Material for the HA course on Wednesday, April 15

Read: [LS] Subsections 2.1.2 & 2.1.3

Exercises: 2.1.13, 2.1.15, 2.1.20, 2.1.21, 2.1.29, 2.1.30, 2.1.36 and 2.1.37

[LS] = Homological Algebra (Bernard Le Stum)