A Crash Course in Representation Theory

This chapter studies representation theory. In order to state the equivariant localization formula of Atiyah–Bott and Berline–Vergne, one will need to know some representation theory. Representation theory “represents” the elements of a group by matrices in such a way that group multiplication becomes matrix multiplication. It is a way of simplifying group theory. The chapter provides the minimal representation theory needed for equivariant cohomology. A real representation of a group G is a group homomorphism. Every representation has at least two invariant subspaces, 0 and V. These are called the trivial invariant subspaces. A representation is said to be irreducible if it has no invariant subspaces other than 0 and V; otherwise, it is reducible.