01.2 Sets, Types, and Universes
1.2 Sets, Types, and Universes Mathematics needs a way to say what kind of thing an object is. Before we can prove a statement, define a function, or compare two objects, we need a domain of discourse. Sets, types, and universes provide different ways to organize that domain. A set is a collection of objects, called elements. We write: x ∈ A to mean that x is an element of...