Functors which preserve sheafification #
In this file, given a Grothendieck topology J
on C
and F : A ⥤ B
,
we define a type class J.PreservesSheafification F
. We say that F
preserves
the sheafification if whenever a morphism of presheaves P₁ ⟶ P₂
induces
an isomorphism on the associated sheaves, then the induced map P₁ ⋙ F ⟶ P₂ ⋙ F
also induces an isomorphism on the associated sheaves. (Note: it suffices to check
this property for the map from any presheaf P
to its associated sheaf, see
GrothendieckTopology.preservesSheafification_iff_of_adjunctions
).
In general, we define Sheaf.composeAndSheafify J F : Sheaf J A ⥤ Sheaf J B
as the functor
which sends a sheaf G
to the sheafification of the composition G.val ⋙ F
.
If J.PreservesSheafification F
, we show that this functor can also be thought of
as the localization of the functor _ ⋙ F
on presheaves: we construct an isomorphism
presheafToSheafCompComposeAndSheafifyIso
between
presheafToSheaf J A ⋙ Sheaf.composeAndSheafify J F
and
(whiskeringRight Cᵒᵖ A B).obj F ⋙ presheafToSheaf J B
.
Moreover, if we assume J.HasSheafCompose F
, we obtain an isomorphism
sheafifyComposeIso J F P : sheafify J (P ⋙ F) ≅ sheafify J P ⋙ F
.
We show that under suitable assumptions, the forgetful functor from a concrete category preserves sheafification; this holds more generally for functors between such concrete categories which commute both with suitable limits and colimits.
TODO #
- construct an isomorphism
Sheaf.composeAndSheafify J F ≅ sheafCompose J F
A functor F : A ⥤ B
preserves the sheafification for the Grothendieck
topology J
on a category C
if whenever a morphism of presheaves f : P₁ ⟶ P₂
in Cᵒᵖ ⥤ A
is such that becomes an iso after sheafification, then it is
also the case of whiskerRight f F : P₁ ⋙ F ⟶ P₂ ⋙ F
.
- le : J.W ≤ J.W.inverseImage ((CategoryTheory.whiskeringRight Cᵒᵖ A B).obj F)
Instances
This is the functor sending a sheaf X : Sheaf J A
to the sheafification
of X.val ⋙ F
.
Equations
- CategoryTheory.Sheaf.composeAndSheafify J F = (CategoryTheory.sheafToPresheaf J A).comp (((CategoryTheory.whiskeringRight Cᵒᵖ A B).obj F).comp (CategoryTheory.presheafToSheaf J B))
Instances For
The canonical natural transformation from
(whiskeringRight Cᵒᵖ A B).obj F ⋙ presheafToSheaf J B
to
presheafToSheaf J A ⋙ Sheaf.composeAndSheafify J F
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The canonical isomorphism between presheafToSheaf J A ⋙ Sheaf.composeAndSheafify J F
and (whiskeringRight Cᵒᵖ A B).obj F ⋙ presheafToSheaf J B
when F : A ⥤ B
preserves sheafification.
Equations
Instances For
The canonical natural transformation
(whiskeringRight Cᵒᵖ A B).obj F ⋙ G₂ ⟶ G₁ ⋙ sheafCompose J F
when F : A ⥤ B
is such that J.HasSheafCompose F
, and that G₁
and G₂
are
left adjoints to the forget functors sheafToPresheaf
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The canonical natural isomorphism
(whiskeringRight Cᵒᵖ A B).obj F ⋙ G₂ ≅ G₁ ⋙ sheafCompose J F
when F : A ⥤ B
preserves sheafification, and that G₁
and G₂
are
left adjoints to the forget functors sheafToPresheaf
.
Equations
- CategoryTheory.sheafComposeNatIso J F adj₁ adj₂ = CategoryTheory.asIso (CategoryTheory.sheafComposeNatTrans J F adj₁ adj₂)
Instances For
The canonical isomorphism sheafify J (P ⋙ F) ≅ sheafify J P ⋙ F
when
F
preserves the sheafification.
Equations
- One or more equations did not get rendered due to their size.