mathlib documentation

category_theory.derived

Left-derived functors #

We define the left-derived functors F.left_derived n : C ⥤ D for any additive functor F out of a category with projective resolutions.

The definition is

projective_resolutions C  F.map_homotopy_category _  homotopy_category.homology_functor D _ n

that is, we pick a projective resolution (thought of as an object of the homotopy category), we apply F objectwise, and compute n-th homology.

We show that these left-derived functors can be calculated on objects using any choice of projective resolution, and on morphisms by any choice of lift to a chain map between chosen projective resolutions.

Similarly we define natural transformations between left-derived functors coming from natural transformations between the original additive functors, and show how to compute the components.

Implementation #

We don't assume the categories involved are abelian (just preadditive, and have equalizers, cokernels, and image maps), or that the functors are right exact. None of these assumptions are needed yet.

It is often convenient, of course, to work with [abelian C] [enough_projectives C] [abelian D] which (assuming the results from category_theory.abelian.projective) are enough to provide all the typeclass hypotheses assumed here.