LeanPool.WhiteheadTheorem.HEP.Cofibration #
Imported Lean Pool material for LeanPool.WhiteheadTheorem.HEP.Cofibration.
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The map i : A ⟶ X is said to have
the "curried HomotopyExtensionProperty" with respect to Y,
if the commutative square
A ---h---> C(I, Y)
| |
i eval₀
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↓ ↓
X ---f---> Y
has a lift H : X → C(I, Y).
- hasLift : CategoryTheory.HasLiftingProperty i (TopCat.PathSpace.eval₀ Y)
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The cocone has a natural transformation from Functor.ofSequence i
to the constant functor at Z. Here we define the first component,
NatTrans.app, of this natural transformation.
(The second component NatTrans.naturality will be obtained by NatTrans.ofSequence.)
X n ------- app n -----> Z
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i n p
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v v
X (n+1) -----> X ---f---> Y
ι (n+1)
Each app gives rise to the next commutative square,
and is a lift for the previous square.
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ofSequenceOfHasLiftingProperty
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Postcompose a cocone cc with a morphism cc.pt ⟶ Y,
giving a cocone whose point is Y. (Does mathlib have this?)
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- cc.postcompose p = { pt := Y', ι := { app := fun (j : J) => CategoryTheory.CategoryStruct.comp (cc.ι.app j) p, naturality := ⋯ } }
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- Functor.ofSequence.coconeDropFirst i cc = { pt := cc.pt, ι := CategoryTheory.NatTrans.ofSequence (fun (n : ℕ) => cc.ι.app (n + 1)) ⋯ }
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Undrop (recover) the first morphism of
Limits.colimit.cocone (Functor.ofSequence fun n ↦ i (n + 1))
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If
A ---f---> B
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i j
| p.o. |
v v
X ---F---> Y
is a pushout square and the left side i has the homotopy extension property,
then the right side j has the homotopy extension property.
- hasCurriedHEP (Y : TopCat) : HasCurriedHEP i Y
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curriedArgSwap
C(I, C(I, Y)) --------------------> C(I, C(I, Y))
| ≃ |
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(exp' I).map (PathSpace.eval₀ Y) PathSpace.eval₀ (TopCat.of C(I, Y))
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v v
C(I, Y) ========================= C(I, Y)
If A ⟶ X is a cofibration, then TopCat.of (A × I) ⟶ TopCat.of (X × I) is a cofibration.
A × I --------> C(I, Y)
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i × id (PathSpace.eval₀ Y)
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v v
X × I ----------> Y
A ---f----> C(I, C(I, Y)) ----curriedArgSwap---> C(I, C(I, Y))
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i (exp' I).map (PathSpace.eval₀ Y) PathSpace.eval₀ (TopCat.of C(I, Y))
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v v v
X ----g----> C(I, Y) =========================== C(I, Y)