Lifting properties and parametrized adjunctions #
Let F : C₁ ⥤ C₂ ⥤ C₃. Given morphisms f₁ : X₁ ⟶ Y₁ in C₁
and f₂ : X₂ ⟶ Y₂ in C₂, we introduce a structure
F.PushoutObjObj f₁ f₂ which contains the data of a
pushout of (F.obj Y₁).obj X₂ and (F.obj X₁).obj Y₂
along (F.obj X₁).obj X₂. If sq₁₂ : F.PushoutObjObj f₁ f₂,
we have a canonical "inclusion" sq₁₂.ι : sq₁₂.pt ⟶ (F.obj Y₁).obj Y₂.
Similarly, if we have a bifunctor G : C₁ᵒᵖ ⥤ C₃ ⥤ C₂, and
morphisms f₁ : X₁ ⟶ Y₁ in C₁ and f₃ : X₃ ⟶ Y₃ in C₃,
we introduce a structure F.PullbackObjObj f₁ f₃ which
contains the data of a pullback of (G.obj (op X₁)).obj X₃
and (G.obj (op Y₁)).obj Y₃ over (G.obj (op X₁)).obj Y₃.
If sq₁₃ : F.PullbackObjObj f₁ f₃, we have a canonical
projection sq₁₃.π : (G.obj Y₁).obj X₃ ⟶ sq₁₃.pt.
Now, if we have a parametrized adjunction adj₂ : F ⊣₂ G,
sq₁₂ : F.PushoutObjObj f₁ f₂ and sq₁₃ : G.PullbackObjObj f₁ f₃,
we show that sq₁₂.ι has the left lifting property with respect to
f₃ if and only if f₂ has the left lifting property with respect
to sq₁₃.π: this is the lemma ParametrizedAdjunction.hasLiftingProperty_iff.
Given a bifunctor F : C₁ ⥤ C₂ ⥤ C₃, and morphisms f₁ : X₁ ⟶ Y₁ in C₁
and f₂ : X₂ ⟶ Y₂ in C₂, this structure contains the data of
a pushout of (F.obj Y₁).obj X₂ and (F.obj X₁).obj Y₂
along (F.obj X₁).obj X₂.
- pt : C₃
the pushout
the first inclusion
the second inclusion
Instances For
The PushoutObjObj structure given by the pushout of the colimits API.
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Instances For
The "inclusion" sq.pt ⟶ (F.obj Y₁).obj Y₂ when
sq : F.PushoutObjObj f₁ f₂.
Instances For
Given sq : F.PushoutObjObj f₁ f₂, flipping the pushout square gives
sq.flip : F.flip.PushoutObjObj f₂ f₁.
Instances For
Given a bifunctor G : C₁ᵒᵖ ⥤ C₃ ⥤ C₂, and morphisms f₁ : X₁ ⟶ Y₁ in C₁
and f₃ : X₃ ⟶ Y₃ in C₃, this structure contains the data of
a pullback of (G.obj (op X₁)).obj X₃
and (G.obj (op Y₁)).obj Y₃ over (G.obj (op X₁)).obj Y₃.
- pt : C₂
the pullback
the first projection
the second projection
- isPullback : IsPullback self.fst self.snd ((G.obj (Opposite.op X₁)).map f₃) ((G.map f₁.op).app Y₃)
Instances For
The PullbackObjObj structure given by the pullback of the limits API.
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Instances For
The projection (G.obj (op Y₁)).obj X₃ ⟶ sq.pt when
sq : G.PullbackObjObj f₁ f₃.
Instances For
Given a parametrized adjunction F ⊣₂ G between bifunctors, and structures
sq₁₂ : F.PushoutObjObj f₁ f₂ and sq₁₃ : G.PullbackObjObj f₁ f₃, there are
as many commutative squares with left map sq₁₂.ι and right map f₃
as commutative squares with left map f₂ and right map sq₁₃.π.
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Instances For
Given a parametrized adjunction F ⊣₂ G between bifunctors, structures
sq₁₂ : F.PushoutObjObj f₁ f₂ and sq₁₃ : G.PullbackObjObj f₁ f₃,
there are as many liftings for the commutative square given by a
map α : Arrow.mk sq₁₂.ι ⟶ Arrow.mk f₃ as there are liftings
for the square given by the corresponding map Arrow.mk f₂ ⟶ Arrow.mk sq₁₃.π.
Equations
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