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Mathlib.CategoryTheory.Limits.FunctorCategory

(Co)limits in functor categories. #

We show that if D has limits, then the functor category C ⥤ D also has limits (CategoryTheory.Limits.functorCategoryHasLimits), and the evaluation functors preserve limits (CategoryTheory.Limits.evaluationPreservesLimits) (and similarly for colimits).

We also show that F : D ⥤ K ⥤ C preserves (co)limits if it does so for each k : K (CategoryTheory.Limits.preservesLimitsOfEvaluation and CategoryTheory.Limits.preservesColimitsOfEvaluation).

The evaluation functors jointly reflect limits: that is, to show a cone is a limit of F it suffices to show that each evaluation cone is a limit. In other words, to prove a cone is limiting you can show it's pointwise limiting.

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    Given a functor F and a collection of limit cones for each diagram X ↦ F X k, we can stitch them together to give a cone for the diagram F. combinedIsLimit shows that the new cone is limiting, and evalCombined shows it is (essentially) made up of the original cones.

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      The stitched together cones each project down to the original given cones (up to iso).

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        The evaluation functors jointly reflect colimits: that is, to show a cocone is a colimit of F it suffices to show that each evaluation cocone is a colimit. In other words, to prove a cocone is colimiting you can show it's pointwise colimiting.

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          Given a functor F and a collection of colimit cocones for each diagram X ↦ F X k, we can stitch them together to give a cocone for the diagram F. combinedIsColimit shows that the new cocone is colimiting, and evalCombined shows it is (essentially) made up of the original cocones.

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            The stitched together cocones each project down to the original given cocones (up to iso).

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              If F : J ⥤ K ⥤ C is a functor into a functor category which has a limit, then the evaluation of that limit at k is the limit of the evaluations of F.obj j at k.

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                If F : J ⥤ K ⥤ C is a functor into a functor category which has a colimit, then the evaluation of that colimit at k is the colimit of the evaluations of F.obj j at k.

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                  The limit of a diagram F : J ⥤ K ⥤ C is isomorphic to the functor given by the individual limits on objects.

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                    For a functor G : J ⥤ K ⥤ C, its limit K ⥤ C is given by (G' : K ⥤ J ⥤ C) ⋙ lim. Note that this does not require K to be small.

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                      The colimit of a diagram F : J ⥤ K ⥤ C is isomorphic to the functor given by the individual colimits on objects.

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                        For a functor G : J ⥤ K ⥤ C, its colimit K ⥤ C is given by (G' : K ⥤ J ⥤ C) ⋙ colim. Note that this does not require K to be small.

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