Lecture 4

Massey: Ch. 1, Sec. 6, and "Step 1" of the proof in Sec. 7

Explain the notion of a triangulation. Then embark in the first step of the proof of the classification of surfaces: every compact surface can be viewed as the quotient of a disk with edges appropriately identified!

Ideas: Give examples of triangulations, and non-examples. Try to conceptualize the elaborate argument in step 1, and give an example of it to get a feel for how it works.