Material for the HA course on Wednesday, April 22

Read: [LS] Subsections 2.1.4, 2.1.5 & 2.1.6

Exercises: 2.1.41, 2.1.46, 2.1.54, 2.1.60, 2.1.61, 2.1.68, 2.1.71, 2.1.73, 2.1.74 and 2.1.75

[LS] = Homological Algebra (Bernard Le Stum)