The homotopy cofiber of a morphism of homological complexes
In this file, we construct the homotopy cofiber of a morphism φ : F ⟶ G
between homological complexes in HomologicalComplex C c. In degree i,
it is isomorphic to (F.X j) ⊞ (G.X i) if there is a j such that c.Rel i j,
and G.X i otherwise. (This is also known as the mapping cone of φ. Under
the name CochainComplex.mappingCone, a specific API shall be developed
for the case of cochain complexes indexed by ℤ.)
When we assume hc : ∀ j, ∃ i, c.Rel i j (which holds in the case of chain complexes,
or cochain complexes indexed by ℤ), then for any homological complex K,
there is a bijection HomologicalComplex.homotopyCofiber.descEquiv φ K hc
between homotopyCofiber φ ⟶ K and the tuples (α, hα) with
α : G ⟶ K and hα : Homotopy (φ ≫ α) 0.
We shall also study the cylinder of a homological complex K: this is the
homotopy cofiber of the morphism biprod.lift (𝟙 K) (-𝟙 K) : K ⟶ K ⊞ K.
Then, a morphism K.cylinder ⟶ M is determined by the data of two
morphisms φ₀ φ₁ : K ⟶ M and a homotopy h : Homotopy φ₀ φ₁,
see cylinder.desc. There is also a homotopy equivalence
cylinder.homotopyEquiv K : HomotopyEquiv K.cylinder K. From the construction of
the cylinder, we deduce the lemma Homotopy.map_eq_of_inverts_homotopyEquivalences
which assert that if a functor inverts homotopy equivalences, then the image of
two homotopic maps are equal.
A morphism of homological complexes φ : F ⟶ G has a homotopy cofiber if for all
indices i and j such that c.Rel i j, the binary biproduct F.X j ⊞ G.X i exists.
- hasBinaryBiproduct (i j : ι) (hij : c.Rel i j) : CategoryTheory.Limits.HasBinaryBiproduct (F.X j) (G.X i)
Instances
The X field of the homological complex homotopyCofiber φ.
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The canonical isomorphism (homotopyCofiber φ).X i ≅ F.X j ⊞ G.X i when c.Rel i j.
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The canonical isomorphism (homotopyCofiber φ).X i ≅ G.X i when ¬ c.Rel i (c.next i).
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The second projection (homotopyCofiber φ).X i ⟶ G.X i.
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The right inclusion G.X i ⟶ (homotopyCofiber φ).X i.
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The first projection (homotopyCofiber φ).X i ⟶ F.X j when c.Rel i j.
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The left inclusion F.X i ⟶ (homotopyCofiber φ).X j when c.Rel j i.
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The d field of the homological complex homotopyCofiber φ.
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The homotopy cofiber of a morphism of homological complexes, also known as the mapping cone.
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The right inclusion G ⟶ homotopyCofiber φ.
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- HomologicalComplex.homotopyCofiber.inr φ = { f := fun (i : ι) => HomologicalComplex.homotopyCofiber.inrX φ i, comm' := ⋯ }
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The composition φ ≫ mappingCone.inr φ is homotopic to 0.
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- HomologicalComplex.homotopyCofiber.inrCompHomotopy φ hc = { hom := fun (i j : ι) => if hij : c.Rel j i then HomologicalComplex.homotopyCofiber.inlX φ i j hij else 0, zero := ⋯, comm := ⋯ }
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The morphism homotopyCofiber φ ⟶ K that is induced by a morphism α : G ⟶ K
and a homotopy hα : Homotopy (φ ≫ α) 0.
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Morphisms homotopyCofiber φ ⟶ K are uniquely determined by
a morphism α : G ⟶ K and a homotopy from φ ≫ α to 0.
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The cylinder object of a homological complex K is the homotopy cofiber
of the morphism biprod.lift (𝟙 K) (-𝟙 K) : K ⟶ K ⊞ K.
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The left inclusion K ⟶ K.cylinder.
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The right inclusion K ⟶ K.cylinder.
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The morphism K.cylinder ⟶ F that is induced by two morphisms φ₀ φ₁ : K ⟶ F
and a homotopy h : Homotopy φ₀ φ₁.
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The projection π : K.cylinder ⟶ K.
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The left inclusion K.X i ⟶ K.cylinder.X j when c.Rel j i.
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The right inclusion (K ⊞ K).X i ⟶ K.cylinder.X i.
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A null homotopic map K.cylinder ⟶ K.cylinder which identifies to
π K ≫ ι₀ K - 𝟙 _, see nullHomotopicMap_eq.
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The obvious homotopy from nullHomotopicMap K to zero.
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The homotopy between π K ≫ ι₀ K and 𝟙 K.cylinder.
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The homotopy equivalence between K.cylinder and K.
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The homotopy between cylinder.ι₀ K and cylinder.ι₁ K.
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If a functor inverts homotopy equivalences, it sends homotopic maps to the same map.