Material for the HA course on Wednesday, May 20

Read:

Exercises: 2.1.66, 2.1.67, 2.1.68, 2.2.3 ,2.2.4 , 2.2.17 ,2.2.19, 2.2.22, 2.2.26 and 2.2.28

[LS] = Homological Algebra (Bernard Le Stum)