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GroupoidModel.Groupoids.Sigma

@[reducible]

For a point x : Γ, (sigma A B).obj x is the groupoidal Grothendieck construction on the composition ι _ x ⋙ B : A.obj x ⥤ Groupoidal A ⥤ Grpd

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    For a morphism f : x ⟶ y in Γ, (sigma A B).map y is a composition of functors. The first functor map (whiskerRight (ιNatTrans f) B) is an equivalence which replaces ι A x with the naturally isomorphic A.map f ⋙ ι A y. The second functor is the action of precomposing A.map f with ι A y ⋙ B on the Grothendieck constructions.

            map ⋯                  pre ⋯
    

    ∫ ι A x ⋙ B ⥤ ∫ A.map f ⋙ ι A y ⋙ B ⥤ ∫ ι A y ⋙ B

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      The formation rule for Σ-types for the ambient natural model base unfolded into operations between functors. See sigmaObj and sigmaMap for the actions of this functor.

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        The left hand side mapPairSectionObjFiber h f is an object in the fiber sigma A B y over y The fiber itself consists of bundles, so (mapPairSectionObjFiber h f).fiber is an object in the fiber B a for an a in the fiber A y. But this a is isomorphic to (pairSectionObjFiber y).base and the functor (ι _ y ⋙ B).map (mapPoint α f) converts the data along this isomorphism.

        The right hand side is (*) in the diagram. sec α B Γ -------> ∫(A) ------------> Grpd

        x (B ⋙ sec α).obj x objPt' h x | f (B ⋙ sec α).map f | - V V | y (B ⋙ sec α).obj y V (*)

        This can be thought of as the action of parallel transport on f or perhaps the path over f, but defined within the fiber over y

        sigma A B x ∋ pairObjFiber h x
        sigma A B f
        V V
        sigma A B y ∋ PairMapFiber
                               _ ⟶ pairObjFiber h y
        
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            Let Γ be a category. For any pair of functors A : Γ ⥤ Grpd and B : ∫(A) ⥤ Grpd, and any "groupoid-term", meaning a functor αβ : Γ ⥤ PGrpd satisfying αβ ⋙ forgetToGrpd = sigma A B : Γ ⥤ Grpd, there is a groupoid-term fst' : Γ ⥤ PGrpd such that fst ⋙ forgetToGrpd = A, thought of as fst' : A. There is a "groupoid-type" dependent' : ∫(fst ⋙ forgetToGrpd) ⥤ Grpd, which is hequal to B modulo fst ⋙ forgetToGrpd being equal to A. And there is a groupoid-term snd' : Γ ⥤ PGrpd satisfying snd' ⋙ forgetToGrpd = sec _ fst rfldependent'.

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              Let Γ be a category. For any pair of functors A : Γ ⥤ Grpd and B : ∫(A) ⥤ Grpd, and any "groupoid-term", meaning a functor αβ : Γ ⥤ PGrpd satisfying αβ ⋙ forgetToGrpd = sigma A B : Γ ⥤ Grpd, there is a groupoid-term fst' : Γ ⥤ PGrpd such that fst ⋙ forgetToGrpd = A, thought of as fst' : A. There is a "groupoid-type" dependent' : ∫(fst ⋙ forgetToGrpd) ⥤ Grpd, which is hequal to B modulo fst ⋙ forgetToGrpd being equal to A. And there is a groupoid-term snd' : Γ ⥤ PGrpd satisfying snd' ⋙ forgetToGrpd = sec _ fst rfldependent'.

              Let Γ be a category. For any pair of functors A : Γ ⥤ Grpd and B : ∫(A) ⥤ Grpd, and any "groupoid-term", meaning a functor αβ : Γ ⥤ PGrpd satisfying αβ ⋙ forgetToGrpd = sigma A B : Γ ⥤ Grpd, there is a groupoid-term fst' : Γ ⥤ PGrpd such that fst ⋙ forgetToGrpd = A, thought of as fst' : A. There is a "groupoid-type" dependent' : ∫(fst ⋙ forgetToGrpd) ⥤ Grpd, which is hequal to B modulo fst ⋙ forgetToGrpd being equal to A. And there is a groupoid-term snd' : Γ ⥤ PGrpd satisfying snd' ⋙ forgetToGrpd = sec _ fst rfldependent'.

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                Let Γ be a category. For any pair of functors A : Γ ⥤ Grpd and B : ∫(A) ⥤ Grpd, and any "groupoid-term", meaning a functor αβ : Γ ⥤ PGrpd satisfying αβ ⋙ forgetToGrpd = sigma A B : Γ ⥤ Grpd, there is a groupoid-term fst' : Γ ⥤ PGrpd such that fst ⋙ forgetToGrpd = A, thought of as fst' : A. There is a "groupoid-type" dependent' : ∫(fst ⋙ forgetToGrpd) ⥤ Grpd, which is hequal to B modulo fst ⋙ forgetToGrpd being equal to A. And there is a groupoid-term snd' : Γ ⥤ PGrpd satisfying snd' ⋙ forgetToGrpd = sec _ fst rfldependent'.

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                  Behavior of the Σ-type former (a natural transformation) on an input. By Yoneda, "an input" is the same as a map from a representable into the domain.

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                    The formation rule for Σ-types for the ambient natural model base If possible, don't use NatTrans.app on this, instead precompose it with maps from representables.

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