## Stream: maths

### Topic: derived functors

#### Scott Morrison (Apr 29 2021 at 13:16):

I still have some moderate sorries to fill in, but I just got

def functor.left_derived (n : ℕ) (F : C ⥤ D) [F.additive] : C ⥤ D :=
projective_resolutions C ⋙ F.map_homotopy_category _ _ ⋙ homotopy_category.homology_functor D _ n


to typecheck. :-)

#### Scott Morrison (Apr 29 2021 at 13:17):

I think I am going to keep going forward, and make sure that I can calculate this using a chosen projective resolution, before going back to the sorries.

#### Scott Morrison (Apr 29 2021 at 13:18):

Tor and Ext and group (co)homology, here we come. :-)

#### Oliver Nash (Apr 29 2021 at 13:20):

Oh wow, this is big. Nice one!

#### Johan Commelin (Apr 29 2021 at 13:43):

@Scott Morrison This is incredible! Really cool!

Last updated: Jun 17 2021 at 17:28 UTC