PineWikiRelations, particularly equivalence relations and their use in constructing the integers and rationals. Readings: §7.2-7.4. See also §18.1 for directed graphs, a way to represent a relation that we will see more of later.
There was some discussion at the end of class of how to construct the real numbers (see construction_of_real_numbers) and more exotic objects like the Surreal_numbers, but you don't need to know about this if you don't want to.