### Notes

Notes will be posted occasionally to supplement the material in lectures and references.

• #### Surfaces and Turns

###### Tuesday, Jan 28, 2020.

In this note we describe how to determine if a two-complex is an oriented surface (possibly with boundary), with polygons oriented according to the orientation of the surface. Some cases may be missed below, so please check the description in addition to implementing it. Assume that we are given a two-complex complex: TwoComplex. First steps Check that the complex is valid, using complex.checkComplex (also check if that method covers all checks).

• #### Quadrangulations: Constructions and Geodesics

###### Wednesday, Jan 29, 2020.

We previously worked with general two-complexes, including those representing surfaces. We now specialize to the first of the three special representations that are most useful to us - non-positive quadrangulations. Here we discuss what these are, how to construct these and the geodesics for these. Non-positive quadrangulations For a closed surface $\Sigma$, a non-positive quadrangulation is a two-complex homeomorphic to $\Sigma$ such that Every face has four sides. The degree of (i.