Wide pullbacks #
We define the category WidePullbackShape, (resp. WidePushoutShape) which is the category
obtained from a discrete category of type J by adjoining a terminal (resp. initial) element.
Limits of this shape are wide pullbacks (pushouts).
The convenience method wideCospan (wideSpan) constructs a functor from this category, hitting
the given morphisms.
We use WidePullbackShape to define ordinary pullbacks (pushouts) by using J := WalkingPair,
which allows easy proofs of some related lemmas.
Furthermore, wide pullbacks are used to show the existence of limits in the slice category.
Namely, if C has wide pullbacks then C/B has limits for any object B in C.
Typeclasses HasWidePullbacks and HasFiniteWidePullbacks assert the existence of wide
pullbacks and finite wide pullbacks.
A wide pullback shape for any type J can be written simply as Option J.
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A wide pushout shape for any type J can be written simply as Option J.
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The type of arrows for the shape indexing a wide pullback.
- id {J : Type w} (X : WidePullbackShape J) : X.Hom X
- term {J : Type w} (j : J) : Hom (some j) none
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An aesop tactic for bulk cases on morphisms in WidePushoutShape
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Construct a functor out of the wide pullback shape given a J-indexed collection of arrows to a fixed object.
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Every diagram is naturally isomorphic (actually, equal) to a wideCospan
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Construct a cone over a wide cospan.
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Wide pullback diagrams of equivalent index types are equivalent.
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Lifting universe and morphism levels preserves wide pullback diagrams.
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Show two functors out of a wide pullback shape are isomorphic by showing their components are isomorphic.
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The type of arrows for the shape indexing a wide pushout.
- id {J : Type w} (X : WidePushoutShape J) : X.Hom X
- init {J : Type w} (j : J) : Hom none (some j)
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An aesop tactic for bulk cases on morphisms in WidePushoutShape
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Construct a functor out of the wide pushout shape given a J-indexed collection of arrows from a fixed object.
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Every diagram is naturally isomorphic (actually, equal) to a wideSpan
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Construct a cocone over a wide span.
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Wide pushout diagrams of equivalent index types are equivalent.
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Lifting universe and morphism levels preserves wide pushout diagrams.
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A category HasWidePullbacks if it has all limits of shape WidePullbackShape J, i.e. if it
has a wide pullback for every collection of morphisms with the same codomain.
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A category HasWidePushouts if it has all colimits of shape WidePushoutShape J, i.e. if it
has a wide pushout for every collection of morphisms with the same domain.
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HasWidePullback B objs arrows means that wideCospan B objs arrows has a limit.
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HasWidePushout B objs arrows means that wideSpan B objs arrows has a colimit.
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A choice of wide pullback.
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A choice of wide pushout.
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The j-th projection from the pullback.
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The unique map to the base from the pullback.
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Lift a collection of morphisms to a morphism to the pullback.
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A wide pullback cone is a cone on the wide cospan formed by a family of morphisms.
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The projection on the components of a wide pullback cone.
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The projection to the base of a wide pullback cone.
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Construct a wide pullback cone from the projections.
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Constructor to show a wide pullback cone is limiting.
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Lift a family of morphisms to a limiting wide pullback cone.
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To show two wide pullback cones are isomorphic, it suffices to give a compatible isomorphism of their cone points.
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Reindex a wide pullback cone.
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Reindexing a pullback cone preserves being limiting.
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The j-th inclusion to the pushout.
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The unique map from the head to the pushout.
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Descend a collection of morphisms to a morphism from the pushout.
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The action on morphisms of the obvious functor
WidePullbackShape_op : WidePullbackShape J ⥤ (WidePushoutShape J)ᵒᵖ
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The obvious functor WidePullbackShape J ⥤ (WidePushoutShape J)ᵒᵖ
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The action on morphisms of the obvious functor
widePushoutShapeOp : WidePushoutShape J ⥤ (WidePullbackShape J)ᵒᵖ
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The obvious functor WidePushoutShape J ⥤ (WidePullbackShape J)ᵒᵖ
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The obvious functor (WidePullbackShape J)ᵒᵖ ⥤ WidePushoutShape J
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The obvious functor (WidePushoutShape J)ᵒᵖ ⥤ WidePullbackShape J
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The inverse of the unit isomorphism of the equivalence
widePushoutShapeOpEquiv : (WidePushoutShape J)ᵒᵖ ≌ WidePullbackShape J
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The counit isomorphism of the equivalence
widePullbackShapeOpEquiv : (WidePullbackShape J)ᵒᵖ ≌ WidePushoutShape J
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The inverse of the unit isomorphism of the equivalence
widePullbackShapeOpEquiv : (WidePullbackShape J)ᵒᵖ ≌ WidePushoutShape J
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The counit isomorphism of the equivalence
widePushoutShapeOpEquiv : (WidePushoutShape J)ᵒᵖ ≌ WidePullbackShape J
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The duality equivalence (WidePushoutShape J)ᵒᵖ ≌ WidePullbackShape J
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The duality equivalence (WidePullbackShape J)ᵒᵖ ≌ WidePushoutShape J
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If a category has wide pushouts on a higher universe level it also has wide pushouts on a lower universe level.
If a category has wide pullbacks on a higher universe level it also has wide pullbacks on a lower universe level.