assumptions are given, we shall also provide simplification lemmas for the unit and counit isomorphisms of equivalence.

(See Equivalence.lean for the general strategy of proof of the Dold-Kan equivalence.)

A basic equivalence A ≌ B' obtained by composing eA : A ≌ A' and e' : A' ≌ B'.

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    An intermediate equivalence A ≌ B' whose functor is F and whose inverse is e'.inverse ⋙ eA.inverse.

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      The counit isomorphism of the equivalence equivalence₁ between A and B'.

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        The unit isomorphism of the equivalence equivalence₁ between A and B'.

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          An intermediate equivalence A ≌ B obtained as the composition of equivalence₁ and the inverse of eB : B ≌ B'.

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            The counit isomorphism of the equivalence equivalence₂ between A and B.

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              The unit isomorphism of the equivalence equivalence₂ between A and B.

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                The equivalence A ≌ B whose functor is F ⋙ eB.inverse and whose inverse functor is G : B ⥤ A.

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                  The isomorphism eB.functor ⋙ e'.inverse ⋙ e'.functor ≅ eB.functor deduced from the counit isomorphism of e'.

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                    The isomorphism eB.functor ⋙ e'.inverse ⋙ e'.functor ≅ eB.functor deduced from the isomorphisms hF : eA.functor ⋙ e'.functor ≅ F, hG : eB.functor ⋙ e'.inverse ≅ G ⋙ eA.functor and the datum of an isomorphism η : G ⋙ F ≅ eB.functor.

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                      The isomorphism eA.functor ≅ F ⋙ e'.inverse deduced from the unit isomorphism of e' and the isomorphism hF : eA.functor ⋙ e'.functor ≅ F.

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