mathlib documentation

category_theory.limits.preserves.functor_category

Preservation of (co)limits in the functor category #

Show that if X ⨯ - preserves colimits in D for any X : D, then the product functor F ⨯ - for F : C ⥤ D preserves colimits.

The idea of the proof is simply that products and colimits in the functor category are computed pointwise, so pointwise preservation implies general preservation.

References #

https://ncatlab.org/nlab/show/commutativity+of+limits+and+colimits#preservation_by_functor_categories_and_localizations

If X × - preserves colimits in D for any X : D, then the product functor F ⨯ - for F : C ⥤ D also preserves colimits.

Note this is (mathematically) a special case of the statement that "if limits commute with colimits in D, then they do as well in C ⥤ D" but the story in Lean is a bit more complex, and this statement isn't directly a special case. That is, even with a formalised proof of the general statement, there would still need to be some work to convert to this version: namely, the natural isomorphism (evaluation C D).obj k ⋙ prod.functor.obj (F.obj k) ≅ prod.functor.obj F ⋙ (evaluation C D).obj k

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