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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- GroupoidModel.FunctorOperation.sigma.fstAux B = { app := fun (x : Γ) => CategoryTheory.Grpd.homOf CategoryTheory.Grothendieck.Groupoidal.forget, naturality := ⋯ }
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fst
projects out the pointed groupoid (A,a)
appearing in (A,B,a : A,b : B a)
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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 rfl ⋙ dependent'
.
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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 rfl ⋙ dependent'
.
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 rfl ⋙ dependent'
.
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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 rfl ⋙ dependent'
.
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fst
projects out the pointed groupoid (A,a)
appearing in (A,B,a : A,b : B a)
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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- GroupoidModel.smallUSigma = { Sig := GroupoidModel.smallUSig, pair := GroupoidModel.smallUPair, Sig_pullback := GroupoidModel.smallU_pb }
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- GroupoidModel.uHomSeqSigmas' 0 ilen_2 = GroupoidModel.smallUSigma
- GroupoidModel.uHomSeqSigmas' 1 ilen_2 = GroupoidModel.smallUSigma
- GroupoidModel.uHomSeqSigmas' 2 ilen_2 = GroupoidModel.smallUSigma
- GroupoidModel.uHomSeqSigmas' 3 ilen_2 = GroupoidModel.smallUSigma
- GroupoidModel.uHomSeqSigmas' n.succ.succ.succ.succ ilen_2 = ⋯.elim