CLASSICAL GEOMETRY — LECTURE NOTES DANNY CALEGARI 1. A CRASH COURSE IN GROUP THEORY A group is an algebraic object which formalizes the mathematical notion which expresses the intuitive idea of symmetry. We start with an abstract definition. Definition . A group is a set G and an operation m : G × G → G called multiplication with the following properties: (1) m is associative. That is, for any a, b, c ∈ G, m(a, m(b, c)) = m(m(a, b), c) and the product can be written unambiguously as abc. (2) There is a unique element e ∈ G called the.
