Topology IV.b - homotopy theory

Homework/semi-flipped class. Please read the beginning of Section 3 in Chapter IV. Definition 3.1 introduces the HEP, Definition 3.2 defines cofibrations, and Example 3.3 is a recap on the fact that the inclusion of a sphere as boundary of a closed ball is a cofibration. I will then start the class with Lemma 3.4. The goal of this week is to study general and basic properties of cofibrations, and then move to Section 4 where we explain how to turn any map into a cofibration and apply this to prove that the CW-approximation we have constructed is unique in a strong sense.

NOTE. I will be away next week, so the lecture will take place on Zoom at the usual link.