The Kan extension functor #
Given a functor L : C ⥤ D
, we define the left Kan extension functor
L.lan : (C ⥤ H) ⥤ (D ⥤ H)
which sends a functor F : C ⥤ H
to its
left Kan extension along L
. This is defined if all F
have such
a left Kan extension. It is shown that L.lan
is the left adjoint to
the functor (D ⥤ H) ⥤ (C ⥤ H)
given by the precomposition
with L
(see Functor.lanAdjunction
).
Similarly, we define the right Kan extension functor
L.ran : (C ⥤ H) ⥤ (D ⥤ H)
which sends a functor F : C ⥤ H
to its
right Kan extension along L
.
The left Kan extension functor (C ⥤ H) ⥤ (D ⥤ H)
along a functor C ⥤ D
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The natural transformation F ⟶ L ⋙ (L.lan).obj G
.
Equations
- L.lanUnit = { app := fun (F : CategoryTheory.Functor C H) => L.leftKanExtensionUnit F, naturality := ⋯ }
Instances For
If there exists a pointwise left Kan extension of F
along L
,
then L.lan.obj G
is a pointwise left Kan extension of F
.
Equations
- L.isPointwiseLeftKanExtensionLeftKanExtensionUnit F = CategoryTheory.Functor.isPointwiseLeftKanExtensionOfIsLeftKanExtension (L.leftKanExtension F) (L.leftKanExtensionUnit F)
Instances For
If a left Kan extension is pointwise, then evaluating it at an object is isomorphic to taking a colimit.
Equations
- L.leftKanExtensionObjIsoColimit F X = (L.isPointwiseLeftKanExtensionLeftKanExtensionUnit F X).isoColimit
Instances For
The left Kan extension of F : C ⥤ H
along a functor L : C ⥤ D
is isomorphic to the
fiberwise colimit of the projection functor on the Grothendieck construction of the costructured
arrow category composed with F
.
Equations
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Instances For
The left Kan extension functor L.Lan
is left adjoint to the precomposition by L
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Composing the left Kan extension of L : C ⥤ D
with colim
on shapes D
is isomorphic
to colim
on shapes C
.
Equations
- L.lanCompColimIso = (CategoryTheory.NatIso.ofComponents (fun (G : CategoryTheory.Functor C H) => ((L.lan.obj G).colimitIsoOfIsLeftKanExtension (L.lanUnit.app G)).symm) ⋯).symm
Instances For
If G : C ⥤ H
admits a left Kan extension along a functor L : C ⥤ D
and H
has colimits of
shape C
and D
, then the colimit of G
is isomorphic to the colimit of a canonical functor
Grothendieck (CostructuredArrow.functor L) ⥤ H
induced by L
and G
.
Equations
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Instances For
The right Kan extension functor (C ⥤ H) ⥤ (D ⥤ H)
along a functor C ⥤ D
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The natural transformation L ⋙ (L.lan).obj G ⟶ L
.
Equations
- L.ranCounit = { app := fun (F : CategoryTheory.Functor C H) => L.rightKanExtensionCounit F, naturality := ⋯ }
Instances For
If there exists a pointwise right Kan extension of F
along L
,
then L.ran.obj G
is a pointwise right Kan extension of F
.
Equations
- L.isPointwiseRightKanExtensionRanCounit F = CategoryTheory.Functor.isPointwiseRightKanExtensionOfIsRightKanExtension (L.ran.obj F) (L.ranCounit.app F)
Instances For
If a right Kan extension is pointwise, then evaluating it at an object is isomorphic to taking a limit.
Equations
- L.ranObjObjIsoLimit F X = (L.isPointwiseRightKanExtensionRanCounit F X).isoLimit
Instances For
The right Kan extension functor L.ran
is right adjoint to the
precomposition by L
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Composing the right Kan extension of L : C ⥤ D
with lim
on shapes D
is isomorphic
to lim
on shapes C
.
Equations
- L.ranCompLimIso = CategoryTheory.NatIso.ofComponents (fun (G : CategoryTheory.Functor C H) => (L.ran.obj G).limitIsoOfIsRightKanExtension (L.ranCounit.app G)) ⋯