Cones and cocones #
We define Cone F
, a cone over a functor F
,
and F.cones : Cᵒᵖ ⥤ Type
, the functor associating to X
the cones over F
with cone point X
.
A cone c
is defined by specifying its cone point c.pt
and a natural transformation c.π
from the constant c.pt
valued functor to F
.
We provide c.w f : c.π.app j ≫ F.map f = c.π.app j'
for any f : j ⟶ j'
as a wrapper for c.π.naturality f
avoiding unneeded identity morphisms.
We define c.extend f
, where c : cone F
and f : Y ⟶ c.pt
for some other Y
,
which replaces the cone point by Y
and inserts f
into each of the components of the cone.
Similarly we have c.whisker F
producing a Cone (E ⋙ F)
We define morphisms of cones, and the category of cones.
We define Cone.postcompose α : cone F ⥤ cone G
for α
a natural transformation F ⟶ G
.
And, of course, we dualise all this to cocones as well.
For more results about the category of cones, see cone_category.lean
.
If F : J ⥤ C
then F.cones
is the functor assigning to an object X : C
the
type of natural transformations from the constant functor with value X
to F
.
An object representing this functor is a limit of F
.
Equations
- F.cones = (CategoryTheory.Functor.const J).op.comp (CategoryTheory.yoneda.obj F)
If F : J ⥤ C
then F.cocones
is the functor assigning to an object (X : C)
the type of natural transformations from F
to the constant functor with value X
.
An object corepresenting this functor is a colimit of F
.
Equations
Functorially associated to each functor J ⥤ C
, we have the C
-presheaf consisting of
cones with a given cone point.
Equations
- One or more equations did not get rendered due to their size.
Contravariantly associated to each functor J ⥤ C
, we have the C
-copresheaf consisting of
cocones with a given cocone point.
Equations
- One or more equations did not get rendered due to their size.
A c : Cone F
is:
Example: if J
is a category coming from a poset then the data required to make
a term of type Cone F
is morphisms πⱼ : c.pt ⟶ F j
for all j : J
and,
for all i ≤ j
in J
, morphisms πᵢⱼ : F i ⟶ F j
such that πᵢ ≫ πᵢⱼ = πᵢ
.
Cone F
is equivalent, via cone.equiv
below, to Σ X, F.cones.obj X
.
- pt : C
An object of
C
A natural transformation from the constant functor at
X
toF
Equations
- One or more equations did not get rendered due to their size.
A c : Cocone F
is
For example, if the source J
of F
is a partially ordered set, then to give
c : Cocone F
is to give a collection of morphisms ιⱼ : F j ⟶ c.pt
and, for
all j ≤ k
in J
, morphisms ιⱼₖ : F j ⟶ F k
such that Fⱼₖ ≫ Fₖ = Fⱼ
for all j ≤ k
.
Cocone F
is equivalent, via Cone.equiv
below, to Σ X, F.cocones.obj X
.
- pt : C
An object of
C
A natural transformation from
F
to the constant functor atpt
Equations
- One or more equations did not get rendered due to their size.
The isomorphism between a cone on F
and an element of the functor F.cones
.
Equations
- One or more equations did not get rendered due to their size.
A map to the vertex of a cone naturally induces a cone by composition.
Equations
- One or more equations did not get rendered due to their size.
A map to the vertex of a cone induces a cone by composition.
Equations
- c.extend f = { pt := X, π := c.extensions.app (Opposite.op X) { down := f } }
Whisker a cone by precomposition of a functor.
Equations
- CategoryTheory.Limits.Cone.whisker E c = { pt := c.pt, π := E.whiskerLeft c.π }
The isomorphism between a cocone on F
and an element of the functor F.cocones
.
Equations
- One or more equations did not get rendered due to their size.
A map from the vertex of a cocone naturally induces a cocone by composition.
Equations
- One or more equations did not get rendered due to their size.
A map from the vertex of a cocone induces a cocone by composition.
Whisker a cocone by precomposition of a functor. See whiskering
for a functorial
version.
Equations
- CategoryTheory.Limits.Cocone.whisker E c = { pt := c.pt, ι := E.whiskerLeft c.ι }
A cone morphism between two cones for the same diagram is a morphism of the cone points which commutes with the cone legs.
A morphism between the two vertex objects of the cones
The triangle consisting of the two natural transformations and
hom
commutes
Equations
- CategoryTheory.Limits.inhabitedConeMorphism A = { default := { hom := CategoryTheory.CategoryStruct.id A.pt, w := ⋯ } }
The category of cones on a given diagram.
Equations
- One or more equations did not get rendered due to their size.
To give an isomorphism between cones, it suffices to give an isomorphism between their vertices which commutes with the cone maps.
Eta rule for cones.
Given a cone morphism whose object part is an isomorphism, produce an isomorphism of cones.
There is a morphism from an extended cone to the original cone.
Equations
- CategoryTheory.Limits.Cones.extend s f = { hom := f, w := ⋯ }
Extending a cone by the identity does nothing.
Extending a cone by a composition is the same as extending the cone twice.
Equations
A cone extended by an isomorphism is isomorphic to the original cone.
Functorially postcompose a cone for F
by a natural transformation F ⟶ G
to give a cone for G
.
Equations
- One or more equations did not get rendered due to their size.
Postcomposing a cone by the composite natural transformation α ≫ β
is the same as
postcomposing by α
and then by β
.
Equations
- One or more equations did not get rendered due to their size.
Postcomposing by the identity does not change the cone up to isomorphism.
Equations
- One or more equations did not get rendered due to their size.
If F
and G
are naturally isomorphic functors, then they have equivalent categories of
cones.
Equations
- One or more equations did not get rendered due to their size.
Whiskering on the left by E : K ⥤ J
gives a functor from Cone F
to Cone (E ⋙ F)
.
Equations
- One or more equations did not get rendered due to their size.
Whiskering by an equivalence gives an equivalence between categories of cones.
Equations
- One or more equations did not get rendered due to their size.
The categories of cones over F
and G
are equivalent if F
and G
are naturally isomorphic
(possibly after changing the indexing category by an equivalence).
Forget the cone structure and obtain just the cone point.
Equations
- One or more equations did not get rendered due to their size.
A functor G : C ⥤ D
sends cones over F
to cones over F ⋙ G
functorially.
Equations
- One or more equations did not get rendered due to their size.
Functoriality is functorial.
Equations
- One or more equations did not get rendered due to their size.
If e : C ≌ D
is an equivalence of categories, then functoriality F e.functor
induces an
equivalence between cones over F
and cones over F ⋙ e.functor
.
Equations
- One or more equations did not get rendered due to their size.
If F
reflects isomorphisms, then Cones.functoriality F
reflects isomorphisms
as well.
A cocone morphism between two cocones for the same diagram is a morphism of the cocone points which commutes with the cocone legs.
A morphism between the (co)vertex objects in
C
The triangle made from the two natural transformations and
hom
commutes
Equations
- CategoryTheory.Limits.inhabitedCoconeMorphism A = { default := { hom := CategoryTheory.CategoryStruct.id A.pt, w := ⋯ } }
Equations
- One or more equations did not get rendered due to their size.
To give an isomorphism between cocones, it suffices to give an isomorphism between their vertices which commutes with the cocone maps.
Eta rule for cocones.
Given a cocone morphism whose object part is an isomorphism, produce an isomorphism of cocones.
There is a morphism from a cocone to its extension.
Equations
- CategoryTheory.Limits.Cocones.extend s f = { hom := f, w := ⋯ }
Extending a cocone by the identity does nothing.
Extending a cocone by a composition is the same as extending the cone twice.
Equations
A cocone extended by an isomorphism is isomorphic to the original cone.
Functorially precompose a cocone for F
by a natural transformation G ⟶ F
to give a cocone
for G
.
Equations
- One or more equations did not get rendered due to their size.
Precomposing a cocone by the composite natural transformation α ≫ β
is the same as
precomposing by β
and then by α
.
Equations
- One or more equations did not get rendered due to their size.
Precomposing by the identity does not change the cocone up to isomorphism.
Equations
- One or more equations did not get rendered due to their size.
If F
and G
are naturally isomorphic functors, then they have equivalent categories of
cocones.
Equations
- One or more equations did not get rendered due to their size.
Whiskering on the left by E : K ⥤ J
gives a functor from Cocone F
to Cocone (E ⋙ F)
.
Equations
- One or more equations did not get rendered due to their size.
Whiskering by an equivalence gives an equivalence between categories of cones.
Equations
- One or more equations did not get rendered due to their size.
The categories of cocones over F
and G
are equivalent if F
and G
are naturally isomorphic
(possibly after changing the indexing category by an equivalence).
Forget the cocone structure and obtain just the cocone point.
Equations
- One or more equations did not get rendered due to their size.
A functor G : C ⥤ D
sends cocones over F
to cocones over F ⋙ G
functorially.
Equations
- One or more equations did not get rendered due to their size.
Functoriality is functorial.
Equations
- One or more equations did not get rendered due to their size.
If e : C ≌ D
is an equivalence of categories, then functoriality F e.functor
induces an
equivalence between cocones over F
and cocones over F ⋙ e.functor
.
Equations
- One or more equations did not get rendered due to their size.
If F
reflects isomorphisms, then Cocones.functoriality F
reflects isomorphisms
as well.
The image of a cone in C under a functor G : C ⥤ D is a cone in D.
Equations
- H.mapCone c = (CategoryTheory.Limits.Cones.functoriality F H).obj c
The construction mapCone
respects functor composition.
Equations
The image of a cocone in C under a functor G : C ⥤ D is a cocone in D.
Equations
- H.mapCocone c = (CategoryTheory.Limits.Cocones.functoriality F H).obj c
The construction mapCocone
respects functor composition.
Equations
Given a cone morphism c ⟶ c'
, construct a cone morphism on the mapped cones functorially.
Equations
- H.mapConeMorphism f = (CategoryTheory.Limits.Cones.functoriality F H).map f
Given a cocone morphism c ⟶ c'
, construct a cocone morphism on the mapped cocones
functorially.
Equations
If H
is an equivalence, we invert H.mapCone
and get a cone for F
from a cone
for F ⋙ H
.
Equations
mapCone
is the left inverse to mapConeInv
.
MapCone
is the right inverse to mapConeInv
.
If H
is an equivalence, we invert H.mapCone
and get a cone for F
from a cone
for F ⋙ H
.
Equations
mapCocone
is the left inverse to mapCoconeInv
.
mapCocone
is the right inverse to mapCoconeInv
.
functoriality F _ ⋙ postcompose (whisker_left F _)
simplifies to functoriality F _
.
Equations
For F : J ⥤ C
, given a cone c : Cone F
, and a natural isomorphism α : H ≅ H'
for functors
H H' : C ⥤ D
, the postcomposition of the cone H.mapCone
using the isomorphism α
is
isomorphic to the cone H'.mapCone
.
mapCone
commutes with postcompose
. In particular, for F : J ⥤ C
, given a cone c : Cone F
, a
natural transformation α : F ⟶ G
and a functor H : C ⥤ D
, we have two obvious ways of producing
a cone over G ⋙ H
, and they are both isomorphic.
Equations
mapCone
commutes with postcomposeEquivalence
functoriality F _ ⋙ precompose (whiskerLeft F _)
simplifies to functoriality F _
.
Equations
For F : J ⥤ C
, given a cocone c : Cocone F
, and a natural isomorphism α : H ≅ H'
for functors
H H' : C ⥤ D
, the precomposition of the cocone H.mapCocone
using the isomorphism α
is
isomorphic to the cocone H'.mapCocone
.
map_cocone
commutes with precompose
. In particular, for F : J ⥤ C
, given a cocone
c : Cocone F
, a natural transformation α : F ⟶ G
and a functor H : C ⥤ D
, we have two obvious
ways of producing a cocone over G ⋙ H
, and they are both isomorphic.
Equations
mapCocone
commutes with precomposeEquivalence
Equations
mapCocone
commutes with whisker
Equations
Change a Cocone F
into a Cone F.op
.
Equations
- c.op = { pt := Opposite.op c.pt, π := CategoryTheory.NatTrans.op c.ι }
Change a Cone F
into a Cocone F.op
.
Equations
- c.op = { pt := Opposite.op c.pt, ι := CategoryTheory.NatTrans.op c.π }
Change a Cocone F.op
into a Cone F
.
Equations
- c.unop = { pt := Opposite.unop c.pt, π := CategoryTheory.NatTrans.removeOp c.ι }
Change a Cone F.op
into a Cocone F
.
Equations
- c.unop = { pt := Opposite.unop c.pt, ι := CategoryTheory.NatTrans.removeOp c.π }
The category of cocones on F
is equivalent to the opposite category of
the category of cones on the opposite of F
.
Equations
- One or more equations did not get rendered due to their size.
Change a cocone on F.leftOp : Jᵒᵖ ⥤ C
to a cocone on F : J ⥤ Cᵒᵖ
.
Equations
- CategoryTheory.Limits.coneOfCoconeLeftOp c = { pt := Opposite.op c.pt, π := CategoryTheory.NatTrans.removeLeftOp c.ι }
Change a cone on F : J ⥤ Cᵒᵖ
to a cocone on F.leftOp : Jᵒᵖ ⥤ C
.
Equations
- CategoryTheory.Limits.coconeLeftOpOfCone c = { pt := Opposite.unop c.pt, ι := CategoryTheory.NatTrans.leftOp c.π }
Change a cone on F.leftOp : Jᵒᵖ ⥤ C
to a cocone on F : J ⥤ Cᵒᵖ
.
Equations
- CategoryTheory.Limits.coconeOfConeLeftOp c = { pt := Opposite.op c.pt, ι := CategoryTheory.NatTrans.removeLeftOp c.π }
Change a cocone on F : J ⥤ Cᵒᵖ
to a cone on F.leftOp : Jᵒᵖ ⥤ C
.
Equations
- CategoryTheory.Limits.coneLeftOpOfCocone c = { pt := Opposite.unop c.pt, π := CategoryTheory.NatTrans.leftOp c.ι }
Change a cocone on F.rightOp : J ⥤ Cᵒᵖ
to a cone on F : Jᵒᵖ ⥤ C
.
Equations
- CategoryTheory.Limits.coneOfCoconeRightOp c = { pt := Opposite.unop c.pt, π := CategoryTheory.NatTrans.removeRightOp c.ι }
Change a cone on F : Jᵒᵖ ⥤ C
to a cocone on F.rightOp : Jᵒᵖ ⥤ C
.
Equations
- CategoryTheory.Limits.coconeRightOpOfCone c = { pt := Opposite.op c.pt, ι := CategoryTheory.NatTrans.rightOp c.π }
Change a cone on F.rightOp : J ⥤ Cᵒᵖ
to a cocone on F : Jᵒᵖ ⥤ C
.
Equations
- CategoryTheory.Limits.coconeOfConeRightOp c = { pt := Opposite.unop c.pt, ι := CategoryTheory.NatTrans.removeRightOp c.π }
Change a cocone on F : Jᵒᵖ ⥤ C
to a cone on F.rightOp : J ⥤ Cᵒᵖ
.
Equations
- CategoryTheory.Limits.coneRightOpOfCocone c = { pt := Opposite.op c.pt, π := CategoryTheory.NatTrans.rightOp c.ι }
Change a cocone on F.unop : J ⥤ C
into a cone on F : Jᵒᵖ ⥤ Cᵒᵖ
.
Equations
- CategoryTheory.Limits.coneOfCoconeUnop c = { pt := Opposite.op c.pt, π := CategoryTheory.NatTrans.removeUnop c.ι }
Change a cone on F : Jᵒᵖ ⥤ Cᵒᵖ
into a cocone on F.unop : J ⥤ C
.
Equations
- CategoryTheory.Limits.coconeUnopOfCone c = { pt := Opposite.unop c.pt, ι := CategoryTheory.NatTrans.unop c.π }
Change a cone on F.unop : J ⥤ C
into a cocone on F : Jᵒᵖ ⥤ Cᵒᵖ
.
Equations
- CategoryTheory.Limits.coconeOfConeUnop c = { pt := Opposite.op c.pt, ι := CategoryTheory.NatTrans.removeUnop c.π }
Change a cocone on F : Jᵒᵖ ⥤ Cᵒᵖ
into a cone on F.unop : J ⥤ C
.
Equations
- CategoryTheory.Limits.coneUnopOfCocone c = { pt := Opposite.unop c.pt, π := CategoryTheory.NatTrans.unop c.ι }
The opposite cocone of the image of a cone is the image of the opposite cocone.
Equations
The opposite cone of the image of a cocone is the image of the opposite cone.