Almost Periodic Functions

Motivated by questions about which functions could be represented by Dirichlet series, Harald Bohr founded the theory of almost periodic functions in the 1920s. This beautiful exposition begins with a discussion of periodic functions before addressing the almost periodic case. An appendix discusses almost periodic functions of a complex variable. This is a beautiful exposition of the theory of ""Almost Periodic Functions"" written by the creator of that theory; translated by H. Cohn.