Material for the HA course on Wednesday, April 29

Read:

Exercises: 1.3.17, 1.3.39, 1.4.12, 1.5.12, 1.5.27, 1.5.41, 2.1.5, 2.1.9, 2.1.13, 2.1.36, 2.1.37, 2.1.54, 2.1.60, 2.1.68

[LS] = Homological Algebra (Bernard Le Stum)